On Tue, Nov 01, 2011 at 01:24:35PM +0100, Laurent Pinchart wrote: > Hi everybody, > > The V4L2 specification documents the v4l2_buffer timestamp field as > > "For input streams this is the system time (as returned by the gettimeofday() > function) when the first data byte was captured." > > The system time is a pretty bad clock source to timestamp buffers, as it can > jump back and forth in time. Using a monotonic clock, as returned by > clock_gettime(CLOCK_MONOTONIC) (or ktime_get_ts() in the kernel), would be > much more useful. > > Several drivers already use a monotonic clock instead of the system clock, > which currently violates the V4L2 specification. As those drivers do the right > thing from a technical point of view, I'd really hate "fixing" them by making > them use gettimeofday(). > > We should instead fix the V4L2 specification to mandate the use of a monotonic > clock (which could then also support hardware timestamps when they are > available). Would such a change be acceptable ? I'm in favour of that. I don't think wall clock timestamps are really useful to begin with. If you really need them, you can always do gettimeofday() in the user space. For any kind of a/v synchronisation where precise timestamps matter the monotonic clock is the way to go. Cheers, -- Sakari Ailus e-mail: sakari.ailus@xxxxxx jabber/XMPP/Gmail: 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
