[v2,2/3] v4l: Clearly document interactions between formats, controls and buffers

Commit Message

Laurent Pinchart Feb. 28, 2017, 3:03 p.m. UTC

V4L2 exposes parameters that influence buffers sizes through the format
ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
not part of the format structure may also influence buffer sizes or
buffer layout in general. One existing such parameter is rotation, which
is implemented by the VIDIOC_ROTATE control and thus exposed through the
V4L2 control ioctls.

The interaction between those parameters and buffers is currently only
partially specified by the V4L2 API. In particular interactions between
controls and buffers isn't specified at all. The behaviour of the
VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
specified.

This commit clearly defines and documents the interactions between
formats, controls and buffers.

The preparatory discussions for the documentation change considered
completely disallowing controls that change the buffer size or layout,
in favour of extending the format API with a new ioctl that would bundle
those controls with format information. The idea has been rejected, as
this would essentially be a restricted version of the upcoming request
API that wouldn't bring any additional value.

Another option we have considered was to mandate the use of the request
API to modify controls that influence buffer size or layout. This has
also been rejected on the grounds that requiring the request API to
change rotation even when streaming is stopped would significantly
complicate implementation of drivers and usage of the V4L2 API for
applications.

Applications will however be required to use the upcoming request API to
change at runtime formats or controls that influence the buffer size or
layout, because of the need to synchronize buffers with the formats and
controls. Otherwise there would be no way to interpret the content of a
buffer correctly.

Signed-off-by: Laurent Pinchart <[e-mail]>
---
 Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

Comments

Sakari Ailus March 2, 2017, 3:37 p.m. UTC | #1

Hi Laurent,

On Tue, Feb 28, 2017 at 05:03:19PM +0200, Laurent Pinchart wrote:
> V4L2 exposes parameters that influence buffers sizes through the format
> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
> not part of the format structure may also influence buffer sizes or
> buffer layout in general. One existing such parameter is rotation, which
> is implemented by the VIDIOC_ROTATE control and thus exposed through the
> V4L2 control ioctls.
> 
> The interaction between those parameters and buffers is currently only
> partially specified by the V4L2 API. In particular interactions between
> controls and buffers isn't specified at all. The behaviour of the
> VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
> specified.
> 
> This commit clearly defines and documents the interactions between
> formats, controls and buffers.
> 
> The preparatory discussions for the documentation change considered
> completely disallowing controls that change the buffer size or layout,
> in favour of extending the format API with a new ioctl that would bundle
> those controls with format information. The idea has been rejected, as
> this would essentially be a restricted version of the upcoming request
> API that wouldn't bring any additional value.
> 
> Another option we have considered was to mandate the use of the request
> API to modify controls that influence buffer size or layout. This has
> also been rejected on the grounds that requiring the request API to
> change rotation even when streaming is stopped would significantly
> complicate implementation of drivers and usage of the V4L2 API for
> applications.
> 
> Applications will however be required to use the upcoming request API to
> change at runtime formats or controls that influence the buffer size or
> layout, because of the need to synchronize buffers with the formats and
> controls. Otherwise there would be no way to interpret the content of a
> buffer correctly.
> 
> Signed-off-by: Laurent Pinchart <[e-mail]>
> ---
>  Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
>  1 file changed, 88 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index ac58966ccb9b..5c58db98ab7a 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the CAPTURE video
>  buffer.
>  
>  
> +Interactions between formats, controls and buffers
> +==================================================
> +
> +V4L2 exposes parameters that influence the buffer size, or the way data is
> +laid out in the buffer. Those parameters are exposed through both formats and
> +controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
> +that modifies the direction in which pixels are stored in the buffer, as well
> +as the buffer size when the selected format includes padding at the end of
> +lines.
> +
> +The set of information needed to interpret the content of a buffer (e.g. the
> +pixel format, the line stride, the tiling orientation or the rotation) is
> +collectively referred to in the rest of this section as the buffer layout.
> +
> +Modifying formats or controls that influence the buffer size or layout require
> +the stream to be stopped. Any attempt at such a modification while the stream
> +is active shall cause the format or control set ioctl to return the ``EBUSY``
> +error code.
> +
> +Controls that only influence the buffer layout can be modified at any time
> +when the stream is stopped. As they don't influence the buffer size, no
> +special handling is needed to synchronize those controls with buffer
> +allocation.
> +
> +Formats and controls that influence the buffer size interact with buffer
> +allocation. As buffer allocation is an expensive operation, drivers should
> +allow format or controls that influence the buffer size to be changed with
> +buffers allocated. A typical ioctl sequence to modify format and controls is
> +
> + #. VIDIOC_STREAMOFF
> + #. VIDIOC_S_FMT
> + #. VIDIOC_S_EXT_CTRLS

Which one do you set first, the format or the controls? Supposedly the user
would have to get the format again after setting the ROTATE control.

> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +Queued buffers must be large enough for the new format or controls.
> +
> +Drivers shall return a ``ENOSPC`` error in response to format change
> +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
> +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
> +currently queued. As a simplification, drivers are allowed to return an error
> +from these ioctls if any buffer is currently queued, without checking the
> +queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
> +:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
> +current format or controls. Together, these requirements ensure that queued
> +buffers will always be large enough for the configured format and controls.
> +
> +Userspace applications can query the buffer size required for a given format
> +and controls by first setting the desired control values and then trying the
> +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
> +buffer size.
> +
> + #. VIDIOC_S_EXT_CTRLS(x)
> + #. VIDIOC_TRY_FMT()
> + #. VIDIOC_S_EXT_CTRLS(y)
> + #. VIDIOC_TRY_FMT()
> +
> +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
> +based on the queried sizes (for instance by allocating a set of buffers large
> +enough for all the desired formats and controls, or by allocating separate set
> +of appropriately sized buffers for each use case).
> +
> +To simplify their implementation, drivers may also require buffers to be
> +reallocated in order to change formats or controls that influence the buffer
> +size. In that case, to perform such changes, userspace applications shall
> +first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
> +is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
> +they are allocated. The format or controls can then be modified, and buffers
> +shall then be reallocated and the stream restarted. A typical ioctl sequence
> +is
> +
> + #. VIDIOC_STREAMOFF
> + #. VIDIOC_REQBUFS(0)
> + #. VIDIOC_S_FMT
> + #. VIDIOC_S_EXT_CTRLS

Same here.

Would it be safe to say that controls are changed first? I wonder if there
could be special cases where this wouldn't apply though. It could ultimately
come down to hardware features: rotation might be only available for certain
formats so you'd need to change the format first to enable rotation.

What you're documenting above is a typical sequence so it doesn't have to be
applicable to all potential hardware. I might mention there could be such
dependencies. I wonder if one exists at the moment. No?

> + #. VIDIOC_REQBUFS(n)
> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
> +value into account to compute the buffer size to allocate. Applications can
> +also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
> +
> +When reallocation is required, any attempt to modify format or controls that
> +influences the buffer size while buffers are allocated shall cause the format
> +or control set ioctl to return the ``EBUSY`` error code.
> +
> +
>  .. c:type:: v4l2_buffer
>  
>  struct v4l2_buffer

Hans Verkuil March 4, 2017, 10:53 a.m. UTC | #2

Hi Laurent,

Here is my review:

On 28/02/17 16:03, Laurent Pinchart wrote:
> V4L2 exposes parameters that influence buffers sizes through the format
> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters

S_SELECTION should be mentioned here as well (more about that later).

> not part of the format structure may also influence buffer sizes or
> buffer layout in general. One existing such parameter is rotation, which
> is implemented by the VIDIOC_ROTATE control and thus exposed through the
> V4L2 control ioctls.
>
> The interaction between those parameters and buffers is currently only
> partially specified by the V4L2 API. In particular interactions between
> controls and buffers isn't specified at all. The behaviour of the
> VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
> specified.
>
> This commit clearly defines and documents the interactions between
> formats, controls and buffers.
>
> The preparatory discussions for the documentation change considered
> completely disallowing controls that change the buffer size or layout,
> in favour of extending the format API with a new ioctl that would bundle
> those controls with format information. The idea has been rejected, as
> this would essentially be a restricted version of the upcoming request
> API that wouldn't bring any additional value.
>
> Another option we have considered was to mandate the use of the request
> API to modify controls that influence buffer size or layout. This has
> also been rejected on the grounds that requiring the request API to
> change rotation even when streaming is stopped would significantly
> complicate implementation of drivers and usage of the V4L2 API for
> applications.
>
> Applications will however be required to use the upcoming request API to
> change at runtime formats or controls that influence the buffer size or
> layout, because of the need to synchronize buffers with the formats and
> controls. Otherwise there would be no way to interpret the content of a
> buffer correctly.
>
> Signed-off-by: Laurent Pinchart <[e-mail]>
> ---
>  Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
>  1 file changed, 88 insertions(+)
>
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index ac58966ccb9b..5c58db98ab7a 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the CAPTURE video
>  buffer.
>
>
> +Interactions between formats, controls and buffers
> +==================================================
> +
> +V4L2 exposes parameters that influence the buffer size, or the way data is
> +laid out in the buffer. Those parameters are exposed through both formats and
> +controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
> +that modifies the direction in which pixels are stored in the buffer, as well
> +as the buffer size when the selected format includes padding at the end of
> +lines.
> +
> +The set of information needed to interpret the content of a buffer (e.g. the
> +pixel format, the line stride, the tiling orientation or the rotation) is
> +collectively referred to in the rest of this section as the buffer layout.
> +
> +Modifying formats or controls that influence the buffer size or layout require
> +the stream to be stopped. Any attempt at such a modification while the stream
> +is active shall cause the format or control set ioctl to return the ``EBUSY``
> +error code.

This is not what happens today: it's not the streaming part that causes EBUSY to
be returned but whether or not buffers are allocated.

Today we do not support changing buffer sizes on the fly, so any attempt to
call an ioctl that would change the buffer size is blocked and EBUSY is returned.
To be precise: drivers call vb2_is_busy() to determine this.

To my knowledge all vb2-using drivers behave like this. There may be old drivers
that do not do this (and these have a high likelyhood of being wrong).

> +
> +Controls that only influence the buffer layout can be modified at any time
> +when the stream is stopped. As they don't influence the buffer size, no
> +special handling is needed to synchronize those controls with buffer
> +allocation.
> +
> +Formats and controls that influence the buffer size interact with buffer
> +allocation. As buffer allocation is an expensive operation, drivers should
> +allow format or controls that influence the buffer size to be changed with
> +buffers allocated. A typical ioctl sequence to modify format and controls is
> +
> + #. VIDIOC_STREAMOFF
> + #. VIDIOC_S_FMT
> + #. VIDIOC_S_EXT_CTRLS
> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +Queued buffers must be large enough for the new format or controls.
> +
> +Drivers shall return a ``ENOSPC`` error in response to format change
> +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
> +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
> +currently queued. As a simplification, drivers are allowed to return an error
> +from these ioctls if any buffer is currently queued, without checking the
> +queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
> +:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
> +current format or controls.

Actually, today qbuf will return -EINVAL (from __verify_length in videobuf2-v4l2.c)
in these cases. I am pretty sure you can't change that to ENOSPC since this has
always been -EINVAL, and so changing this would break the ABI.

Trying to change the format while buffers are allocated will return EBUSY today.

If you are trying to document what will happen when drivers allow format changes
on the fly then this is not at all clear from what you write here.

So:

If the driver does not support changing the format while buffers are queued, then
it will return EBUSY (true for almost (?) all drivers today). If it does support
this, then it will behave as described above, except for the ENOSPC error in QBUF.

Note that the meaning of ENOSPC should also be explicitly documented in the ioctls
that can return this.

 > Together, these requirements ensure that queued
> +buffers will always be large enough for the configured format and controls.
> +
> +Userspace applications can query the buffer size required for a given format
> +and controls by first setting the desired control values and then trying the
> +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
> +buffer size.
> +
> + #. VIDIOC_S_EXT_CTRLS(x)
> + #. VIDIOC_TRY_FMT()
> + #. VIDIOC_S_EXT_CTRLS(y)
> + #. VIDIOC_TRY_FMT()
> +
> +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
> +based on the queried sizes (for instance by allocating a set of buffers large
> +enough for all the desired formats and controls, or by allocating separate set
> +of appropriately sized buffers for each use case).
> +
> +To simplify their implementation, drivers may also require buffers to be
> +reallocated in order to change formats or controls that influence the buffer
> +size. In that case, to perform such changes, userspace applications shall
> +first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
> +is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
> +they are allocated. The format or controls can then be modified, and buffers
> +shall then be reallocated and the stream restarted. A typical ioctl sequence
> +is
> +
> + #. VIDIOC_STREAMOFF
> + #. VIDIOC_REQBUFS(0)
> + #. VIDIOC_S_FMT
> + #. VIDIOC_S_EXT_CTRLS
> + #. VIDIOC_REQBUFS(n)
> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
> +value into account to compute the buffer size to allocate. Applications can
> +also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
> +
> +When reallocation is required, any attempt to modify format or controls that
> +influences the buffer size while buffers are allocated shall cause the format
> +or control set ioctl to return the ``EBUSY`` error code.

Ah, here you describe the 99% situation. This should come first. You've been working
on the 1% that allows changing formats while buffers are allocated so that feels
all-important to you, but in reality almost all drivers use the 'simplified'
implementation. Describe that first, then go on describing what will happen for
your driver. That will make much more sense to me (as you can tell from the preceding
comments) and I'm sure to the end-user as well.

What should be mentioned here as well is that S_SELECTION can also implicitly change
the format, specifically if there is no scaler or if the scaler has limitations.

Also there are a few ioctls that can reset selections and formats when called:
S_INPUT/S_OUTPUT, S_STD and S_DV_TIMINGS.

I think this should be mentioned here. It can be just in passing, no need to go
in-depth on that. As long as people are aware of it.

The documentation of S_STD and S_DV_TIMINGS should also be updated saying that if
the new std or timings differ from the existing std or timings, then the format
will also change and selection rectangles will be reset to the defaults. Setting
the std/dv_timings with the current std or timings will not do anything: there
are applications that do this even when streaming so this should be allowed.

Unfortunately, this is not documented but it really should.

If you don't have time to update the S_STD/DV_TIMINGS ioctls, then let me know and
I will do that.

> +
> +
>  .. c:type:: v4l2_buffer
>
>  struct v4l2_buffer
>

Regards,

	Hans

Hans Verkuil March 4, 2017, 10:57 a.m. UTC | #3

On 02/03/17 16:37, Sakari Ailus wrote:
> Hi Laurent,
>
> On Tue, Feb 28, 2017 at 05:03:19PM +0200, Laurent Pinchart wrote:
>> V4L2 exposes parameters that influence buffers sizes through the format
>> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
>> not part of the format structure may also influence buffer sizes or
>> buffer layout in general. One existing such parameter is rotation, which
>> is implemented by the VIDIOC_ROTATE control and thus exposed through the
>> V4L2 control ioctls.
>>
>> The interaction between those parameters and buffers is currently only
>> partially specified by the V4L2 API. In particular interactions between
>> controls and buffers isn't specified at all. The behaviour of the
>> VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
>> specified.
>>
>> This commit clearly defines and documents the interactions between
>> formats, controls and buffers.
>>
>> The preparatory discussions for the documentation change considered
>> completely disallowing controls that change the buffer size or layout,
>> in favour of extending the format API with a new ioctl that would bundle
>> those controls with format information. The idea has been rejected, as
>> this would essentially be a restricted version of the upcoming request
>> API that wouldn't bring any additional value.
>>
>> Another option we have considered was to mandate the use of the request
>> API to modify controls that influence buffer size or layout. This has
>> also been rejected on the grounds that requiring the request API to
>> change rotation even when streaming is stopped would significantly
>> complicate implementation of drivers and usage of the V4L2 API for
>> applications.
>>
>> Applications will however be required to use the upcoming request API to
>> change at runtime formats or controls that influence the buffer size or
>> layout, because of the need to synchronize buffers with the formats and
>> controls. Otherwise there would be no way to interpret the content of a
>> buffer correctly.
>>
>> Signed-off-by: Laurent Pinchart <[e-mail]>
>> ---
>>  Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
>>  1 file changed, 88 insertions(+)
>>
>> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
>> index ac58966ccb9b..5c58db98ab7a 100644
>> --- a/Documentation/media/uapi/v4l/buffer.rst
>> +++ b/Documentation/media/uapi/v4l/buffer.rst
>> @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the CAPTURE video
>>  buffer.
>>
>>
>> +Interactions between formats, controls and buffers
>> +==================================================
>> +
>> +V4L2 exposes parameters that influence the buffer size, or the way data is
>> +laid out in the buffer. Those parameters are exposed through both formats and
>> +controls. One example of such a control is the ``V4L2_CI, and also documented in the documentation of
those ioctls (if that isn't done already). The latter can be done later, or D_ROTATE`` control
>> +that modifies the direction in which pixels are stored in the buffer, as well
>> +as the buffer size when the selected format includes padding at the end of
>> +lines.
>> +
>> +The set of information needed to interpret the content of a buffer (e.g. the
>> +pixel format, the line stride, the tiling orientation or the rotation) is
>> +collectively referred to in the rest of this section as the buffer layout.
>> +
>> +Modifying formats or controls that influence the buffer size or layout require
>> +the stream to be stopped. Any attempt at such a modification while the stream
>> +is active shall cause the format or control set ioctl to return the ``EBUSY``
>> +error code.
>> +
>> +Controls that only influence the buffer layout can be modified at any time
>> +when the stream is stopped. As they don't influence the buffer size, no
>> +special handling is needed to synchronize those controls with buffer
>> +allocation.
>> +
>> +Formats and controls that influence the buffer size interact with buffer
>> +allocation. As buffer allocation is an expensive operation, drivers should
>> +allow format or controls that influence the buffer size to be changed with
>> +buffers allocated. A typical ioctl sequence to modify format and controls is
>> +
>> + #. VIDIOC_STREAMOFF
>> + #. VIDIOC_S_FMT
>> + #. VIDIOC_S_EXT_CTRLS
>
> Which one do you set first, the format or the controls? Supposedly the user
> would have to get the format again after setting the ROTATE control.
>
>> + #. VIDIOC_QBUF
>> + #. VIDIOC_STREAMON
>> +
>> +Queued buffers must be large enough for the new format or controls.
>> +
>> +Drivers shall return a ``ENOSPC`` error in response to format change
>> +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
>> +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
>> +currently queued. As a simplification, drivers are allowed to return an error
>> +from these ioctls if any buffer is currently queued, without checking the
>> +queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
>> +:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
>> +current format or controls. Together, these requirements ensure that queued
>> +buffers will always be large enough for the configured format and controls.
>> +
>> +Userspace applications can query the buffer size required for a given format
>> +and controls by first setting the desired control values and then trying the
>> +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
>> +buffer size.
>> +
>> + #. VIDIOC_S_EXT_CTRLS(x)
>> + #. VIDIOC_TRY_FMT()
>> + #. VIDIOC_S_EXT_CTRLS(y)
>> + #. VIDIOC_TRY_FMT()
>> +
>> +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
>> +based on the queried sizes (for instance by allocating a set of buffers large
>> +enough for all the desired formats and controls, or by allocating separate set
>> +of appropriately sized buffers for each use case).
>> +
>> +To simplify their implementation, drivers may also require buffers to be
>> +reallocated in order to change formats or controls that influence the buffer
>> +size. In that case, to perform such changes, userspace applications shall
>> +first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
>> +is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
>> +they are allocated. The format or controls can then be modified, and buffers
>> +shall then be reallocated and the stream restarted. A typical ioctl sequence
>> +is
>> +
>> + #. VIDIOC_STREAMOFF
>> + #. VIDIOC_REQBUFS(0)
>> + #. VIDIOC_S_FMT
>> + #. VIDIOC_S_EXT_CTRLS
>
> Same here.
>
> Would it be safe to say that controls are changed first? I wonder if there
> could be special cases where this wouldn't apply though. It could ultimately
> come down to hardware features: rotation might be only available for certain
> formats so you'd need to change the format first to enable rotation.
>
> What you're documenting above is a typical sequence so it doesn't have to be
> applicable to all potential hardware. I might mention there could be such
> dependencies. I wonder if one exists at the moment. No?

The way V4L2 works is that the last ioctl called gets 'preference'. So the
driver should attempt to satisfy the ioctl, even if that means undoing previous
ioctls. In other words, V4L2 allows any order, but the end-result might be
different depending on the hardware capabilities.

Regards,

	Hans

>
>> + #. VIDIOC_REQBUFS(n)
>> + #. VIDIOC_QBUF
>> + #. VIDIOC_STREAMON
>> +
>> +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
>> +value into account to compute the buffer size to allocate. Applications can
>> +also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
>> +
>> +When reallocation is required, any attempt to modify format or controls that
>> +influences the buffer size while buffers are allocated shall cause the format
>> +or control set ioctl to return the ``EBUSY`` error code.
>> +
>> +
>>  .. c:type:: v4l2_buffer
>>
>>  struct v4l2_buffer
>

Sakari Ailus March 4, 2017, 1:48 p.m. UTC | #4

Hi Hans,

On Sat, Mar 04, 2017 at 11:57:32AM +0100, Hans Verkuil wrote:
...
> >>+To simplify their implementation, drivers may also require buffers to be
> >>+reallocated in order to change formats or controls that influence the buffer
> >>+size. In that case, to perform such changes, userspace applications shall
> >>+first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
> >>+is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
> >>+they are allocated. The format or controls can then be modified, and buffers
> >>+shall then be reallocated and the stream restarted. A typical ioctl sequence
> >>+is
> >>+
> >>+ #. VIDIOC_STREAMOFF
> >>+ #. VIDIOC_REQBUFS(0)
> >>+ #. VIDIOC_S_FMT
> >>+ #. VIDIOC_S_EXT_CTRLS
> >
> >Same here.
> >
> >Would it be safe to say that controls are changed first? I wonder if there
> >could be special cases where this wouldn't apply though. It could ultimately
> >come down to hardware features: rotation might be only available for certain
> >formats so you'd need to change the format first to enable rotation.
> >
> >What you're documenting above is a typical sequence so it doesn't have to be
> >applicable to all potential hardware. I might mention there could be such
> >dependencies. I wonder if one exists at the moment. No?
> 
> The way V4L2 works is that the last ioctl called gets 'preference'. So the
> driver should attempt to satisfy the ioctl, even if that means undoing previous
> ioctls. In other words, V4L2 allows any order, but the end-result might be
> different depending on the hardware capabilities.

Indeed. But the above sequence suggests that formats are set before
controls. I suggested to clarify that part.

Sakari Ailus March 4, 2017, 2:37 p.m. UTC | #5

Hi Hans (and Laurent).

On Sat, Mar 04, 2017 at 11:53:45AM +0100, Hans Verkuil wrote:
> Hi Laurent,
> 
> Here is my review:
> 
> On 28/02/17 16:03, Laurent Pinchart wrote:
> >V4L2 exposes parameters that influence buffers sizes through the format
> >ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
> 
> S_SELECTION should be mentioned here as well (more about that later).
> 
> >not part of the format structure may also influence buffer sizes or
> >buffer layout in general. One existing such parameter is rotation, which
> >is implemented by the VIDIOC_ROTATE control and thus exposed through the
> >V4L2 control ioctls.
> >
> >The interaction between those parameters and buffers is currently only
> >partially specified by the V4L2 API. In particular interactions between
> >controls and buffers isn't specified at all. The behaviour of the
> >VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
> >specified.
> >
> >This commit clearly defines and documents the interactions between
> >formats, controls and buffers.
> >
> >The preparatory discussions for the documentation change considered
> >completely disallowing controls that change the buffer size or layout,
> >in favour of extending the format API with a new ioctl that would bundle
> >those controls with format information. The idea has been rejected, as
> >this would essentially be a restricted version of the upcoming request
> >API that wouldn't bring any additional value.
> >
> >Another option we have considered was to mandate the use of the request
> >API to modify controls that influence buffer size or layout. This has
> >also been rejected on the grounds that requiring the request API to
> >change rotation even when streaming is stopped would significantly
> >complicate implementation of drivers and usage of the V4L2 API for
> >applications.
> >
> >Applications will however be required to use the upcoming request API to
> >change at runtime formats or controls that influence the buffer size or
> >layout, because of the need to synchronize buffers with the formats and
> >controls. Otherwise there would be no way to interpret the content of a
> >buffer correctly.
> >
> >Signed-off-by: Laurent Pinchart <[e-mail]>
> >---
> > Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
> > 1 file changed, 88 insertions(+)
> >
> >diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> >index ac58966ccb9b..5c58db98ab7a 100644
> >--- a/Documentation/media/uapi/v4l/buffer.rst
> >+++ b/Documentation/media/uapi/v4l/buffer.rst
> >@@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the CAPTURE video
> > buffer.
> >
> >
> >+Interactions between formats, controls and buffers
> >+==================================================
> >+
> >+V4L2 exposes parameters that influence the buffer size, or the way data is
> >+laid out in the buffer. Those parameters are exposed through both formats and
> >+controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
> >+that modifies the direction in which pixels are stored in the buffer, as well
> >+as the buffer size when the selected format includes padding at the end of
> >+lines.
> >+
> >+The set of information needed to interpret the content of a buffer (e.g. the
> >+pixel format, the line stride, the tiling orientation or the rotation) is
> >+collectively referred to in the rest of this section as the buffer layout.
> >+
> >+Modifying formats or controls that influence the buffer size or layout require
> >+the stream to be stopped. Any attempt at such a modification while the stream
> >+is active shall cause the format or control set ioctl to return the ``EBUSY``
> >+error code.
> 
> This is not what happens today: it's not the streaming part that causes EBUSY to
> be returned but whether or not buffers are allocated.
> 
> Today we do not support changing buffer sizes on the fly, so any attempt to
> call an ioctl that would change the buffer size is blocked and EBUSY is returned.
> To be precise: drivers call vb2_is_busy() to determine this.

It certainly shouldn't be like that. Not allowing S_FMT() while there are
buffers allocated makes CREATE_BUFS entirely useless.

> 
> To my knowledge all vb2-using drivers behave like this. There may be old drivers
> that do not do this (and these have a high likelyhood of being wrong).

What's really needed is that the driver verifies that the buffer is large
enough to be used for a given format. vb2_is_busy() shouldn't be used to
check whether setting format is allowed.

> 
> >+
> >+Controls that only influence the buffer layout can be modified at any time
> >+when the stream is stopped. As they don't influence the buffer size, no
> >+special handling is needed to synchronize those controls with buffer
> >+allocation.
> >+
> >+Formats and controls that influence the buffer size interact with buffer
> >+allocation. As buffer allocation is an expensive operation, drivers should
> >+allow format or controls that influence the buffer size to be changed with
> >+buffers allocated. A typical ioctl sequence to modify format and controls is
> >+
> >+ #. VIDIOC_STREAMOFF
> >+ #. VIDIOC_S_FMT
> >+ #. VIDIOC_S_EXT_CTRLS
> >+ #. VIDIOC_QBUF
> >+ #. VIDIOC_STREAMON
> >+
> >+Queued buffers must be large enough for the new format or controls.
> >+
> >+Drivers shall return a ``ENOSPC`` error in response to format change
> >+(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
> >+:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
> >+currently queued. As a simplification, drivers are allowed to return an error
> >+from these ioctls if any buffer is currently queued, without checking the
> >+queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
> >+:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
> >+current format or controls.
> 
> Actually, today qbuf will return -EINVAL (from __verify_length in videobuf2-v4l2.c)
> in these cases. I am pretty sure you can't change that to ENOSPC since this has
> always been -EINVAL, and so changing this would break the ABI.

*If* today drivers return -EBUSY on S_FMT if there are buffers allocated,
you can't have this happening in the first place: it is a new condition so
using a new error code is possible. I would not expect applications to try
that either for the same reason.

It is rather that applications designed for a particular device are more
likely to attempt this (e.g. capturing a still image after streaming
viewfinder first).

I realised -EBUSY is not even documented for S_FMT; I'll post a patch to fix
that.

> 
> Trying to change the format while buffers are allocated will return EBUSY today.
> 
> If you are trying to document what will happen when drivers allow format changes
> on the fly then this is not at all clear from what you write here.
> 
> So:
> 
> If the driver does not support changing the format while buffers are queued, then
> it will return EBUSY (true for almost (?) all drivers today). If it does support
> this, then it will behave as described above, except for the ENOSPC error in QBUF.
> 
> Note that the meaning of ENOSPC should also be explicitly documented in the ioctls
> that can return this.
> 
> > Together, these requirements ensure that queued
> >+buffers will always be large enough for the configured format and controls.
> >+
> >+Userspace applications can query the buffer size required for a given format
> >+and controls by first setting the desired control values and then trying the
> >+desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
> >+buffer size.
> >+
> >+ #. VIDIOC_S_EXT_CTRLS(x)
> >+ #. VIDIOC_TRY_FMT()
> >+ #. VIDIOC_S_EXT_CTRLS(y)
> >+ #. VIDIOC_TRY_FMT()
> >+
> >+The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
> >+based on the queried sizes (for instance by allocating a set of buffers large
> >+enough for all the desired formats and controls, or by allocating separate set
> >+of appropriately sized buffers for each use case).
> >+
> >+To simplify their implementation, drivers may also require buffers to be
> >+reallocated in order to change formats or controls that influence the buffer
> >+size. In that case, to perform such changes, userspace applications shall
> >+first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
> >+is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
> >+they are allocated. The format or controls can then be modified, and buffers
> >+shall then be reallocated and the stream restarted. A typical ioctl sequence
> >+is
> >+
> >+ #. VIDIOC_STREAMOFF
> >+ #. VIDIOC_REQBUFS(0)
> >+ #. VIDIOC_S_FMT
> >+ #. VIDIOC_S_EXT_CTRLS
> >+ #. VIDIOC_REQBUFS(n)
> >+ #. VIDIOC_QBUF
> >+ #. VIDIOC_STREAMON
> >+
> >+The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
> >+value into account to compute the buffer size to allocate. Applications can
> >+also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
> >+
> >+When reallocation is required, any attempt to modify format or controls that
> >+influences the buffer size while buffers are allocated shall cause the format
> >+or control set ioctl to return the ``EBUSY`` error code.
> 
> Ah, here you describe the 99% situation. This should come first. You've been working
> on the 1% that allows changing formats while buffers are allocated so that feels
> all-important to you, but in reality almost all drivers use the 'simplified'
> implementation. Describe that first, then go on describing what will happen for
> your driver. That will make much more sense to me (as you can tell from the preceding
> comments) and I'm sure to the end-user as well.
> 
> What should be mentioned here as well is that S_SELECTION can also implicitly change
> the format, specifically if there is no scaler or if the scaler has limitations.

I think it'd be fine to mention that, but the effect is indeed implicit
through the change in format.

> 
> Also there are a few ioctls that can reset selections and formats when called:
> S_INPUT/S_OUTPUT, S_STD and S_DV_TIMINGS.
> 
> I think this should be mentioned here. It can be just in passing, no need to go
> in-depth on that. As long as people are aware of it.
> 
> The documentation of S_STD and S_DV_TIMINGS should also be updated saying that if
> the new std or timings differ from the existing std or timings, then the format
> will also change and selection rectangles will be reset to the defaults. Setting
> the std/dv_timings with the current std or timings will not do anything: there
> are applications that do this even when streaming so this should be allowed.
> 
> Unfortunately, this is not documented but it really should.
> 
> If you don't have time to update the S_STD/DV_TIMINGS ioctls, then let me know and
> I will do that.

Laurent Pinchart March 5, 2017, 12:52 p.m. UTC | #6

Hi Sakari,

On Saturday 04 Mar 2017 15:48:54 Sakari Ailus wrote:
> On Sat, Mar 04, 2017 at 11:57:32AM +0100, Hans Verkuil wrote:
> ...
> 
> >>> +To simplify their implementation, drivers may also require buffers to
> >>> be +reallocated in order to change formats or controls that influence
> >>> the buffer +size. In that case, to perform such changes, userspace
> >>> applications shall +first stop the video stream with the
> >>> :c:func:`VIDIOC_STREAMOFF` ioctl if it +is running and free all buffers
> >>> with the :c:func:`VIDIOC_REQBUFS` ioctl if +they are allocated. The
> >>> format or controls can then be modified, and buffers +shall then be
> >>> reallocated and the stream restarted. A typical ioctl sequence +is
> >>> +
> >>> + #. VIDIOC_STREAMOFF
> >>> + #. VIDIOC_REQBUFS(0)
> >>> + #. VIDIOC_S_FMT
> >>> + #. VIDIOC_S_EXT_CTRLS
> >>
> >> Same here.
> >>
> >> Would it be safe to say that controls are changed first? I wonder if
> >> there could be special cases where this wouldn't apply though. It could
> >> ultimately come down to hardware features: rotation might be only
> >> available for certain formats so you'd need to change the format first
> >> to enable rotation.
> >>
> >> What you're documenting above is a typical sequence so it doesn't have to
> >> be applicable to all potential hardware. I might mention there could be
> >> such dependencies. I wonder if one exists at the moment. No?
> > 
> > The way V4L2 works is that the last ioctl called gets 'preference'. So the
> > driver should attempt to satisfy the ioctl, even if that means undoing
> > previous ioctls. In other words, V4L2 allows any order, but the
> > end-result might be different depending on the hardware capabilities.
> 
> Indeed. But the above sequence suggests that formats are set before
> controls. I suggested to clarify that part.

I agree with both of you. I'll clarify that this is just an example and that 
formats and controls can be set in a different order (or even interleaved).

Laurent Pinchart March 5, 2017, 2:35 p.m. UTC | #7

Hi Hans,

On Saturday 04 Mar 2017 11:53:45 Hans Verkuil wrote:
> On 28/02/17 16:03, Laurent Pinchart wrote:
> > V4L2 exposes parameters that influence buffers sizes through the format
> > ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
> 
> S_SELECTION should be mentioned here as well (more about that later).

I'll update that according to the conversation we had on IRC about this topic.

> > not part of the format structure may also influence buffer sizes or
> > buffer layout in general. One existing such parameter is rotation, which
> > is implemented by the VIDIOC_ROTATE control and thus exposed through the
> > V4L2 control ioctls.
> > 
> > The interaction between those parameters and buffers is currently only
> > partially specified by the V4L2 API. In particular interactions between
> > controls and buffers isn't specified at all. The behaviour of the
> > VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
> > specified.
> > 
> > This commit clearly defines and documents the interactions between
> > formats, controls and buffers.
> > 
> > The preparatory discussions for the documentation change considered
> > completely disallowing controls that change the buffer size or layout,
> > in favour of extending the format API with a new ioctl that would bundle
> > those controls with format information. The idea has been rejected, as
> > this would essentially be a restricted version of the upcoming request
> > API that wouldn't bring any additional value.
> > 
> > Another option we have considered was to mandate the use of the request
> > API to modify controls that influence buffer size or layout. This has
> > also been rejected on the grounds that requiring the request API to
> > change rotation even when streaming is stopped would significantly
> > complicate implementation of drivers and usage of the V4L2 API for
> > applications.
> > 
> > Applications will however be required to use the upcoming request API to
> > change at runtime formats or controls that influence the buffer size or
> > layout, because of the need to synchronize buffers with the formats and
> > controls. Otherwise there would be no way to interpret the content of a
> > buffer correctly.
> > 
> > Signed-off-by: Laurent Pinchart
> > <[e-mail]>
> > ---
> > 
> >  Documentation/media/uapi/v4l/buffer.rst | 88 ++++++++++++++++++++++++++++
> >  1 file changed, 88 insertions(+)
> > 
> > diff --git a/Documentation/media/uapi/v4l/buffer.rst
> > b/Documentation/media/uapi/v4l/buffer.rst index
> > ac58966ccb9b..5c58db98ab7a 100644
> > --- a/Documentation/media/uapi/v4l/buffer.rst
> > +++ b/Documentation/media/uapi/v4l/buffer.rst
> > @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the
> > CAPTURE video> 
> >  buffer.
> > 
> > +Interactions between formats, controls and buffers
> > +==================================================
> > +
> > +V4L2 exposes parameters that influence the buffer size, or the way data
> > is
> > +laid out in the buffer. Those parameters are exposed through both formats
> > and +controls. One example of such a control is the ``V4L2_CID_ROTATE``
> > control +that modifies the direction in which pixels are stored in the
> > buffer, as well +as the buffer size when the selected format includes
> > padding at the end of +lines.
> > +
> > +The set of information needed to interpret the content of a buffer (e.g.
> > the +pixel format, the line stride, the tiling orientation or the
> > rotation) is +collectively referred to in the rest of this section as the
> > buffer layout. +
> > +Modifying formats or controls that influence the buffer size or layout
> > require +the stream to be stopped. Any attempt at such a modification
> > while the stream +is active shall cause the format or control set ioctl
> > to return the ``EBUSY`` +error code.
> 
> This is not what happens today: it's not the streaming part that causes
> EBUSY to be returned but whether or not buffers are allocated.

Note that this patch documents the recommended behaviour we want to achieve. 
It can differ from the behaviour implemented today as long as today's 
behaviour (at least the behaviour of the drivers that we don't consider as 
non-compliant, which is the vast majority) is not contradicted by this patch. 
In other words, we can recommend here a more generic versatile and powerful 
behaviour as long as the more restricted behaviour implemented today is also 
allowed.

> Today we do not support changing buffer sizes on the fly, so any attempt to
> call an ioctl that would change the buffer size is blocked and EBUSY is
> returned. To be precise: drivers call vb2_is_busy() to determine this.
> 
> To my knowledge all vb2-using drivers behave like this. There may be old
> drivers that do not do this (and these have a high likelyhood of being
> wrong).

Please also note that the above description doesn't contradict the behaviour 
implemented today. It explains that changing formats and controls is not 
allowed while streaming, which is a subset of the restriction implemented by 
many drivers through vb2_is_busy() of not allowing changes when buffers are 
allocated.

Please see below for more about this in response to your "here's the 99% case" 
comment.

> > +
> > +Controls that only influence the buffer layout can be modified at any
> > time
> > +when the stream is stopped. As they don't influence the buffer size, no
> > +special handling is needed to synchronize those controls with buffer
> > +allocation.
> > +
> > +Formats and controls that influence the buffer size interact with buffer
> > +allocation. As buffer allocation is an expensive operation, drivers
> > should
> > +allow format or controls that influence the buffer size to be changed
> > with
> > +buffers allocated. A typical ioctl sequence to modify format and controls
> > is +
> > + #. VIDIOC_STREAMOFF
> > + #. VIDIOC_S_FMT
> > + #. VIDIOC_S_EXT_CTRLS
> > + #. VIDIOC_QBUF
> > + #. VIDIOC_STREAMON
> > +
> > +Queued buffers must be large enough for the new format or controls.
> > +
> > +Drivers shall return a ``ENOSPC`` error in response to format change
> > +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
> > +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
> > +currently queued. As a simplification, drivers are allowed to return an
> > error +from these ioctls if any buffer is currently queued, without
> > checking the +queued buffers sizes. Drivers shall also return a
> > ``ENOSPC`` error from the +:c:func:`VIDIOC_QBUF` ioctl if the buffer
> > being queued is too small for the +current format or controls.
> 
> Actually, today qbuf will return -EINVAL (from __verify_length in
> videobuf2-v4l2.c) in these cases.

No, __verify_length() only checks that the bytesused value doesn't exceed the 
buffer length (and only for output buffers, not for capture buffers). 
videobuf2 isn't format-aware, so it doesn't check the buffer length against 
formats and controls.

> I am pretty sure you can't change that to ENOSPC since this has always been
> -EINVAL, and so changing this would break the ABI.

As videobuf2 doesn't perform any such check today, I'm not changing the return 
value :-)

> Trying to change the format while buffers are allocated will return EBUSY
> today.
> 
> If you are trying to document what will happen when drivers allow format
> changes on the fly then this is not at all clear from what you write here.
> 
> So:
> 
> If the driver does not support changing the format while buffers are queued,
> then it will return EBUSY (true for almost (?) all drivers today). If it
> does support this, then it will behave as described above, except for the
> ENOSPC error in QBUF.
> 
> Note that the meaning of ENOSPC should also be explicitly documented in the
> ioctls that can return this.

Agreed.

> > Together, these requirements ensure that queued
> > 
> > +buffers will always be large enough for the configured format and
> > controls. +
> > +Userspace applications can query the buffer size required for a given
> > format +and controls by first setting the desired control values and then
> > trying the +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will
> > return the required +buffer size.
> > +
> > + #. VIDIOC_S_EXT_CTRLS(x)
> > + #. VIDIOC_TRY_FMT()
> > + #. VIDIOC_S_EXT_CTRLS(y)
> > + #. VIDIOC_TRY_FMT()
> > +
> > +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate
> > buffers +based on the queried sizes (for instance by allocating a set of
> > buffers large +enough for all the desired formats and controls, or by
> > allocating separate set +of appropriately sized buffers for each use
> > case).
> > +
> > +To simplify their implementation, drivers may also require buffers to be
> > +reallocated in order to change formats or controls that influence the
> > buffer +size. In that case, to perform such changes, userspace
> > applications shall +first stop the video stream with the
> > :c:func:`VIDIOC_STREAMOFF` ioctl if it +is running and free all buffers
> > with the :c:func:`VIDIOC_REQBUFS` ioctl if +they are allocated. The
> > format or controls can then be modified, and buffers +shall then be
> > reallocated and the stream restarted. A typical ioctl sequence +is
> > +
> > + #. VIDIOC_STREAMOFF
> > + #. VIDIOC_REQBUFS(0)
> > + #. VIDIOC_S_FMT
> > + #. VIDIOC_S_EXT_CTRLS
> > + #. VIDIOC_REQBUFS(n)
> > + #. VIDIOC_QBUF
> > + #. VIDIOC_STREAMON
> > +
> > +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and
> > control +value into account to compute the buffer size to allocate.
> > Applications can +also retrieve the size by calling the
> > :c:func:`VIDIOC_G_FMT` ioctl if needed. +
> > +When reallocation is required, any attempt to modify format or controls
> > that +influences the buffer size while buffers are allocated shall cause
> > the format +or control set ioctl to return the ``EBUSY`` error code.
> 
> Ah, here you describe the 99% situation. This should come first. You've been
> working on the 1% that allows changing formats while buffers are allocated
> so that feels all-important to you, but in reality almost all drivers use
> the 'simplified' implementation. Describe that first, then go on describing
> what will happen for your driver. That will make much more sense to me (as
> you can tell from the preceding comments) and I'm sure to the end-user as
> well.

This was actually my first, non published version. After discussing it with 
Sakari, we decided to change the description. As Sakari mentioned, the 
CREATE_BUFS ioctl becomes quite pointless if we don't allow changes with 
buffers allocated. We obviously can't mandate it, as most drivers don't allow 
it today, but Sakari and I thought we should recommend it. That's what I tried 
to convey in this patch by describing the recommended option first.

The approach I took here was to say

- formats and controls can't be changed while streaming in any case
- when buffers are allocated but the stream is off, drivers can allow changes
- drivers are not required to allow changes while buffers are allocated

What's your opinion about that ?

> What should be mentioned here as well is that S_SELECTION can also
> implicitly change the format, specifically if there is no scaler or if the
> scaler has limitations.
>
> Also there are a few ioctls that can reset selections and formats when
> called: S_INPUT/S_OUTPUT, S_STD and S_DV_TIMINGS.
>
> I think this should be mentioned here. It can be just in passing, no need to
> go in-depth on that. As long as people are aware of it.

Agreed, I'll mention all of these.

> The documentation of S_STD and S_DV_TIMINGS should also be updated saying
> that if the new std or timings differ from the existing std or timings,
> then the format will also change and selection rectangles will be reset to
> the defaults. Setting the std/dv_timings with the current std or timings
> will not do anything: there are applications that do this even when
> streaming so this should be allowed.
> 
> Unfortunately, this is not documented but it really should.
> 
> If you don't have time to update the S_STD/DV_TIMINGS ioctls, then let me
> know and I will do that.

I likely won't have time to do that before leaving for holidays so I'd 
appreciate if you could update the documentation of those ioctls. I would also 
appreciate if we could get this patch merged (or rather a more recent version 
thereof) without being blocked by the S_STD/DV_TIMINGS documentation update 
:-)

> > +
> > +
> > 
> >  .. c:type:: v4l2_buffer
> >  
> >  struct v4l2_buffer

I'll post a v3 now that addresses all your comments except the one about 
describing the most restrictive option first. I'm not completely against

Hans Verkuil March 6, 2017, 9:27 a.m. UTC | #8

On 04/03/17 15:37, Sakari Ailus wrote:
> Hi Hans (and Laurent).
>
> On Sat, Mar 04, 2017 at 11:53:45AM +0100, Hans Verkuil wrote:
>> Hi Laurent,
>>
>> Here is my review:
>>
>> On 28/02/17 16:03, Laurent Pinchart wrote:
>>> V4L2 exposes parameters that influence buffers sizes through the format
>>> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
>>
>> S_SELECTION should be mentioned here as well (more about that later).
>>
>>> not part of the format structure may also influence buffer sizes or
>>> buffer layout in general. One existing such parameter is rotation, which
>>> is implemented by the VIDIOC_ROTATE control and thus exposed through the
>>> V4L2 control ioctls.
>>>
>>> The interaction between those parameters and buffers is currently only
>>> partially specified by the V4L2 API. In particular interactions between
>>> controls and buffers isn't specified at all. The behaviour of the
>>> VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
>>> specified.
>>>
>>> This commit clearly defines and documents the interactions between
>>> formats, controls and buffers.
>>>
>>> The preparatory discussions for the documentation change considered
>>> completely disallowing controls that change the buffer size or layout,
>>> in favour of extending the format API with a new ioctl that would bundle
>>> those controls with format information. The idea has been rejected, as
>>> this would essentially be a restricted version of the upcoming request
>>> API that wouldn't bring any additional value.
>>>
>>> Another option we have considered was to mandate the use of the request
>>> API to modify controls that influence buffer size or layout. This has
>>> also been rejected on the grounds that requiring the request API to
>>> change rotation even when streaming is stopped would significantly
>>> complicate implementation of drivers and usage of the V4L2 API for
>>> applications.
>>>
>>> Applications will however be required to use the upcoming request API to
>>> change at runtime formats or controls that influence the buffer size or
>>> layout, because of the need to synchronize buffers with the formats and
>>> controls. Otherwise there would be no way to interpret the content of a
>>> buffer correctly.
>>>
>>> Signed-off-by: Laurent Pinchart <[e-mail]>
>>> ---
>>> Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
>>> 1 file changed, 88 insertions(+)
>>>
>>> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
>>> index ac58966ccb9b..5c58db98ab7a 100644
>>> --- a/Documentation/media/uapi/v4l/buffer.rst
>>> +++ b/Documentation/media/uapi/v4l/buffer.rst
>>> @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the CAPTURE video
>>> buffer.
>>>
>>>
>>> +Interactions between formats, controls and buffers
>>> +==================================================
>>> +
>>> +V4L2 exposes parameters that influence the buffer size, or the way data is
>>> +laid out in the buffer. Those parameters are exposed through both formats and
>>> +controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
>>> +that modifies the direction in which pixels are stored in the buffer, as well
>>> +as the buffer size when the selected format includes padding at the end of
>>> +lines.
>>> +
>>> +The set of information needed to interpret the content of a buffer (e.g. the
>>> +pixel format, the line stride, the tiling orientation or the rotation) is
>>> +collectively referred to in the rest of this section as the buffer layout.
>>> +
>>> +Modifying formats or controls that influence the buffer size or layout require
>>> +the stream to be stopped. Any attempt at such a modification while the stream
>>> +is active shall cause the format or control set ioctl to return the ``EBUSY``
>>> +error code.
>>
>> This is not what happens today: it's not the streaming part that causes EBUSY to
>> be returned but whether or not buffers are allocated.
>>
>> Today we do not support changing buffer sizes on the fly, so any attempt to
>> call an ioctl that would change the buffer size is blocked and EBUSY is returned.
>> To be precise: drivers call vb2_is_busy() to determine this.
>
> It certainly shouldn't be like that. Not allowing S_FMT() while there are
> buffers allocated makes CREATE_BUFS entirely useless.

Well, CREATE_BUFS can still be used to make large buffers in which the image
is composed. But yes, CREATE_BUFS has limited usefulness today, but the simple
fact is that nobody wrote drivers that really can do on the fly format changes.

>
>>
>> To my knowledge all vb2-using drivers behave like this. There may be old drivers
>> that do not do this (and these have a high likelyhood of being wrong).
>
> What's really needed is that the driver verifies that the buffer is large
> enough to be used for a given format. vb2_is_busy() shouldn't be used to
> check whether setting format is allowed.

Again, there are no drivers that support this. Using vb2_is_busy() prevents
userspace from trying this, since without proper driver support this *will*
fail.

Of course, drivers that do support this will not test vb2_is_busy but the
vb2_is_streaming instead (and with the request API even that can be dropped).

>
>>
>>> +
>>> +Controls that only influence the buffer layout can be modified at any time
>>> +when the stream is stopped. As they don't influence the buffer size, no
>>> +special handling is needed to synchronize those controls with buffer
>>> +allocation.
>>> +
>>> +Formats and controls that influence the buffer size interact with buffer
>>> +allocation. As buffer allocation is an expensive operation, drivers should
>>> +allow format or controls that influence the buffer size to be changed with
>>> +buffers allocated. A typical ioctl sequence to modify format and controls is
>>> +
>>> + #. VIDIOC_STREAMOFF
>>> + #. VIDIOC_S_FMT
>>> + #. VIDIOC_S_EXT_CTRLS
>>> + #. VIDIOC_QBUF
>>> + #. VIDIOC_STREAMON
>>> +
>>> +Queued buffers must be large enough for the new format or controls.
>>> +
>>> +Drivers shall return a ``ENOSPC`` error in response to format change
>>> +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
>>> +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
>>> +currently queued. As a simplification, drivers are allowed to return an error
>>> +from these ioctls if any buffer is currently queued, without checking the
>>> +queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
>>> +:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
>>> +current format or controls.
>>
>> Actually, today qbuf will return -EINVAL (from __verify_length in videobuf2-v4l2.c)
>> in these cases. I am pretty sure you can't change that to ENOSPC since this has
>> always been -EINVAL, and so changing this would break the ABI.
>
> *If* today drivers return -EBUSY on S_FMT if there are buffers allocated,
> you can't have this happening in the first place: it is a new condition so
> using a new error code is possible. I would not expect applications to try
> that either for the same reason.

Yes, this can happen when you pass in USERPTR or DMABUF buffers. It indeed can't
happen with MMAP buffers.

> It is rather that applications designed for a particular device are more
> likely to attempt this (e.g. capturing a still image after streaming
> viewfinder first).
>
> I realised -EBUSY is not even documented for S_FMT; I'll post a patch to fix
> that.
>
>>
>> Trying to change the format while buffers are allocated will return EBUSY today.
>>
>> If you are trying to document what will happen when drivers allow format changes
>> on the fly then this is not at all clear from what you write here.
>>
>> So:
>>
>> If the driver does not support changing the format while buffers are queued, then
>> it will return EBUSY (true for almost (?) all drivers today). If it does support
>> this, then it will behave as described above, except for the ENOSPC error in QBUF.
>>
>> Note that the meaning of ENOSPC should also be explicitly documented in the ioctls
>> that can return this.
>>
>>> Together, these requirements ensure that queued
>>> +buffers will always be large enough for the configured format and controls.
>>> +
>>> +Userspace applications can query the buffer size required for a given format
>>> +and controls by first setting the desired control values and then trying the
>>> +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
>>> +buffer size.
>>> +
>>> + #. VIDIOC_S_EXT_CTRLS(x)
>>> + #. VIDIOC_TRY_FMT()
>>> + #. VIDIOC_S_EXT_CTRLS(y)
>>> + #. VIDIOC_TRY_FMT()
>>> +
>>> +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
>>> +based on the queried sizes (for instance by allocating a set of buffers large
>>> +enough for all the desired formats and controls, or by allocating separate set
>>> +of appropriately sized buffers for each use case).
>>> +
>>> +To simplify their implementation, drivers may also require buffers to be
>>> +reallocated in order to change formats or controls that influence the buffer
>>> +size. In that case, to perform such changes, userspace applications shall
>>> +first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
>>> +is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
>>> +they are allocated. The format or controls can then be modified, and buffers
>>> +shall then be reallocated and the stream restarted. A typical ioctl sequence
>>> +is
>>> +
>>> + #. VIDIOC_STREAMOFF
>>> + #. VIDIOC_REQBUFS(0)
>>> + #. VIDIOC_S_FMT
>>> + #. VIDIOC_S_EXT_CTRLS
>>> + #. VIDIOC_REQBUFS(n)
>>> + #. VIDIOC_QBUF
>>> + #. VIDIOC_STREAMON
>>> +
>>> +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
>>> +value into account to compute the buffer size to allocate. Applications can
>>> +also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
>>> +
>>> +When reallocation is required, any attempt to modify format or controls that
>>> +influences the buffer size while buffers are allocated shall cause the format
>>> +or control set ioctl to return the ``EBUSY`` error code.
>>
>> Ah, here you describe the 99% situation. This should come first. You've been working
>> on the 1% that allows changing formats while buffers are allocated so that feels
>> all-important to you, but in reality almost all drivers use the 'simplified'
>> implementation. Describe that first, then go on describing what will happen for
>> your driver. That will make much more sense to me (as you can tell from the preceding
>> comments) and I'm sure to the end-user as well.
>>
>> What should be mentioned here as well is that S_SELECTION can also implicitly change
>> the format, specifically if there is no scaler or if the scaler has limitations.
>
> I think it'd be fine to mention that, but the effect is indeed implicit
> through the change in format.
>
>>
>> Also there are a few ioctls that can reset selections and formats when called:
>> S_INPUT/S_OUTPUT, S_STD and S_DV_TIMINGS.
>>
>> I think this should be mentioned here. It can be just in passing, no need to go
>> in-depth on that. As long as people are aware of it.
>>
>> The documentation of S_STD and S_DV_TIMINGS should also be updated saying that if
>> the new std or timings differ from the existing std or timings, then the format
>> will also change and selection rectangles will be reset to the defaults. Setting
>> the std/dv_timings with the current std or timings will not do anything: there
>> are applications that do this even when streaming so this should be allowed.
>>
>> Unfortunately, this is not documented but it really should.
>>
>> If you don't have time to update the S_STD/DV_TIMINGS ioctls, then let me know and
>> I will do that.
>

Regards,

	Hans

Hans Verkuil March 6, 2017, 9:41 a.m. UTC | #9

On 05/03/17 15:35, Laurent Pinchart wrote:
> Hi Hans,
>
> On Saturday 04 Mar 2017 11:53:45 Hans Verkuil wrote:
>> On 28/02/17 16:03, Laurent Pinchart wrote:
>>> V4L2 exposes parameters that influence buffers sizes through the format
>>> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
>>
>> S_SELECTION should be mentioned here as well (more about that later).
>
> I'll update that according to the conversation we had on IRC about this topic.
>
>>> not part of the format structure may also influence buffer sizes or
>>> buffer layout in general. One existing such parameter is rotation, which
>>> is implemented by the VIDIOC_ROTATE control and thus exposed through the
>>> V4L2 control ioctls.
>>>
>>> The interaction between those parameters and buffers is currently only
>>> partially specified by the V4L2 API. In particular interactions between
>>> controls and buffers isn't specified at all. The behaviour of the
>>> VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
>>> specified.
>>>
>>> This commit clearly defines and documents the interactions between
>>> formats, controls and buffers.
>>>
>>> The preparatory discussions for the documentation change considered
>>> completely disallowing controls that change the buffer size or layout,
>>> in favour of extending the format API with a new ioctl that would bundle
>>> those controls with format information. The idea has been rejected, as
>>> this would essentially be a restricted version of the upcoming request
>>> API that wouldn't bring any additional value.
>>>
>>> Another option we have considered was to mandate the use of the request
>>> API to modify controls that influence buffer size or layout. This has
>>> also been rejected on the grounds that requiring the request API to
>>> change rotation even when streaming is stopped would significantly
>>> complicate implementation of drivers and usage of the V4L2 API for
>>> applications.
>>>
>>> Applications will however be required to use the upcoming request API to
>>> change at runtime formats or controls that influence the buffer size or
>>> layout, because of the need to synchronize buffers with the formats and
>>> controls. Otherwise there would be no way to interpret the content of a
>>> buffer correctly.
>>>
>>> Signed-off-by: Laurent Pinchart
>>> <[e-mail]>
>>> ---
>>>
>>>  Documentation/media/uapi/v4l/buffer.rst | 88 ++++++++++++++++++++++++++++
>>>  1 file changed, 88 insertions(+)
>>>
>>> diff --git a/Documentation/media/uapi/v4l/buffer.rst
>>> b/Documentation/media/uapi/v4l/buffer.rst index
>>> ac58966ccb9b..5c58db98ab7a 100644
>>> --- a/Documentation/media/uapi/v4l/buffer.rst
>>> +++ b/Documentation/media/uapi/v4l/buffer.rst
>>> @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the
>>> CAPTURE video>
>>>  buffer.
>>>
>>> +Interactions between formats, controls and buffers
>>> +==================================================
>>> +
>>> +V4L2 exposes parameters that influence the buffer size, or the way data
>>> is
>>> +laid out in the buffer. Those parameters are exposed through both formats
>>> and +controls. One example of such a control is the ``V4L2_CID_ROTATE``
>>> control +that modifies the direction in which pixels are stored in the
>>> buffer, as well +as the buffer size when the selected format includes
>>> padding at the end of +lines.
>>> +
>>> +The set of information needed to interpret the content of a buffer (e.g.
>>> the +pixel format, the line stride, the tiling orientation or the
>>> rotation) is +collectively referred to in the rest of this section as the
>>> buffer layout. +
>>> +Modifying formats or controls that influence the buffer size or layout
>>> require +the stream to be stopped. Any attempt at such a modification
>>> while the stream +is active shall cause the format or control set ioctl
>>> to return the ``EBUSY`` +error code.
>>
>> This is not what happens today: it's not the streaming part that causes
>> EBUSY to be returned but whether or not buffers are allocated.
>
> Note that this patch documents the recommended behaviour we want to achieve.
> It can differ from the behaviour implemented today as long as today's
> behaviour (at least the behaviour of the drivers that we don't consider as
> non-compliant, which is the vast majority) is not contradicted by this patch.
> In other words, we can recommend here a more generic versatile and powerful
> behaviour as long as the more restricted behaviour implemented today is also
> allowed.
>
>> Today we do not support changing buffer sizes on the fly, so any attempt to
>> call an ioctl that would change the buffer size is blocked and EBUSY is
>> returned. To be precise: drivers call vb2_is_busy() to determine this.
>>
>> To my knowledge all vb2-using drivers behave like this. There may be old
>> drivers that do not do this (and these have a high likelyhood of being
>> wrong).
>
> Please also note that the above description doesn't contradict the behaviour
> implemented today. It explains that changing formats and controls is not
> allowed while streaming, which is a subset of the restriction implemented by
> many drivers through vb2_is_busy() of not allowing changes when buffers are
> allocated.
>
> Please see below for more about this in response to your "here's the 99% case"
> comment.
>
>>> +
>>> +Controls that only influence the buffer layout can be modified at any
>>> time
>>> +when the stream is stopped. As they don't influence the buffer size, no
>>> +special handling is needed to synchronize those controls with buffer
>>> +allocation.
>>> +
>>> +Formats and controls that influence the buffer size interact with buffer
>>> +allocation. As buffer allocation is an expensive operation, drivers
>>> should
>>> +allow format or controls that influence the buffer size to be changed
>>> with
>>> +buffers allocated. A typical ioctl sequence to modify format and controls
>>> is +
>>> + #. VIDIOC_STREAMOFF
>>> + #. VIDIOC_S_FMT
>>> + #. VIDIOC_S_EXT_CTRLS
>>> + #. VIDIOC_QBUF
>>> + #. VIDIOC_STREAMON
>>> +
>>> +Queued buffers must be large enough for the new format or controls.
>>> +
>>> +Drivers shall return a ``ENOSPC`` error in response to format change
>>> +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
>>> +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
>>> +currently queued. As a simplification, drivers are allowed to return an
>>> error +from these ioctls if any buffer is currently queued, without
>>> checking the +queued buffers sizes. Drivers shall also return a
>>> ``ENOSPC`` error from the +:c:func:`VIDIOC_QBUF` ioctl if the buffer
>>> being queued is too small for the +current format or controls.
>>
>> Actually, today qbuf will return -EINVAL (from __verify_length in
>> videobuf2-v4l2.c) in these cases.
>
> No, __verify_length() only checks that the bytesused value doesn't exceed the
> buffer length (and only for output buffers, not for capture buffers).
> videobuf2 isn't format-aware, so it doesn't check the buffer length against
> formats and controls.

True, sorry about that.

However, buf_prepare *does* check for that (see uvc_buffer_prepare()) and
returns -EINVAL. So my comment stands.

>
>> I am pretty sure you can't change that to ENOSPC since this has always been
>> -EINVAL, and so changing this would break the ABI.
>
> As videobuf2 doesn't perform any such check today, I'm not changing the return
> value :-)

Sorry, no. Most drivers test this in buf_prepare (and any that do not are
almost certainly buggy) and return -EINVAL.

>
>> Trying to change the format while buffers are allocated will return EBUSY
>> today.
>>
>> If you are trying to document what will happen when drivers allow format
>> changes on the fly then this is not at all clear from what you write here.
>>
>> So:
>>
>> If the driver does not support changing the format while buffers are queued,
>> then it will return EBUSY (true for almost (?) all drivers today). If it
>> does support this, then it will behave as described above, except for the
>> ENOSPC error in QBUF.
>>
>> Note that the meaning of ENOSPC should also be explicitly documented in the
>> ioctls that can return this.
>
> Agreed.
>
>>> Together, these requirements ensure that queued
>>>
>>> +buffers will always be large enough for the configured format and
>>> controls. +
>>> +Userspace applications can query the buffer size required for a given
>>> format +and controls by first setting the desired control values and then
>>> trying the +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will
>>> return the required +buffer size.
>>> +
>>> + #. VIDIOC_S_EXT_CTRLS(x)
>>> + #. VIDIOC_TRY_FMT()
>>> + #. VIDIOC_S_EXT_CTRLS(y)
>>> + #. VIDIOC_TRY_FMT()
>>> +
>>> +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate
>>> buffers +based on the queried sizes (for instance by allocating a set of
>>> buffers large +enough for all the desired formats and controls, or by
>>> allocating separate set +of appropriately sized buffers for each use
>>> case).
>>> +
>>> +To simplify their implementation, drivers may also require buffers to be
>>> +reallocated in order to change formats or controls that influence the
>>> buffer +size. In that case, to perform such changes, userspace
>>> applications shall +first stop the video stream with the
>>> :c:func:`VIDIOC_STREAMOFF` ioctl if it +is running and free all buffers
>>> with the :c:func:`VIDIOC_REQBUFS` ioctl if +they are allocated. The
>>> format or controls can then be modified, and buffers +shall then be
>>> reallocated and the stream restarted. A typical ioctl sequence +is
>>> +
>>> + #. VIDIOC_STREAMOFF
>>> + #. VIDIOC_REQBUFS(0)
>>> + #. VIDIOC_S_FMT
>>> + #. VIDIOC_S_EXT_CTRLS
>>> + #. VIDIOC_REQBUFS(n)
>>> + #. VIDIOC_QBUF
>>> + #. VIDIOC_STREAMON
>>> +
>>> +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and
>>> control +value into account to compute the buffer size to allocate.
>>> Applications can +also retrieve the size by calling the
>>> :c:func:`VIDIOC_G_FMT` ioctl if needed. +
>>> +When reallocation is required, any attempt to modify format or controls
>>> that +influences the buffer size while buffers are allocated shall cause
>>> the format +or control set ioctl to return the ``EBUSY`` error code.
>>
>> Ah, here you describe the 99% situation. This should come first. You've been
>> working on the 1% that allows changing formats while buffers are allocated
>> so that feels all-important to you, but in reality almost all drivers use
>> the 'simplified' implementation. Describe that first, then go on describing
>> what will happen for your driver. That will make much more sense to me (as
>> you can tell from the preceding comments) and I'm sure to the end-user as
>> well.
>
> This was actually my first, non published version. After discussing it with
> Sakari, we decided to change the description. As Sakari mentioned, the
> CREATE_BUFS ioctl becomes quite pointless if we don't allow changes with
> buffers allocated. We obviously can't mandate it, as most drivers don't allow
> it today, but Sakari and I thought we should recommend it. That's what I tried
> to convey in this patch by describing the recommended option first.
>
> The approach I took here was to say
>
> - formats and controls can't be changed while streaming in any case
> - when buffers are allocated but the stream is off, drivers can allow changes
> - drivers are not required to allow changes while buffers are allocated
>
> What's your opinion about that ?

I disagree. Drivers needs explicit support to be able to change format when
buffers are allocated, but streaming is off: they need to check if the
allocated buffers are big enough for the new format. BTW: we need a vb2 helper
function for that.

No driver does that today. Very few drivers have any need for that, it's only
the more complex SoC drivers that would need this, and even then it is of
limited functionality: it is really the request API that would make this truly
useful.

So you should really describe the common situation, then the complex one.
Anything else is very confusing for the average reader (heck, it confused me!).

The recommended way to organize documentation is to start with the simple, common
case, then work your way up to the more complex situations. That should be done
here as well.

>
>> What should be mentioned here as well is that S_SELECTION can also
>> implicitly change the format, specifically if there is no scaler or if the
>> scaler has limitations.
>>
>> Also there are a few ioctls that can reset selections and formats when
>> called: S_INPUT/S_OUTPUT, S_STD and S_DV_TIMINGS.
>>
>> I think this should be mentioned here. It can be just in passing, no need to
>> go in-depth on that. As long as people are aware of it.
>
> Agreed, I'll mention all of these.
>
>> The documentation of S_STD and S_DV_TIMINGS should also be updated saying
>> that if the new std or timings differ from the existing std or timings,
>> then the format will also change and selection rectangles will be reset to
>> the defaults. Setting the std/dv_timings with the current std or timings
>> will not do anything: there are applications that do this even when
>> streaming so this should be allowed.
>>
>> Unfortunately, this is not documented but it really should.
>>
>> If you don't have time to update the S_STD/DV_TIMINGS ioctls, then let me
>> know and I will do that.
>
> I likely won't have time to do that before leaving for holidays so I'd
> appreciate if you could update the documentation of those ioctls. I would also
> appreciate if we could get this patch merged (or rather a more recent version
> thereof) without being blocked by the S_STD/DV_TIMINGS documentation update
> :-)

No problem.

>
>>> +
>>> +
>>>
>>>  .. c:type:: v4l2_buffer
>>>
>>>  struct v4l2_buffer
>
> I'll post a v3 now that addresses all your comments except the one about
> describing the most restrictive option first. I'm not completely against
>

Hmm, missing text here?

Regards,

	Hans

Patch

diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
index ac58966ccb9b..5c58db98ab7a 100644
--- a/Documentation/media/uapi/v4l/buffer.rst
+++ b/Documentation/media/uapi/v4l/buffer.rst
@@ -34,6 +34,94 @@  flags are copied from the OUTPUT video buffer to the CAPTURE video
 buffer.
 
 
+Interactions between formats, controls and buffers
+==================================================
+
+V4L2 exposes parameters that influence the buffer size, or the way data is
+laid out in the buffer. Those parameters are exposed through both formats and
+controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
+that modifies the direction in which pixels are stored in the buffer, as well
+as the buffer size when the selected format includes padding at the end of
+lines.
+
+The set of information needed to interpret the content of a buffer (e.g. the
+pixel format, the line stride, the tiling orientation or the rotation) is
+collectively referred to in the rest of this section as the buffer layout.
+
+Modifying formats or controls that influence the buffer size or layout require
+the stream to be stopped. Any attempt at such a modification while the stream
+is active shall cause the format or control set ioctl to return the ``EBUSY``
+error code.
+
+Controls that only influence the buffer layout can be modified at any time
+when the stream is stopped. As they don't influence the buffer size, no
+special handling is needed to synchronize those controls with buffer
+allocation.
+
+Formats and controls that influence the buffer size interact with buffer
+allocation. As buffer allocation is an expensive operation, drivers should
+allow format or controls that influence the buffer size to be changed with
+buffers allocated. A typical ioctl sequence to modify format and controls is
+
+ #. VIDIOC_STREAMOFF
+ #. VIDIOC_S_FMT
+ #. VIDIOC_S_EXT_CTRLS
+ #. VIDIOC_QBUF
+ #. VIDIOC_STREAMON
+
+Queued buffers must be large enough for the new format or controls.
+
+Drivers shall return a ``ENOSPC`` error in response to format change
+(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
+:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
+currently queued. As a simplification, drivers are allowed to return an error
+from these ioctls if any buffer is currently queued, without checking the
+queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
+:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
+current format or controls. Together, these requirements ensure that queued
+buffers will always be large enough for the configured format and controls.
+
+Userspace applications can query the buffer size required for a given format
+and controls by first setting the desired control values and then trying the
+desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
+buffer size.
+
+ #. VIDIOC_S_EXT_CTRLS(x)
+ #. VIDIOC_TRY_FMT()
+ #. VIDIOC_S_EXT_CTRLS(y)
+ #. VIDIOC_TRY_FMT()
+
+The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
+based on the queried sizes (for instance by allocating a set of buffers large
+enough for all the desired formats and controls, or by allocating separate set
+of appropriately sized buffers for each use case).
+
+To simplify their implementation, drivers may also require buffers to be
+reallocated in order to change formats or controls that influence the buffer
+size. In that case, to perform such changes, userspace applications shall
+first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
+is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
+they are allocated. The format or controls can then be modified, and buffers
+shall then be reallocated and the stream restarted. A typical ioctl sequence
+is
+
+ #. VIDIOC_STREAMOFF
+ #. VIDIOC_REQBUFS(0)
+ #. VIDIOC_S_FMT
+ #. VIDIOC_S_EXT_CTRLS
+ #. VIDIOC_REQBUFS(n)
+ #. VIDIOC_QBUF
+ #. VIDIOC_STREAMON
+
+The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
+value into account to compute the buffer size to allocate. Applications can
+also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
+
+When reallocation is required, any attempt to modify format or controls that
+influences the buffer size while buffers are allocated shall cause the format
+or control set ioctl to return the ``EBUSY`` error code.
+
+
 .. c:type:: v4l2_buffer
 
 struct v4l2_buffer