Hi Sakari, On Thursday 13 September 2012 23:38:14 Sakari Ailus wrote: > On Fri, Sep 07, 2012 at 03:29:12PM +0200, Hans Verkuil wrote: > > From: Hans Verkuil <hans.verkuil@xxxxxxxxx> > > > > Add a new flag that tells userspace that the monotonic clock is used > > for timestamps and update the documentation accordingly. > > > > We decided on this new flag during the 2012 Media Workshop. > > > > Signed-off-by: Hans Verkuil <hans.verkuil@xxxxxxxxx> > > --- > > > > Documentation/DocBook/media/v4l/io.xml | 10 +++++++--- > > Documentation/DocBook/media/v4l/vidioc-dqevent.xml | 3 ++- > > Documentation/DocBook/media/v4l/vidioc-querycap.xml | 7 +++++++ > > include/linux/videodev2.h | 1 + > > 4 files changed, 17 insertions(+), 4 deletions(-) > > > > diff --git a/Documentation/DocBook/media/v4l/io.xml > > b/Documentation/DocBook/media/v4l/io.xml index 2dc39d8..b680d66 100644 > > --- a/Documentation/DocBook/media/v4l/io.xml > > +++ b/Documentation/DocBook/media/v4l/io.xml > > @@ -582,10 +582,14 @@ applications when an output stream.</entry> > > > > <entry>struct timeval</entry> > > <entry><structfield>timestamp</structfield></entry> > > <entry></entry> > > > > - <entry><para>For input streams this is the > > + <entry><para>This is either the > > > > system time (as returned by the <function>gettimeofday()</function> > > > > -function) when the first data byte was captured. For output streams > > -the data will not be displayed before this time, secondary to the > > +function) or a monotonic timestamp (as returned by the > > +<function>clock_gettime(CLOCK_MONOTONIC, &ts)</function> function). > > +A monotonic timestamp is used if the > > <constant>V4L2_CAP_MONOTONIC_TS</constant> +capability is set, otherwise > > the system time is used. > > +For input streams this is the timestamp when the first data byte was > > captured. +For output streams the data will not be displayed before this > > time, secondary to the > I have an alternative proposal. > > The type of the desired timestamps depend on the use case, not the driver > used to capture the buffers. Thus we could also give the choice to the user > by means of e.g. a control. Or a buffer flag. I will need something similar to select device-specific timestamps. However, for wall clock vs. monotonic clock, I don't think there's a reason to let applications decide to use the wall clock. It would be a broken use case. I don't think we should let applications decide in this case. On the other hand, reporting a timespec instead of a timeval would be a good idea. I'm tempted. > If we'd make it configurable (applications would still get the wallclock > time unless they ask for monotonic time), we'd have a chance to add more > precision by using struct timespec (ns precision) instead of struct timeval > (us precision). struct timespec is also used by V4L2 events. > > Adding support for CLOCK_MONOTONIC_RAW would also be a non-issue. > > Additional helper function could be used to generate the timestamp of the > desired type. > > What do you think? -- Regards, Laurent Pinchart -- 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
