diff options
| -rw-r--r-- | Documentation/DocBook/media/v4l/io.xml | 56 |
1 files changed, 12 insertions, 44 deletions
diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml index 1fb11e8..89544e4 100644 --- a/Documentation/DocBook/media/v4l/io.xml +++ b/Documentation/DocBook/media/v4l/io.xml @@ -339,8 +339,8 @@ returns immediately with an &EAGAIN; when no buffer is available. The queues as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the &v4l2-buffer; -<structfield>timestamp</structfield> of captured buffers, or set the -field before enqueuing buffers for output.</para> +<structfield>timestamp</structfield> of captured or outputted buffers. +</para> <para>Drivers implementing memory mapping I/O must support the <constant>VIDIOC_REQBUFS</constant>, @@ -457,7 +457,7 @@ queues and unlocks all buffers as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the &v4l2-buffer; <structfield>timestamp</structfield> of captured -buffers, or set the field before enqueuing buffers for output.</para> +or outputted buffers.</para> <para>Drivers implementing user pointer I/O must support the <constant>VIDIOC_REQBUFS</constant>, @@ -620,8 +620,7 @@ returns immediately with an &EAGAIN; when no buffer is available. The unlocks all buffers as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the &v4l2-buffer; -<structfield>timestamp</structfield> of captured buffers, or set the field -before enqueuing buffers for output.</para> +<structfield>timestamp</structfield> of captured or outputted buffers.</para> <para>Drivers implementing DMABUF importing I/O must support the <constant>VIDIOC_REQBUFS</constant>, <constant>VIDIOC_QBUF</constant>, @@ -654,38 +653,11 @@ 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 +717,9 @@ 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> |
