Hi Laurent, On Mon, Jan 28, 2013 at 08:56:21PM +0100, Laurent Pinchart wrote: > On Monday 28 January 2013 10:55:14 Hans Verkuil wrote: > > On Fri January 25 2013 19:03:29 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. > > > > > > Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxx> > > > --- > > > > > > Documentation/DocBook/media/v4l/io.xml | 27 ++++++++++++++------------- > > > 1 files changed, 14 insertions(+), 13 deletions(-) > > > > > > diff --git a/Documentation/DocBook/media/v4l/io.xml > > > b/Documentation/DocBook/media/v4l/io.xml index 2c4646d..3b8bf61 100644 > > > --- a/Documentation/DocBook/media/v4l/io.xml > > > +++ b/Documentation/DocBook/media/v4l/io.xml > > > @@ -654,19 +654,20 @@ 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>On 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. > > > > add: " (or transmitted for video output devices)" > > The uvcvideo driver currently uses monotonic timestamps corresponding to the > start of the frame :-) Ah, I had almost forgotten this! :-) What would you think about changing it? :-) I guess uvc is a little special since it receives packets, not frames. -- Cheers, Sakari Ailus e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
