[RFC,3/4] Documentation: move dma-buf documentation to rst

Commit Message

Sumit Semwal Aug. 11, 2016, 10:47 a.m. UTC

Branch out dma-buf related documentation into its own rst file to allow
adding it to the sphinx documentation generated.

While at it, move dma-buf-sharing.txt into rst as the dma-buf guide too.

Signed-off-by: Sumit Semwal <[e-mail]>
---
 Documentation/DocBook/device-drivers.tmpl |  37 ---
 Documentation/dma-buf/guide.rst           | 503 ++++++++++++++++++++++++++++++
 Documentation/dma-buf/intro.rst           |  76 +++++
 MAINTAINERS                               |   2 +-
 4 files changed, 580 insertions(+), 38 deletions(-)
 create mode 100644 Documentation/dma-buf/guide.rst
 create mode 100644 Documentation/dma-buf/intro.rst

Comments

Jani Nikula Aug. 11, 2016, 11:47 a.m. UTC | #1

On Thu, 11 Aug 2016, Sumit Semwal <[e-mail]> wrote:
> diff --git a/Documentation/dma-buf/guide.rst b/Documentation/dma-buf/guide.rst
> new file mode 100644
> index 000000000000..fd3534fdccb3
> --- /dev/null
> +++ b/Documentation/dma-buf/guide.rst
> @@ -0,0 +1,503 @@
> +
> +.. _dma-buf-guide:
> +
> +============================
> +DMA Buffer Sharing API Guide
> +============================
> +
> +Sumit Semwal - [e-mail], [e-mail]

Please use the format

:author: Sumit Semwal <[e-mail]>

---

While on this subject, please excuse me for hijacking the thread a bit.

Personally, I believe it would be better to leave out authorship notes
from documentation and source files in collaborative projects. Of
course, it is only fair that people who deserve credit get the
credit. Listing the authors in the file is often the natural thing to
do, and superficially seems fair.

However, when do you add more names to the list? When has someone
contributed enough to warrant that? Is it fair that the original authors
keep getting the credit for the contributions of others? After a while,
perhaps there is next to nothing left of the original contributions, but
the bar is really high for removing anyone from the authors. Listing the
authors gives the impression this is *their* file, while everyone should
feel welcome to contribute, and everyone who contributes should feel
ownership.

IMHO we would be better off using just the git history for the credits.


BR,
Jani.


PS. I am no saint here, I've got a couple of authors lines myself. I
promise not to add more. I certainly won't chastise anyone for adding
theirs.

Markus Heiser Aug. 11, 2016, 11:58 a.m. UTC | #2

Hi Sumit, 

I haven't compiled your patch yet, just my 2cent about the
reStructuredText (reST) ASCII markup ...

Here are some handy links about reST and the Sphinx markup constructs,
we have not yet added to the documentation (sorry):

* reST primer:    http://www.sphinx-doc.org/en/stable/rest.html
* reST quickref:  http://docutils.sourceforge.net/docs/user/rst/quickref.html
* Sphinx markup constructs: http://www.sphinx-doc.org/en/stable/markup/index.html
* Sphinx domains: http://www.sphinx-doc.org/en/stable/domains.html
* Sphinx cross references:  http://www.sphinx-doc.org/en/stable/markup/inline.html#cross-referencing-arbitrary-locations

Am 11.08.2016 um 12:47 schrieb Sumit Semwal <[e-mail]>:

> Branch out dma-buf related documentation into its own rst file to allow
> adding it to the sphinx documentation generated.
> 
> While at it, move dma-buf-sharing.txt into rst as the dma-buf guide too.
> 
> Signed-off-by: Sumit Semwal <[e-mail]>
... 
> +dma-buf operations for device dma only
> +======================================
> +
> +The dma_buf buffer sharing API usage contains the following steps:
> +
> +1. Exporter announces that it wishes to export a buffer
> +2. Userspace gets the file descriptor associated with the exported buffer, and
> +   passes it around to potential buffer-users based on use case
> +3. Each buffer-user 'connects' itself to the buffer
> +4. When needed, buffer-user requests access to the buffer from exporter
> +5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
> +6. when buffer-user is done using this buffer completely, it 'disconnects'
> +   itself from the buffer.
> +
> +
> +1. Exporter's announcement of buffer export
> +-------------------------------------------

I can't recommend numbering the (sub-) sections explicit, even if you here 
wanted to numerate the steps. Most often section numbers are controlled by
the subordinate toctree directive and this might not fit to the step
numbers.

* http://www.sphinx-doc.org/en/stable/markup/toctree.html?highlight=toc#the-toc-tree

> +
> +   The buffer exporter announces its wish to export a buffer. In this, it
> +   connects its own private buffer data, provides implementation for operations
> +   that can be performed on the exported :c:type:`dma_buf`, and flags for the file
> +   associated with this buffer. All these fields are filled in struct
> +   :c:type:`dma_buf_export_info`, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.
> +

In restructuredText whitespace are markups. 

Do not indent if you don't want to create a (indented) block. I recommend to drop
the two spaces in front of the paragraphs.

IMHO you have to decide if you like to use sectioning (drop the to spaces)
or stay with the two spaces and use an enumeration list. If you wan't to stay
with the enumeration write:

|1. Exporter's announcement of buffer export
|
|   The buffer exporter announces its wish to export a buffer. In this, it
|   connects its own private buffer ..
|   ...


> +   Interface:
> +      :c:type:`DEFINE_DMA_BUF_EXPORT_INFO(exp_info) <DEFINE_DMA_BUF_EXPORT_INFO>`

If haven't tested your patch, but I guess this will result in a Warning.

the markup ":c:type" is a reference to a typedef description

* http://www.sphinx-doc.org/en/stable/domains.html#role-c:type 

If the description is given, you can use the short form

:c:type:`DEFINE_DMA_BUF_EXPORT_INFO`

but I think, this is a function, not a typedef.

> +
> +      :c:func:`struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)<dma_buf_export>`

short form to refer    

 :c:func:`dma_buf_export()`

> +
> +   If this succeeds, dma_buf_export allocates a dma_buf structure, and
> +   returns a pointer to the same. It also associates an anonymous file with this
> +   buffer, so it can be exported. On failure to allocate the dma_buf object,
> +   it returns NULL.
> +
> +   'exp_name' in struct dma_buf_export_info is the name of exporter - to
> +   facilitate information while debugging. It is set to KBUILD_MODNAME by

If you want to render a constant in a monospace font you can use the
inline markup ``KBUILD_MODNAME``, but if you want.

> +   default, so exporters don't have to provide a specific name, if they don't
> +   wish to.
> +
> +   DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
> +   zeroes it out and pre-populates exp_name in it.
> +
> +2. Userspace gets a handle to pass around to potential buffer-users
> +-------------------------------------------------------------------
> +
> +   Userspace entity requests for a file-descriptor (fd) which is a handle to the
> +   anonymous file associated with the buffer. It can then share the fd with other
> +   drivers and/or processes.
> +
> +   Interface:
> +      :c:func:`int dma_buf_fd(struct dma_buf *dmabuf, int flags) <dma_buf_fd>`
> +
> +   This API installs an fd for the anonymous file associated with this buffer;
> +   returns either 'fd', or error.

I recommend to markup source code objects uniform with ``fd``.

> +
> +3. Each buffer-user 'connects' itself to the buffer
> +---------------------------------------------------
> +
> +   Each buffer-user now gets a reference to the buffer, using the fd passed to
> +   it.
> +
> +   Interface:
> +      :c:func:`struct dma_buf *dma_buf_get(int fd) <dma_buf_get>`
> +
> +   This API will return a reference to the dma_buf, and increment refcount for
> +   it.
> +
> +   After this, the buffer-user needs to attach its device with the buffer, which
> +   helps the exporter to know of device buffer constraints.
> +
> +   Interface:
> +      :c:func:`struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev) <dma_buf_attach>`
> +
> +   This API returns reference to an attachment structure, which is then used
> +   for scatterlist operations. It will optionally call the 'attach' dma_buf
> +   operation, if provided by the exporter.
> +
> +   The dma-buf sharing framework does the bookkeeping bits related to managing
> +   the list of all attachments to a buffer.
> +
> +.. note:: Until this stage, the buffer-exporter has the option to choose not to
> +   actually allocate the backing storage for this buffer, but wait for the
> +   first buffer-user to request use of buffer for allocation.

Use newlines ... which are markups in reST ;)

.. note:: 

 Until this stage, the buffer-exporter has the option to choose not to
 actually allocate the backing storage for this buffer, but wait for the
 first buffer-user to request use of buffer for allocation.


> +6. when buffer-user is done using this buffer, it 'disconnects' itself from the buffer.
> +---------------------------------------------------------------------------------------
> +
> +   After the buffer-user has no more interest in using this buffer, it should
> +   disconnect itself from the buffer:
> +
> +   * it first detaches itself from the buffer.
> +
> +   Interface:
> +      :c:func:`void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *dmabuf_attach) <dma_buf_detach>`
> +
> +   This API removes the attachment from the list in dmabuf, and optionally calls
> +   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
> +
> +   * Then, the buffer-user returns the buffer reference to exporter.
> +
> +   Interface:
> +     :c:func:`void dma_buf_put(struct dma_buf *dmabuf) <dma_buf_put>`
> +
> +   This API then reduces the refcount for this buffer.
> +
> +   If, as a result of this call, the refcount becomes 0, the 'release' file
> +   operation related to this fd is called. It calls the dmabuf->ops->release()
> +   operation in turn, and frees the memory allocated for dmabuf when exported.
> +
> +NOTES:
> +------
> +

I can't recommend to use colons in titles.

> +* **Importance of attach-detach and {map,unmap}_dma_buf operation pairs**
> +
> +   The attach-detach calls allow the exporter to figure out backing-storage

only 2 spaces are needed here ... whitespaces are markups ;) 

> +   constraints for the currently-interested devices. This allows preferential
> +   allocation, and/or migration of pages across different types of storage
> +   available, if possible.
> +
> +   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
> +   to allow just-in-time backing of storage, and migration mid-way through a
> +   use-case.

> +
> +* **Migration of backing storage if needed**
> +

I can't recommend this style, using a list and a first  bold line to emulate
something what looks like a subsection .. use subsections or leave the bold line.



> +   If after
> +
> +   * at least one map_dma_buf has happened,
> +   * and the backing storage has been allocated for this buffer,
> +
> +   another new buffer-user intends to attach itself to this buffer, it might
> +   be allowed, if possible for the exporter.
> +
> +   In case it is allowed by the exporter:
> +
> +   * if the new buffer-user has stricter 'backing-storage constraints', and the
> +     exporter can handle these constraints, the exporter can just stall on the
> +     map_dma_buf until all outstanding access is completed (as signalled by
> +     unmap_dma_buf).
> +
> +   * Once all users have finished accessing and have unmapped this buffer, the
> +     exporter could potentially move the buffer to the stricter backing-storage,
> +     and then allow further {map,unmap}_dma_buf operations from any buffer-user
> +     from the migrated backing-storage.
> +
> +   * If the exporter cannot fulfill the backing-storage constraints of the new
> +     buffer-user device as requested, dma_buf_attach() would return an error to
> +     denote non-compatibility of the new buffer-sharing request with the current
> +     buffer.
> +
> +
> +   If the exporter chooses not to allow an attach() operation once a
> +   map_dma_buf() API has been called, it simply returns an error.
> +
> +Kernel cpu access to a dma-buf buffer object
> +============================================
> +
> +The motivation to allow cpu access from the kernel to a dma-buf object from the
> +importers side are:
> +
> +* fallback operations, e.g. if the devices is connected to a usb bus and the
> +  kernel needs to shuffle the data around first before sending it away.
> +* full transparency for existing users on the importer side, i.e. userspace
> +  should not notice the difference between a normal object from that subsystem
> +  and an imported one backed by a dma-buf. This is really important for drm
> +  opengl drivers that expect to still use all the existing upload/download
> +  paths.

I is recommended to separate blocks (in this case the list item blocks) with
a newline. E.g.

* first lorem
 ipsum

* second lorem
 ipsum

If you have only one-liners, it is OK to write

* first
* second

> +
> +Access to a dma_buf from the kernel context involves three steps:
> +
> +1. Prepare access, which invalidate any necessary caches and make the object
> +   available for cpu access.
> +2. Access the object page-by-page with the dma_buf map apis
> +3. Finish access, which will flush any necessary cpu caches and free reserved
> +   resources.
> +:`void dma_buf_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direction dir) <dma_buf_end_cpu_access>`
> +
> +
> +Direct Userspace Access/mmap Support
> +====================================
> +
> +Being able to mmap an export dma-buf buffer object has 2 main use-cases:
> +* CPU fallback processing in a pipeline and
> +* supporting existing mmap interfaces in importers.
> +

Insert a newline in front of the list.

> +1. CPU fallback processing in a pipeline
> +----------------------------------------
> +
> +   In many processing pipelines it is sometimes required that the cpu can access
> +   the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
> +   the need to handle this specially in userspace frameworks for buffer sharing
> +   it's ideal if the dma_buf fd itself can be used to access the backing storage
> +   from userspace using mmap.
> +
> +   Furthermore Android's ION framework already supports this (and is otherwise
> +   rather similar to dma-buf from a userspace consumer side with using fds as
> +   handles, too). So it's beneficial to support this in a similar fashion on
> +   dma-buf to have a good transition path for existing Android userspace.
> +
> +   No special interfaces, userspace simply calls mmap on the dma-buf fd, making
> +   sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
> +   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
> +   -EAGAIN or -EINTR, in which case it must be restarted.
> +
> +   Some systems might need some sort of cache coherency management e.g. when
> +   CPU and GPU domains are being accessed through dma-buf at the same time. To
> +   circumvent this problem there are begin/end coherency markers, that forward
> +   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
> +   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
> +   would be used like following:
> +
> +     * mmap dma-buf fd
> +     * for each drawing/upload cycle in CPU
> +
> +       1. SYNC_START ioctl,
> +       2. read/write to mmap area
> +       3. SYNC_END ioctl.
> +
> +       This can be repeated as often as you want (with the new data being
> +       consumed by the GPU or say scanout device)
> +     * munmap once you don't need the buffer any more
> +

see above, use newline to separate last list item from the one before

> +    For correctness and optimal performance, it is always required to use
> +    SYNC_START and SYNC_END before and after, respectively, when accessing the
> +    mapped address. Userspace cannot rely on coherent access, even when there
> +    are systems where it just works without calling these ioctls.
> +
> 
> +Introduction
> +------------
> +
> +The dma-buf subsystem provides the framework for sharing buffers for
> +hardware (DMA) access across multiple device drivers and subsystems, and
> +for synchronizing asynchronous hardware access.
> +
> +This is used, for example, by drm "prime" multi-GPU support, but is of
> +course not limited to GPU use cases.
> +
> +The three main components of this are:
> +
> +* dma-buf_: represents an sg_table, and is exposed to userspace as a file
> +  descriptor to allow passing between devices,
> +
> +* fence_: which provides a mechanism to signal when one device has finished
> +  access, and
> +
> +* reservation_: manages the shared or exclusive fence(s) associated with the
> +  buffer.
> +
> +Please refer to :ref:`DMA buffer sharing guide <dma-buf-guide>` for more details.

alternative you can use the short form :ref:`dma-buf-guide`, depends on the
context from which you refer and the targets title.

But over all, I wan't say: thanks for your work :)

-- Markus --

Markus Heiser Aug. 11, 2016, 12:12 p.m. UTC | #3

Am 11.08.2016 um 13:58 schrieb Markus Heiser <[e-mail]>:
>> +.. note:: Until this stage, the buffer-exporter has the option to choose not to
>> +   actually allocate the backing storage for this buffer, but wait for the
>> +   first buffer-user to request use of buffer for allocation.
> 
> Use newlines ... which are markups in reST ;)
> 
> .. note:: 
> 
> Until this stage, the buffer-exporter has the option to choose not to
> actually allocate the backing storage for this buffer, but wait for the
> first buffer-user to request use of buffer for allocation.
> 

Sorry, my f... apple mail drops leading whitespaces ...

|.. note::
|
|   Until this stage, the buffer-exporter has the option to choose not to
|   actually allocate the backing storage for this buffer, but wait for the

>> +Kernel cpu access to a dma-buf buffer object
>> +============================================
>> +
>> +The motivation to allow cpu access from the kernel to a dma-buf object from the
>> +importers side are:
>> +
>> +* fallback operations, e.g. if the devices is connected to a usb bus and the
>> +  kernel needs to shuffle the data around first before sending it away.
>> +* full transparency for existing users on the importer side, i.e. userspace
>> +  should not notice the difference between a normal object from that subsystem
>> +  and an imported one backed by a dma-buf. This is really important for drm
>> +  opengl drivers that expect to still use all the existing upload/download
>> +  paths.
> 
> I is recommended to separate blocks (in this case the list item blocks) with
> a newline. E.g.
> 
> * first lorem
> ipsum
> 
> * second lorem
> ipsum
> 
> If you have only one-liners, it is OK to write
> 
> * first
> * second
> 

same here, leading whitespace are droped by the mail client.

|* first lorem
|  ipsum
|
|* second lorem
|  ipsum

Sorry for disorientation. For a snatch I forgot, that 
that Apple & MS have a mistaken idea of "plain text" ;)


-- Markus --

Sumit Semwal Aug. 11, 2016, 12:31 p.m. UTC | #4

Hello Jani,

On 11 August 2016 at 17:17, Jani Nikula <[e-mail]> wrote:
> On Thu, 11 Aug 2016, Sumit Semwal <[e-mail]> wrote:
>> diff --git a/Documentation/dma-buf/guide.rst b/Documentation/dma-buf/guide.rst
>> new file mode 100644
>> index 000000000000..fd3534fdccb3
>> --- /dev/null
>> +++ b/Documentation/dma-buf/guide.rst
>> @@ -0,0 +1,503 @@
>> +
>> +.. _dma-buf-guide:
>> +
>> +============================
>> +DMA Buffer Sharing API Guide
>> +============================
>> +
>> +Sumit Semwal - [e-mail], [e-mail]
>
> Please use the format
>
> :author: Sumit Semwal <[e-mail]>
>
Thanks very much for reviewing!
> ---
>
> While on this subject, please excuse me for hijacking the thread a bit.
>
> Personally, I believe it would be better to leave out authorship notes
> from documentation and source files in collaborative projects. Of
> course, it is only fair that people who deserve credit get the
> credit. Listing the authors in the file is often the natural thing to
> do, and superficially seems fair.
>
> However, when do you add more names to the list? When has someone
> contributed enough to warrant that? Is it fair that the original authors
> keep getting the credit for the contributions of others? After a while,
> perhaps there is next to nothing left of the original contributions, but
> the bar is really high for removing anyone from the authors. Listing the
> authors gives the impression this is *their* file, while everyone should
> feel welcome to contribute, and everyone who contributes should feel
> ownership.
>
> IMHO we would be better off using just the git history for the credits.
>
:) - I totally agree with your stand; this patch was an (almost)
direct conversion from the earlier format, hence this patch.

But yes, I will remove it in the next iteration.
>
> BR,
> Jani.
>
>
BR,
Sumit.

> PS. I am no saint here, I've got a couple of authors lines myself. I
> promise not to add more. I certainly won't chastise anyone for adding
> theirs.
>
>
> --
> Jani Nikula, Intel Open Source Technology Center

Sumit Semwal Aug. 11, 2016, 12:49 p.m. UTC | #5

Hi Markus,

On 11 August 2016 at 17:28, Markus Heiser <[e-mail]> wrote:
> Hi Sumit,
>
> I haven't compiled your patch yet, just my 2cent about the
> reStructuredText (reST) ASCII markup ...
>
Thanks very much for your detailed review comments - highly appreciated!

> Here are some handy links about reST and the Sphinx markup constructs,
> we have not yet added to the documentation (sorry):
>
> * reST primer:    http://www.sphinx-doc.org/en/stable/rest.html
> * reST quickref:  http://docutils.sourceforge.net/docs/user/rst/quickref.html
> * Sphinx markup constructs: http://www.sphinx-doc.org/en/stable/markup/index.html
> * Sphinx domains: http://www.sphinx-doc.org/en/stable/domains.html
> * Sphinx cross references:  http://www.sphinx-doc.org/en/stable/markup/inline.html#cross-referencing-arbitrary-locations
>
> Am 11.08.2016 um 12:47 schrieb Sumit Semwal <[e-mail]>:
>
>> Branch out dma-buf related documentation into its own rst file to allow
>> adding it to the sphinx documentation generated.
>>
>> While at it, move dma-buf-sharing.txt into rst as the dma-buf guide too.
>>
>> Signed-off-by: Sumit Semwal <[e-mail]>
> ...
>> +dma-buf operations for device dma only
>> +======================================
>> +
>> +The dma_buf buffer sharing API usage contains the following steps:
>> +
>> +1. Exporter announces that it wishes to export a buffer
>> +2. Userspace gets the file descriptor associated with the exported buffer, and
>> +   passes it around to potential buffer-users based on use case
>> +3. Each buffer-user 'connects' itself to the buffer
>> +4. When needed, buffer-user requests access to the buffer from exporter
>> +5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
>> +6. when buffer-user is done using this buffer completely, it 'disconnects'
>> +   itself from the buffer.
>> +
>> +
>> +1. Exporter's announcement of buffer export
>> +-------------------------------------------
>
> I can't recommend numbering the (sub-) sections explicit, even if you here
> wanted to numerate the steps. Most often section numbers are controlled by
> the subordinate toctree directive and this might not fit to the step
> numbers.
>
> * http://www.sphinx-doc.org/en/stable/markup/toctree.html?highlight=toc#the-toc-tree
>
>> +
>> +   The buffer exporter announces its wish to export a buffer. In this, it
>> +   connects its own private buffer data, provides implementation for operations
>> +   that can be performed on the exported :c:type:`dma_buf`, and flags for the file
>> +   associated with this buffer. All these fields are filled in struct
>> +   :c:type:`dma_buf_export_info`, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.
>> +
>
> In restructuredText whitespace are markups.
>
> Do not indent if you don't want to create a (indented) block. I recommend to drop
> the two spaces in front of the paragraphs.
>
> IMHO you have to decide if you like to use sectioning (drop the to spaces)
> or stay with the two spaces and use an enumeration list. If you wan't to stay
> with the enumeration write:
>
> |1. Exporter's announcement of buffer export
> |
> |   The buffer exporter announces its wish to export a buffer. In this, it
> |   connects its own private buffer ..
> |   ...
>
>
Understood; will correct.

>> +   Interface:
>> +      :c:type:`DEFINE_DMA_BUF_EXPORT_INFO(exp_info) <DEFINE_DMA_BUF_EXPORT_INFO>`
>
> If haven't tested your patch, but I guess this will result in a Warning.
>
> the markup ":c:type" is a reference to a typedef description
>
> * http://www.sphinx-doc.org/en/stable/domains.html#role-c:type
>
> If the description is given, you can use the short form
>
> :c:type:`DEFINE_DMA_BUF_EXPORT_INFO`
>
> but I think, this is a function, not a typedef.
>
I did test it before sending out the RFC, and didn't get any warnings.
This, btw, is a macro, so I guess that's why no warning.

While on this topic, in my experimentation, if I just use the short
form, the resultant text doesn't include the "(exp_info)" part as
shown above.

>> +
>> +      :c:func:`struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)<dma_buf_export>`
>
> short form to refer
>
>  :c:func:`dma_buf_export()`
>
When I last tried the short form, it only gave me a link to the full
definition, and I wanted to have the arguments also shown here, so
that the explanation below was easier. Though I think I tried without
the (), so will try again and let you know.

>> +
>> +   If this succeeds, dma_buf_export allocates a dma_buf structure, and
>> +   returns a pointer to the same. It also associates an anonymous file with this
>> +   buffer, so it can be exported. On failure to allocate the dma_buf object,
>> +   it returns NULL.
>> +
>> +   'exp_name' in struct dma_buf_export_info is the name of exporter - to
>> +   facilitate information while debugging. It is set to KBUILD_MODNAME by
>
> If you want to render a constant in a monospace font you can use the
> inline markup ``KBUILD_MODNAME``, but if you want.
>
Thanks, will do.

>> +   default, so exporters don't have to provide a specific name, if they don't
>> +   wish to.
>> +
>> +   DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
>> +   zeroes it out and pre-populates exp_name in it.
>> +
>> +2. Userspace gets a handle to pass around to potential buffer-users
>> +-------------------------------------------------------------------
>> +
>> +   Userspace entity requests for a file-descriptor (fd) which is a handle to the
>> +   anonymous file associated with the buffer. It can then share the fd with other
>> +   drivers and/or processes.
>> +
>> +   Interface:
>> +      :c:func:`int dma_buf_fd(struct dma_buf *dmabuf, int flags) <dma_buf_fd>`
>> +
>> +   This API installs an fd for the anonymous file associated with this buffer;
>> +   returns either 'fd', or error.
>
> I recommend to markup source code objects uniform with ``fd``.
>
Ok.
>> +
>> +3. Each buffer-user 'connects' itself to the buffer
>> +---------------------------------------------------
>> +
>> +   Each buffer-user now gets a reference to the buffer, using the fd passed to
>> +   it.
>> +
>> +   Interface:
>> +      :c:func:`struct dma_buf *dma_buf_get(int fd) <dma_buf_get>`
>> +
>> +   This API will return a reference to the dma_buf, and increment refcount for
>> +   it.
>> +
>> +   After this, the buffer-user needs to attach its device with the buffer, which
>> +   helps the exporter to know of device buffer constraints.
>> +
>> +   Interface:
>> +      :c:func:`struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev) <dma_buf_attach>`
>> +
>> +   This API returns reference to an attachment structure, which is then used
>> +   for scatterlist operations. It will optionally call the 'attach' dma_buf
>> +   operation, if provided by the exporter.
>> +
>> +   The dma-buf sharing framework does the bookkeeping bits related to managing
>> +   the list of all attachments to a buffer.
>> +
>> +.. note:: Until this stage, the buffer-exporter has the option to choose not to
>> +   actually allocate the backing storage for this buffer, but wait for the
>> +   first buffer-user to request use of buffer for allocation.
>
> Use newlines ... which are markups in reST ;)
>
> .. note::
>
>  Until this stage, the buffer-exporter has the option to choose not to
>  actually allocate the backing storage for this buffer, but wait for the
>  first buffer-user to request use of buffer for allocation.
>
>
Ok :), will correct.

>> +6. when buffer-user is done using this buffer, it 'disconnects' itself from the buffer.
>> +---------------------------------------------------------------------------------------
>> +
>> +   After the buffer-user has no more interest in using this buffer, it should
>> +   disconnect itself from the buffer:
>> +
>> +   * it first detaches itself from the buffer.
>> +
>> +   Interface:
>> +      :c:func:`void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *dmabuf_attach) <dma_buf_detach>`
>> +
>> +   This API removes the attachment from the list in dmabuf, and optionally calls
>> +   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
>> +
>> +   * Then, the buffer-user returns the buffer reference to exporter.
>> +
>> +   Interface:
>> +     :c:func:`void dma_buf_put(struct dma_buf *dmabuf) <dma_buf_put>`
>> +
>> +   This API then reduces the refcount for this buffer.
>> +
>> +   If, as a result of this call, the refcount becomes 0, the 'release' file
>> +   operation related to this fd is called. It calls the dmabuf->ops->release()
>> +   operation in turn, and frees the memory allocated for dmabuf when exported.
>> +
>> +NOTES:
>> +------
>> +
>
> I can't recommend to use colons in titles.
>
Ok, will remove them.

>> +* **Importance of attach-detach and {map,unmap}_dma_buf operation pairs**
>> +
>> +   The attach-detach calls allow the exporter to figure out backing-storage
>
> only 2 spaces are needed here ... whitespaces are markups ;)
>
Right, got the message;)

>> +   constraints for the currently-interested devices. This allows preferential
>> +   allocation, and/or migration of pages across different types of storage
>> +   available, if possible.
>> +
>> +   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
>> +   to allow just-in-time backing of storage, and migration mid-way through a
>> +   use-case.
>
>> +
>> +* **Migration of backing storage if needed**
>> +
>
> I can't recommend this style, using a list and a first  bold line to emulate
> something what looks like a subsection .. use subsections or leave the bold line.
>
Yes, that's right; will correct.
>
>
>> +   If after
>> +
>> +   * at least one map_dma_buf has happened,
>> +   * and the backing storage has been allocated for this buffer,
>> +
>> +   another new buffer-user intends to attach itself to this buffer, it might
>> +   be allowed, if possible for the exporter.
>> +
>> +   In case it is allowed by the exporter:
>> +
>> +   * if the new buffer-user has stricter 'backing-storage constraints', and the
>> +     exporter can handle these constraints, the exporter can just stall on the
>> +     map_dma_buf until all outstanding access is completed (as signalled by
>> +     unmap_dma_buf).
>> +
>> +   * Once all users have finished accessing and have unmapped this buffer, the
>> +     exporter could potentially move the buffer to the stricter backing-storage,
>> +     and then allow further {map,unmap}_dma_buf operations from any buffer-user
>> +     from the migrated backing-storage.
>> +
>> +   * If the exporter cannot fulfill the backing-storage constraints of the new
>> +     buffer-user device as requested, dma_buf_attach() would return an error to
>> +     denote non-compatibility of the new buffer-sharing request with the current
>> +     buffer.
>> +
>> +
>> +   If the exporter chooses not to allow an attach() operation once a
>> +   map_dma_buf() API has been called, it simply returns an error.
>> +
>> +Kernel cpu access to a dma-buf buffer object
>> +============================================
>> +
>> +The motivation to allow cpu access from the kernel to a dma-buf object from the
>> +importers side are:
>> +
>> +* fallback operations, e.g. if the devices is connected to a usb bus and the
>> +  kernel needs to shuffle the data around first before sending it away.
>> +* full transparency for existing users on the importer side, i.e. userspace
>> +  should not notice the difference between a normal object from that subsystem
>> +  and an imported one backed by a dma-buf. This is really important for drm
>> +  opengl drivers that expect to still use all the existing upload/download
>> +  paths.
>
> I is recommended to separate blocks (in this case the list item blocks) with
> a newline. E.g.
>
> * first lorem
>  ipsum
>
> * second lorem
>  ipsum
>
> If you have only one-liners, it is OK to write
>
> * first
> * second
>

Understood, good tip for me.
>> +
>> +Access to a dma_buf from the kernel context involves three steps:
>> +
>> +1. Prepare access, which invalidate any necessary caches and make the object
>> +   available for cpu access.
>> +2. Access the object page-by-page with the dma_buf map apis
>> +3. Finish access, which will flush any necessary cpu caches and free reserved
>> +   resources.
>> +:`void dma_buf_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direction dir) <dma_buf_end_cpu_access>`
>> +
>> +
>> +Direct Userspace Access/mmap Support
>> +====================================
>> +
>> +Being able to mmap an export dma-buf buffer object has 2 main use-cases:
>> +* CPU fallback processing in a pipeline and
>> +* supporting existing mmap interfaces in importers.
>> +
>
> Insert a newline in front of the list.
>
Ok.

>> +1. CPU fallback processing in a pipeline
>> +----------------------------------------
>> +
>> +   In many processing pipelines it is sometimes required that the cpu can access
>> +   the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
>> +   the need to handle this specially in userspace frameworks for buffer sharing
>> +   it's ideal if the dma_buf fd itself can be used to access the backing storage
>> +   from userspace using mmap.
>> +
>> +   Furthermore Android's ION framework already supports this (and is otherwise
>> +   rather similar to dma-buf from a userspace consumer side with using fds as
>> +   handles, too). So it's beneficial to support this in a similar fashion on
>> +   dma-buf to have a good transition path for existing Android userspace.
>> +
>> +   No special interfaces, userspace simply calls mmap on the dma-buf fd, making
>> +   sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
>> +   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
>> +   -EAGAIN or -EINTR, in which case it must be restarted.
>> +
>> +   Some systems might need some sort of cache coherency management e.g. when
>> +   CPU and GPU domains are being accessed through dma-buf at the same time. To
>> +   circumvent this problem there are begin/end coherency markers, that forward
>> +   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
>> +   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
>> +   would be used like following:
>> +
>> +     * mmap dma-buf fd
>> +     * for each drawing/upload cycle in CPU
>> +
>> +       1. SYNC_START ioctl,
>> +       2. read/write to mmap area
>> +       3. SYNC_END ioctl.
>> +
>> +       This can be repeated as often as you want (with the new data being
>> +       consumed by the GPU or say scanout device)
>> +     * munmap once you don't need the buffer any more
>> +
>
> see above, use newline to separate last list item from the one before
>
Ok.
>> +    For correctness and optimal performance, it is always required to use
>> +    SYNC_START and SYNC_END before and after, respectively, when accessing the
>> +    mapped address. Userspace cannot rely on coherent access, even when there
>> +    are systems where it just works without calling these ioctls.
>> +
>>
>> +Introduction
>> +------------
>> +
>> +The dma-buf subsystem provides the framework for sharing buffers for
>> +hardware (DMA) access across multiple device drivers and subsystems, and
>> +for synchronizing asynchronous hardware access.
>> +
>> +This is used, for example, by drm "prime" multi-GPU support, but is of
>> +course not limited to GPU use cases.
>> +
>> +The three main components of this are:
>> +
>> +* dma-buf_: represents an sg_table, and is exposed to userspace as a file
>> +  descriptor to allow passing between devices,
>> +
>> +* fence_: which provides a mechanism to signal when one device has finished
>> +  access, and
>> +
>> +* reservation_: manages the shared or exclusive fence(s) associated with the
>> +  buffer.
>> +
>> +Please refer to :ref:`DMA buffer sharing guide <dma-buf-guide>` for more details.
>
> alternative you can use the short form :ref:`dma-buf-guide`, depends on the
> context from which you refer and the targets title.
>
Will try that out.

> But over all, I wan't say: thanks for your work :)
>
Thanks to you too, appreciate you taking time to read through this whole doc! :)
> -- Markus --
>
>
BR,
Sumit.

Patch

diff --git a/Documentation/DocBook/device-drivers.tmpl b/Documentation/DocBook/device-drivers.tmpl
index 99cdc05bbb7a..a93fbffa9742 100644
--- a/Documentation/DocBook/device-drivers.tmpl
+++ b/Documentation/DocBook/device-drivers.tmpl
@@ -128,43 +128,6 @@  X!Edrivers/base/interface.c
 !Edrivers/base/platform.c
 !Edrivers/base/bus.c
      </sect1>
-     <sect1>
-       <title>Buffer Sharing and Synchronization</title>
-       <para>
-         The dma-buf subsystem provides the framework for sharing buffers
-         for hardware (DMA) access across multiple device drivers and
-         subsystems, and for synchronizing asynchronous hardware access.
-       </para>
-       <para>
-         This is used, for example, by drm "prime" multi-GPU support, but
-         is of course not limited to GPU use cases.
-       </para>
-       <para>
-         The three main components of this are: (1) dma-buf, representing
-         a sg_table and exposed to userspace as a file descriptor to allow
-         passing between devices, (2) fence, which provides a mechanism
-         to signal when one device as finished access, and (3) reservation,
-         which manages the shared or exclusive fence(s) associated with
-         the buffer.
-       </para>
-       <sect2><title>dma-buf</title>
-!Edrivers/dma-buf/dma-buf.c
-!Iinclude/linux/dma-buf.h
-       </sect2>
-       <sect2><title>reservation</title>
-!Pdrivers/dma-buf/reservation.c Reservation Object Overview
-!Edrivers/dma-buf/reservation.c
-!Iinclude/linux/reservation.h
-       </sect2>
-       <sect2><title>fence</title>
-!Edrivers/dma-buf/fence.c
-!Iinclude/linux/fence.h
-!Edrivers/dma-buf/seqno-fence.c
-!Iinclude/linux/seqno-fence.h
-!Edrivers/dma-buf/sync_file.c
-!Iinclude/linux/sync_file.h
-       </sect2>
-     </sect1>
      <sect1><title>Device Drivers DMA Management</title>
 !Edrivers/base/dma-coherent.c
 !Edrivers/base/dma-mapping.c
diff --git a/Documentation/dma-buf/guide.rst b/Documentation/dma-buf/guide.rst
new file mode 100644
index 000000000000..fd3534fdccb3
--- /dev/null
+++ b/Documentation/dma-buf/guide.rst
@@ -0,0 +1,503 @@ 
+
+.. _dma-buf-guide:
+
+============================
+DMA Buffer Sharing API Guide
+============================
+
+Sumit Semwal - [e-mail], [e-mail]
+
+This document serves as a guide to device-driver writers on what is the dma-buf
+buffer sharing API, how to use it for exporting and using shared buffers.
+
+Any device driver which wishes to be a part of DMA buffer sharing, can do so as
+either the 'exporter' of buffers, or the 'user' of buffers.
+
+Say a driver A wants to use buffers created by driver B, then we call B as the
+exporter, and A as buffer-user.
+
+The exporter
+
+* implements and manages operations for the buffer via
+  :c:type:`struct dma_buf_ops <dma_buf_ops>`
+* allows other users to share the buffer by using dma_buf sharing APIs,
+* manages the details of buffer allocation,
+* decides about the actual backing storage where this allocation happens,
+* takes care of any migration of scatterlist - for all (shared) users of this
+  buffer,
+
+The buffer-user
+
+* is one of (many) sharing users of the buffer.
+* doesn't need to worry about how the buffer is allocated, or where.
+* needs a mechanism to get access to the scatterlist that makes up this buffer
+  in memory, mapped into its own address space, so it can access the same area
+  of memory.
+
+dma-buf operations for device dma only
+======================================
+
+The dma_buf buffer sharing API usage contains the following steps:
+
+1. Exporter announces that it wishes to export a buffer
+2. Userspace gets the file descriptor associated with the exported buffer, and
+   passes it around to potential buffer-users based on use case
+3. Each buffer-user 'connects' itself to the buffer
+4. When needed, buffer-user requests access to the buffer from exporter
+5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
+6. when buffer-user is done using this buffer completely, it 'disconnects'
+   itself from the buffer.
+
+
+1. Exporter's announcement of buffer export
+-------------------------------------------
+
+   The buffer exporter announces its wish to export a buffer. In this, it
+   connects its own private buffer data, provides implementation for operations
+   that can be performed on the exported :c:type:`dma_buf`, and flags for the file
+   associated with this buffer. All these fields are filled in struct
+   :c:type:`dma_buf_export_info`, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.
+
+   Interface:
+      :c:type:`DEFINE_DMA_BUF_EXPORT_INFO(exp_info) <DEFINE_DMA_BUF_EXPORT_INFO>`
+
+      :c:func:`struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)<dma_buf_export>`
+
+   If this succeeds, dma_buf_export allocates a dma_buf structure, and
+   returns a pointer to the same. It also associates an anonymous file with this
+   buffer, so it can be exported. On failure to allocate the dma_buf object,
+   it returns NULL.
+
+   'exp_name' in struct dma_buf_export_info is the name of exporter - to
+   facilitate information while debugging. It is set to KBUILD_MODNAME by
+   default, so exporters don't have to provide a specific name, if they don't
+   wish to.
+
+   DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
+   zeroes it out and pre-populates exp_name in it.
+
+2. Userspace gets a handle to pass around to potential buffer-users
+-------------------------------------------------------------------
+
+   Userspace entity requests for a file-descriptor (fd) which is a handle to the
+   anonymous file associated with the buffer. It can then share the fd with other
+   drivers and/or processes.
+
+   Interface:
+      :c:func:`int dma_buf_fd(struct dma_buf *dmabuf, int flags) <dma_buf_fd>`
+
+   This API installs an fd for the anonymous file associated with this buffer;
+   returns either 'fd', or error.
+
+3. Each buffer-user 'connects' itself to the buffer
+---------------------------------------------------
+
+   Each buffer-user now gets a reference to the buffer, using the fd passed to
+   it.
+
+   Interface:
+      :c:func:`struct dma_buf *dma_buf_get(int fd) <dma_buf_get>`
+
+   This API will return a reference to the dma_buf, and increment refcount for
+   it.
+
+   After this, the buffer-user needs to attach its device with the buffer, which
+   helps the exporter to know of device buffer constraints.
+
+   Interface:
+      :c:func:`struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev) <dma_buf_attach>`
+
+   This API returns reference to an attachment structure, which is then used
+   for scatterlist operations. It will optionally call the 'attach' dma_buf
+   operation, if provided by the exporter.
+
+   The dma-buf sharing framework does the bookkeeping bits related to managing
+   the list of all attachments to a buffer.
+
+.. note:: Until this stage, the buffer-exporter has the option to choose not to
+   actually allocate the backing storage for this buffer, but wait for the
+   first buffer-user to request use of buffer for allocation.
+
+
+4. When needed, buffer-user requests access to the buffer
+---------------------------------------------------------
+
+   Whenever a buffer-user wants to use the buffer for any DMA, it asks for
+   access to the buffer using dma_buf_map_attachment API. At least one attach to
+   the buffer must have happened before map_dma_buf can be called.
+
+   Interface:
+      :c:func:`struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, enum dma_data_direction) <dma_buf_map_attachment>`
+
+   This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
+   "dma_buf->ops->" indirection from the users of this interface.
+
+   In struct :c:type:`dma_buf_ops`, map_dma_buf is defined as
+      ``struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, enum dma_data_direction);``
+
+   It is one of the buffer operations that must be implemented by the exporter.
+   It should return the sg_table containing scatterlist for this buffer, mapped
+   into caller's address space.
+
+   If this is being called for the first time, the exporter can now choose to
+   scan through the list of attachments for this buffer, collate the requirements
+   of the attached devices, and choose an appropriate backing storage for the
+   buffer.
+
+   Based on enum :c:type:`dma_data_direction`, it might be possible to have multiple users
+   accessing at the same time (for reading, maybe), or any other kind of sharing
+   that the exporter might wish to make available to buffer-users.
+
+   map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
+
+
+5. When finished, the buffer-user notifies end-of-DMA to exporter
+-----------------------------------------------------------------
+
+   Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
+   the exporter using the dma_buf_unmap_attachment API.
+
+   Interface:
+      :c:func:`void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *) <dma_buf_unmap_attachment>`
+
+   This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
+   "dma_buf->ops->" indirection from the users of this interface.
+
+   In struct dma_buf_ops, unmap_dma_buf is defined as
+      ``void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *, enum dma_data_direction)``
+
+   unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
+   map_dma_buf, this API also must be implemented by the exporter.
+
+
+6. when buffer-user is done using this buffer, it 'disconnects' itself from the buffer.
+---------------------------------------------------------------------------------------
+
+   After the buffer-user has no more interest in using this buffer, it should
+   disconnect itself from the buffer:
+
+   * it first detaches itself from the buffer.
+
+   Interface:
+      :c:func:`void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *dmabuf_attach) <dma_buf_detach>`
+
+   This API removes the attachment from the list in dmabuf, and optionally calls
+   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
+
+   * Then, the buffer-user returns the buffer reference to exporter.
+
+   Interface:
+     :c:func:`void dma_buf_put(struct dma_buf *dmabuf) <dma_buf_put>`
+
+   This API then reduces the refcount for this buffer.
+
+   If, as a result of this call, the refcount becomes 0, the 'release' file
+   operation related to this fd is called. It calls the dmabuf->ops->release()
+   operation in turn, and frees the memory allocated for dmabuf when exported.
+
+NOTES:
+------
+
+* **Importance of attach-detach and {map,unmap}_dma_buf operation pairs**
+
+   The attach-detach calls allow the exporter to figure out backing-storage
+   constraints for the currently-interested devices. This allows preferential
+   allocation, and/or migration of pages across different types of storage
+   available, if possible.
+
+   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
+   to allow just-in-time backing of storage, and migration mid-way through a
+   use-case.
+
+* **Migration of backing storage if needed**
+
+   If after
+
+   * at least one map_dma_buf has happened,
+   * and the backing storage has been allocated for this buffer,
+
+   another new buffer-user intends to attach itself to this buffer, it might
+   be allowed, if possible for the exporter.
+
+   In case it is allowed by the exporter:
+
+   * if the new buffer-user has stricter 'backing-storage constraints', and the
+     exporter can handle these constraints, the exporter can just stall on the
+     map_dma_buf until all outstanding access is completed (as signalled by
+     unmap_dma_buf).
+
+   * Once all users have finished accessing and have unmapped this buffer, the
+     exporter could potentially move the buffer to the stricter backing-storage,
+     and then allow further {map,unmap}_dma_buf operations from any buffer-user
+     from the migrated backing-storage.
+
+   * If the exporter cannot fulfill the backing-storage constraints of the new
+     buffer-user device as requested, dma_buf_attach() would return an error to
+     denote non-compatibility of the new buffer-sharing request with the current
+     buffer.
+
+
+   If the exporter chooses not to allow an attach() operation once a
+   map_dma_buf() API has been called, it simply returns an error.
+
+Kernel cpu access to a dma-buf buffer object
+============================================
+
+The motivation to allow cpu access from the kernel to a dma-buf object from the
+importers side are:
+
+* fallback operations, e.g. if the devices is connected to a usb bus and the
+  kernel needs to shuffle the data around first before sending it away.
+* full transparency for existing users on the importer side, i.e. userspace
+  should not notice the difference between a normal object from that subsystem
+  and an imported one backed by a dma-buf. This is really important for drm
+  opengl drivers that expect to still use all the existing upload/download
+  paths.
+
+Access to a dma_buf from the kernel context involves three steps:
+
+1. Prepare access, which invalidate any necessary caches and make the object
+   available for cpu access.
+2. Access the object page-by-page with the dma_buf map apis
+3. Finish access, which will flush any necessary cpu caches and free reserved
+   resources.
+
+1. Prepare access
+-----------------
+
+   Before an importer can access a dma_buf object with the cpu from the kernel
+   context, it needs to notify the exporter of the access that is about to
+   happen.
+
+   Interface:
+      :c:func:`int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction) <dma_buf_begin_cpu_access>`
+
+   This allows the exporter to ensure that the memory is actually available for
+   cpu access - the exporter might need to allocate or swap-in and pin the
+   backing storage. The exporter also needs to ensure that cpu access is
+   coherent for the access direction. The direction can be used by the exporter
+   to optimize the cache flushing, i.e. access with a different direction (read
+   instead of write) might return stale or even bogus data (e.g. when the
+   exporter needs to copy the data to temporary storage).
+
+   This step might fail, e.g. in oom conditions.
+
+2. Accessing the buffer
+-----------------------
+
+   To support dma_buf objects residing in highmem cpu access is page-based using
+   an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
+   PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
+   a pointer in kernel virtual address space. Afterwards the chunk needs to be
+   unmapped again. There is no limit on how often a given chunk can be mapped
+   and unmapped, i.e. the importer does not need to call begin_cpu_access again
+   before mapping the same chunk again.
+
+   Interfaces:
+      :c:func:`void *dma_buf_kmap(struct dma_buf *, unsigned long) <dma_buf_kmap>`
+
+      :c:func:`void dma_buf_kunmap(struct dma_buf *, unsigned long, void *) <dma_buf_kunmap>`
+
+   There are also atomic variants of these interfaces. Like for kmap they
+   facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
+   the callback) is allowed to block when using these.
+
+   Interfaces:
+      :c:func:`void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long) <dma_buf_kmap_atomic>`
+
+      :c:func:`void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *) <dma_buf_kunmap_atomic>`
+
+   For importers all the restrictions of using kmap apply, like the limited
+   supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
+   atomic dma_buf kmaps at the same time (in any given process context).
+
+   dma_buf kmap calls outside of the range specified in begin_cpu_access are
+   undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
+   the partial chunks at the beginning and end but may return stale or bogus
+   data outside of the range (in these partial chunks).
+
+   Note that these calls need to always succeed. The exporter needs to complete
+   any preparations that might fail in begin_cpu_access.
+
+   For some cases the overhead of kmap can be too high, a vmap interface
+   is introduced. This interface should be used very carefully, as vmalloc
+   space is a limited resources on many architectures.
+
+   Interfaces:
+      :c:func:`void *dma_buf_vmap(struct dma_buf *dmabuf) <dma_buf_vmap>`
+
+      :c:func:`void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr) <dma_buf_vunmap>`
+
+   The vmap call can fail if there is no vmap support in the exporter, or if it
+   runs out of vmalloc space. Fallback to kmap should be implemented. Note that
+   the dma-buf layer keeps a reference count for all vmap access and calls down
+   into the exporter's vmap function only when no vmapping exists, and only
+   unmaps it once. Protection against concurrent vmap/vunmap calls is provided
+   by taking the dma_buf->lock mutex.
+
+3. Finish access
+----------------
+
+   When the importer is done accessing the CPU, it needs to announce this to
+   the exporter (to facilitate cache flushing and unpinning of any pinned
+   resources). The result of any dma_buf kmap calls after end_cpu_access is
+   undefined.
+
+   Interface:
+      :c:func:`void dma_buf_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direction dir) <dma_buf_end_cpu_access>`
+
+
+Direct Userspace Access/mmap Support
+====================================
+
+Being able to mmap an export dma-buf buffer object has 2 main use-cases:
+* CPU fallback processing in a pipeline and
+* supporting existing mmap interfaces in importers.
+
+1. CPU fallback processing in a pipeline
+----------------------------------------
+
+   In many processing pipelines it is sometimes required that the cpu can access
+   the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
+   the need to handle this specially in userspace frameworks for buffer sharing
+   it's ideal if the dma_buf fd itself can be used to access the backing storage
+   from userspace using mmap.
+
+   Furthermore Android's ION framework already supports this (and is otherwise
+   rather similar to dma-buf from a userspace consumer side with using fds as
+   handles, too). So it's beneficial to support this in a similar fashion on
+   dma-buf to have a good transition path for existing Android userspace.
+
+   No special interfaces, userspace simply calls mmap on the dma-buf fd, making
+   sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
+   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
+   -EAGAIN or -EINTR, in which case it must be restarted.
+
+   Some systems might need some sort of cache coherency management e.g. when
+   CPU and GPU domains are being accessed through dma-buf at the same time. To
+   circumvent this problem there are begin/end coherency markers, that forward
+   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
+   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
+   would be used like following:
+
+     * mmap dma-buf fd
+     * for each drawing/upload cycle in CPU
+
+       1. SYNC_START ioctl,
+       2. read/write to mmap area
+       3. SYNC_END ioctl.
+
+       This can be repeated as often as you want (with the new data being
+       consumed by the GPU or say scanout device)
+     * munmap once you don't need the buffer any more
+
+    For correctness and optimal performance, it is always required to use
+    SYNC_START and SYNC_END before and after, respectively, when accessing the
+    mapped address. Userspace cannot rely on coherent access, even when there
+    are systems where it just works without calling these ioctls.
+
+2. Supporting existing mmap interfaces in importers
+---------------------------------------------------
+
+   Similar to the motivation for kernel cpu access it is again important that
+   the userspace code of a given importing subsystem can use the same interfaces
+   with a imported dma-buf buffer object as with a native buffer object. This is
+   especially important for drm where the userspace part of contemporary OpenGL,
+   X, and other drivers is huge, and reworking them to use a different way to
+   mmap a buffer rather invasive.
+
+   The assumption in the current dma-buf interfaces is that redirecting the
+   initial mmap is all that's needed. A survey of some of the existing
+   subsystems shows that no driver seems to do any nefarious thing like syncing
+   up with outstanding asynchronous processing on the device or allocating
+   special resources at fault time. So hopefully this is good enough, since
+   adding interfaces to intercept pagefaults and allow pte shootdowns would
+   increase the complexity quite a bit.
+
+   Interface:
+      :c:func:`int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *, unsigned long) <dma_buf_mmap>`
+
+   If the importing subsystem simply provides a special-purpose mmap call to set
+   up a mapping in userspace, calling do_mmap with dma_buf->file will equally
+   achieve that for a dma-buf object.
+
+3. Implementation notes for exporters
+-------------------------------------
+
+   Because dma-buf buffers have invariant size over their lifetime, the dma-buf
+   core checks whether a vma is too large and rejects such mappings. The
+   exporter hence does not need to duplicate this check.
+
+   Because existing importing subsystems might presume coherent mappings for
+   userspace, the exporter needs to set up a coherent mapping. If that's not
+   possible, it needs to fake coherency by manually shooting down ptes when
+   leaving the cpu domain and flushing caches at fault time. Note that all the
+   dma_buf files share the same anon inode, hence the exporter needs to replace
+   the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
+   required. This is because the kernel uses the underlying inode's address_space
+   for vma tracking (and hence pte tracking at shootdown time with
+   unmap_mapping_range).
+
+   If the above shootdown dance turns out to be too expensive in certain
+   scenarios, we can extend dma-buf with a more explicit cache tracking scheme
+   for userspace mappings. But the current assumption is that using mmap is
+   always a slower path, so some inefficiencies should be acceptable.
+
+   Exporters that shoot down mappings (for any reasons) shall not do any
+   synchronization at fault time with outstanding device operations.
+   Synchronization is an orthogonal issue to sharing the backing storage of a
+   buffer and hence should not be handled by dma-buf itself. This is explicitly
+   mentioned here because many people seem to want something like this, but if
+   different exporters handle this differently, buffer sharing can fail in
+   interesting ways depending upong the exporter (if userspace starts depending
+   upon this implicit synchronization).
+
+Other Interfaces Exposed to Userspace on the dma-buf FD
+-------------------------------------------------------
+
+* Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+  llseek operation will report -EINVAL.
+
+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+  cases. Userspace can use this to detect support for discovering the dma-buf
+  size using llseek.
+
+Miscellaneous notes
+-------------------
+
+* Any exporters or users of the dma-buf buffer sharing framework must have
+  a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
+* In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
+  on the file descriptor.  This is not just a resource leak, but a
+  potential security hole.  It could give the newly exec'd application
+  access to buffers, via the leaked fd, to which it should otherwise
+  not be permitted access.
+
+  The problem with doing this via a separate fcntl() call, versus doing it
+  atomically when the fd is created, is that this is inherently racy in a
+  multi-threaded app (See `here <https://lwn.net/Articles/236486/>`_). The issue
+  is made worse when it is library code opening/creating the file descriptor,
+  as the application may not even be aware of the fd's.
+
+  To avoid this problem, userspace must have a way to request O_CLOEXEC
+  flag be set when the dma-buf fd is created.  So any API provided by
+  the exporting driver to create a dmabuf fd must provide a way to let
+  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
+
+- If an exporter needs to manually flush caches and hence needs to fake
+  coherency for mmap support, it needs to be able to zap all the ptes pointing
+  at the backing storage. Now linux mm needs a struct address_space associated
+  with the struct file stored in vma->vm_file to do that with the function
+  unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
+  with the anon_file struct file, i.e. all dma_bufs share the same file.
+
+  Hence exporters need to setup their own file (and address_space) association
+  by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
+  callback. In the specific case of a gem driver the exporter could use the
+  shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
+  zap ptes by unmapping the corresponding range of the struct address_space
+  associated with their own file.
+
diff --git a/Documentation/dma-buf/intro.rst b/Documentation/dma-buf/intro.rst
new file mode 100644
index 000000000000..36359bbc9865
--- /dev/null
+++ b/Documentation/dma-buf/intro.rst
@@ -0,0 +1,76 @@ 
+==================================
+Buffer Sharing and Synchronization
+==================================
+
+
+Introduction
+------------
+
+The dma-buf subsystem provides the framework for sharing buffers for
+hardware (DMA) access across multiple device drivers and subsystems, and
+for synchronizing asynchronous hardware access.
+
+This is used, for example, by drm "prime" multi-GPU support, but is of
+course not limited to GPU use cases.
+
+The three main components of this are:
+
+* dma-buf_: represents an sg_table, and is exposed to userspace as a file
+  descriptor to allow passing between devices,
+
+* fence_: which provides a mechanism to signal when one device has finished
+  access, and
+
+* reservation_: manages the shared or exclusive fence(s) associated with the
+  buffer.
+
+Please refer to :ref:`DMA buffer sharing guide <dma-buf-guide>` for more details.
+
+.. _dma-buf:
+
+dma-buf
+~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :export:
+
+.. kernel-doc:: include/linux/dma-buf.h
+   :internal:
+
+.. _fence:
+
+fence
+~~~~~
+
+.. kernel-doc:: drivers/dma-buf/fence.c
+   :export:
+
+.. kernel-doc:: include/linux/fence.h
+   :internal:
+
+.. kernel-doc:: drivers/dma-buf/seqno-fence.c
+   :export:
+
+.. kernel-doc:: include/linux/seqno-fence.h
+   :internal:
+
+.. kernel-doc:: drivers/dma-buf/sync_file.c
+   :export:
+
+.. kernel-doc:: include/linux/sync_file.h
+   :internal:
+
+.. _reservation:
+
+reservation
+~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/reservation.c
+   :doc: Reservation Object Overview
+
+.. kernel-doc:: drivers/dma-buf/reservation.c
+   :export:
+
+.. kernel-doc:: include/linux/reservation.h
+   :internal:
+
diff --git a/MAINTAINERS b/MAINTAINERS
index c92defa5be2c..d58b5c4068b6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3816,7 +3816,7 @@  F:	drivers/dma-buf/
 F:	include/linux/dma-buf*
 F:	include/linux/reservation.h
 F:	include/linux/*fence.h
-F:	Documentation/dma-buf-sharing.txt
+F:	Documentation/dma-buf/
 T:	git git://git.linaro.org/people/sumitsemwal/linux-dma-buf.git
 
 DMA GENERIC OFFLOAD ENGINE SUBSYSTEM