[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 <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

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>