| Message ID | 1377703438-21026-1-git-send-email-sakari.ailus@iki.fi (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 08/28/2013 05:23 PM, Sakari Ailus wrote: > Document that monotonic timestamps are taken after the corresponding frame > has been received, not when the reception has begun. This corresponds to the > reality of current drivers: the timestamp is naturally taken when the > hardware triggers an interrupt to tell the driver to handle the received > frame. > > Remove the note on timestamp accuracy as it is fairly subjective what is > actually an unstable timestamp. > > Also remove explanation that output buffer timestamps can be used to delay > outputting a frame. > > Remove the footnote saying we always use realtime clock. > > Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi> Acked-by: Hans Verkuil <hans.verkuil@cisco.com> Regards, Hans > --- > since v4: > - Fixes according to Hans's comments. > > Documentation/DocBook/media/v4l/io.xml | 47 ++++++-------------------------- > 1 file changed, 8 insertions(+), 39 deletions(-) > > diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml > index 2c4c068..9720606 100644 > --- a/Documentation/DocBook/media/v4l/io.xml > +++ b/Documentation/DocBook/media/v4l/io.xml > @@ -654,38 +654,11 @@ plane, are stored in struct <structname>v4l2_plane</structname> instead. > In that case, struct <structname>v4l2_buffer</structname> contains an array of > plane structures.</para> > > - <para>Nominally timestamps refer to the first data byte transmitted. > -In practice however the wide range of hardware covered by the V4L2 API > -limits timestamp accuracy. Often an interrupt routine will > -sample the system clock shortly after the field or frame was stored > -completely in memory. So applications must expect a constant > -difference up to one field or frame period plus a small (few scan > -lines) random error. The delay and error can be much > -larger due to compression or transmission over an external bus when > -the frames are not properly stamped by the sender. This is frequently > -the case with USB cameras. Here timestamps refer to the instant the > -field or frame was received by the driver, not the capture time. These > -devices identify by not enumerating any video standards, see <xref > -linkend="standard" />.</para> > - > - <para>Similar limitations apply to output timestamps. Typically > -the video hardware locks to a clock controlling the video timing, the > -horizontal and vertical synchronization pulses. At some point in the > -line sequence, possibly the vertical blanking, an interrupt routine > -samples the system clock, compares against the timestamp and programs > -the hardware to repeat the previous field or frame, or to display the > -buffer contents.</para> > - > - <para>Apart of limitations of the video device and natural > -inaccuracies of all clocks, it should be noted system time itself is > -not perfectly stable. It can be affected by power saving cycles, > -warped to insert leap seconds, or even turned back or forth by the > -system administrator affecting long term measurements. <footnote> > - <para>Since no other Linux multimedia > -API supports unadjusted time it would be foolish to introduce here. We > -must use a universally supported clock to synchronize different media, > -hence time of day.</para> > - </footnote></para> > + <para>For timestamp types that are sampled from the system clock > +(V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC) it is guaranteed that the timestamp is > +taken after the complete frame has been received (or transmitted in > +case of video output devices). For other kinds of > +timestamps this may vary depending on the driver.</para> > > <table frame="none" pgwide="1" id="v4l2-buffer"> > <title>struct <structname>v4l2_buffer</structname></title> > @@ -745,13 +718,9 @@ applications when an output stream.</entry> > byte was captured, as returned by the > <function>clock_gettime()</function> function for the relevant > clock id; see <constant>V4L2_BUF_FLAG_TIMESTAMP_*</constant> in > - <xref linkend="buffer-flags" />. For output streams the data > - will not be displayed before this time, secondary to the nominal > - frame rate determined by the current video standard in enqueued > - order. Applications can for example zero this field to display > - frames as soon as possible. The driver stores the time at which > - the first data byte was actually sent out in the > - <structfield>timestamp</structfield> field. This permits > + <xref linkend="buffer-flags" />. For output streams the driver > + stores the time at which the last data byte was actually sent out > + in the <structfield>timestamp</structfield> field. This permits > applications to monitor the drift between the video and system > clock.</para></entry> > </row> > -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml index 2c4c068..9720606 100644 --- a/Documentation/DocBook/media/v4l/io.xml +++ b/Documentation/DocBook/media/v4l/io.xml @@ -654,38 +654,11 @@ plane, are stored in struct <structname>v4l2_plane</structname> instead. In that case, struct <structname>v4l2_buffer</structname> contains an array of plane structures.</para> - <para>Nominally timestamps refer to the first data byte transmitted. -In practice however the wide range of hardware covered by the V4L2 API -limits timestamp accuracy. Often an interrupt routine will -sample the system clock shortly after the field or frame was stored -completely in memory. So applications must expect a constant -difference up to one field or frame period plus a small (few scan -lines) random error. The delay and error can be much -larger due to compression or transmission over an external bus when -the frames are not properly stamped by the sender. This is frequently -the case with USB cameras. Here timestamps refer to the instant the -field or frame was received by the driver, not the capture time. These -devices identify by not enumerating any video standards, see <xref -linkend="standard" />.</para> - - <para>Similar limitations apply to output timestamps. Typically -the video hardware locks to a clock controlling the video timing, the -horizontal and vertical synchronization pulses. At some point in the -line sequence, possibly the vertical blanking, an interrupt routine -samples the system clock, compares against the timestamp and programs -the hardware to repeat the previous field or frame, or to display the -buffer contents.</para> - - <para>Apart of limitations of the video device and natural -inaccuracies of all clocks, it should be noted system time itself is -not perfectly stable. It can be affected by power saving cycles, -warped to insert leap seconds, or even turned back or forth by the -system administrator affecting long term measurements. <footnote> - <para>Since no other Linux multimedia -API supports unadjusted time it would be foolish to introduce here. We -must use a universally supported clock to synchronize different media, -hence time of day.</para> - </footnote></para> + <para>For timestamp types that are sampled from the system clock +(V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC) it is guaranteed that the timestamp is +taken after the complete frame has been received (or transmitted in +case of video output devices). For other kinds of +timestamps this may vary depending on the driver.</para> <table frame="none" pgwide="1" id="v4l2-buffer"> <title>struct <structname>v4l2_buffer</structname></title> @@ -745,13 +718,9 @@ applications when an output stream.</entry> byte was captured, as returned by the <function>clock_gettime()</function> function for the relevant clock id; see <constant>V4L2_BUF_FLAG_TIMESTAMP_*</constant> in - <xref linkend="buffer-flags" />. For output streams the data - will not be displayed before this time, secondary to the nominal - frame rate determined by the current video standard in enqueued - order. Applications can for example zero this field to display - frames as soon as possible. The driver stores the time at which - the first data byte was actually sent out in the - <structfield>timestamp</structfield> field. This permits + <xref linkend="buffer-flags" />. For output streams the driver + stores the time at which the last data byte was actually sent out + in the <structfield>timestamp</structfield> field. This permits applications to monitor the drift between the video and system clock.</para></entry> </row>
Document that monotonic timestamps are taken after the corresponding frame has been received, not when the reception has begun. This corresponds to the reality of current drivers: the timestamp is naturally taken when the hardware triggers an interrupt to tell the driver to handle the received frame. Remove the note on timestamp accuracy as it is fairly subjective what is actually an unstable timestamp. Also remove explanation that output buffer timestamps can be used to delay outputting a frame. Remove the footnote saying we always use realtime clock. Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi> --- since v4: - Fixes according to Hans's comments. Documentation/DocBook/media/v4l/io.xml | 47 ++++++-------------------------- 1 file changed, 8 insertions(+), 39 deletions(-)