From c6c092135d4f61b038a41685147c79e966c2399c Mon Sep 17 00:00:00 2001 From: Sakari Ailus Date: Fri, 25 Jan 2013 15:00:07 -0300 Subject: [PATCH] [media] v4l: Document timestamp behaviour to correspond to reality 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 Acked-by: Hans Verkuil Signed-off-by: Mauro Carvalho Chehab --- Documentation/DocBook/media/v4l/io.xml | 56 ++++++-------------------- 1 file changed, 12 insertions(+), 44 deletions(-) diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml index 1fb11e8d2a26..89544e4495a9 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; -timestamp of captured buffers, or set the -field before enqueuing buffers for output. +timestamp of captured or outputted buffers. + Drivers implementing memory mapping I/O must support the VIDIOC_REQBUFS, @@ -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; timestamp of captured -buffers, or set the field before enqueuing buffers for output. +or outputted buffers. Drivers implementing user pointer I/O must support the VIDIOC_REQBUFS, @@ -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; -timestamp of captured buffers, or set the field -before enqueuing buffers for output. +timestamp of captured or outputted buffers. Drivers implementing DMABUF importing I/O must support the VIDIOC_REQBUFS, VIDIOC_QBUF, @@ -654,38 +653,11 @@ plane, are stored in struct v4l2_plane instead. In that case, struct v4l2_buffer contains an array of plane structures. - 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 . - - 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. - - 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. - 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. - + 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. struct <structname>v4l2_buffer</structname> @@ -745,13 +717,9 @@ applications when an output stream. byte was captured, as returned by the clock_gettime() function for the relevant clock id; see V4L2_BUF_FLAG_TIMESTAMP_* in - . 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 - timestamp field. This permits + . For output streams the driver + stores the time at which the last data byte was actually sent out + in the timestamp field. This permits applications to monitor the drift between the video and system clock. -- 2.25.1