Hi Sakari, On Wednesday 05 November 2014 18:11:47 Sakari Ailus wrote: > On Wed, Nov 05, 2014 at 10:13:45AM +0000, Paulo Assis wrote: > > 2014-11-04 23:32 GMT+00:00 Sakari Ailus <sakari.ailus@xxxxxx>: > >> Sakari Ailus wrote: > >>> yavta does, for example, print both the monotonic timestamp from the > >>> buffer and the time when the buffer has been dequeued: > >>> > >>> <URL:http://git.ideasonboard.org/yavta.git> > >>> > >>> $ yavta -c /dev/video0 > >>> > >>> should do it. The first timestamp is the buffer timestamp, and the > >>> latter is the one is taken when the buffer is dequeued (by yavta). > > > > I've done exaclty this with guvcview, and uvcvideo timestamps are > > completly unreliable, in some devices they may have just a bit of > > jitter, but in others, values go back and forth in time, making them > > totally unusable. > > Honestly I wouldn't trust device firmware to provide correct > > timestamps, or at least I would have the driver perform a couple of > > tests to make sure these are at least reasonable: within an expected > > interval (maybe comparing it to a reference monotonic clock) or at the > > very least making sure the current frame timestamp is not lower than > > the previous one. > > Using the hardware timestamps provides much better accuracy than the > software ones --- the real time capabilities of the USB aren't exactly the > same as on some other busses. > > Freel free to try the follow-up patches; I've only compile tested them so > far. > > It might be possible to add some heuristics to detect bad implementations > but perhaps we could simply flag them for now. If heuristics would be used, > then one would likely have a few bad timestamps every time the device is > accessed the first time anyway. Besides, the timestamp type changes as a > result. > > I wonder what Laurent thinks. :-) I've been toying with the idea of a heuristic, but decided to delay the implementation until needed. I'd like to find the root cause of the issue first before deciding how to fix it. -- 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
