Hi Niels, On Monday 31 Oct 2016 14:42:54 Niels Möller wrote: > Hi, > > I'm tracking down a problem in Chrome, where video streams captured > from a Logitech c930e camera get bogus timestamps. Chrome started > using camera timestamps on linux a few months ago. I've noted commit > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id= > 5d0fd3c806b9e932010931ae67dbb482020e0882 > > "[media] uvcvideo: Disable hardware timestamps by default" > > but I'm running with a kernel which doesn't have that change. > > First, let me say that for our purposes, the hairy syncing to the > "SOF" clock done by uvc_video_clock_update is not that useful. > Ideally, I would prefer if the v4l2_buffer of a captured frame > included both > > * untranslated pts timestamp from the camera device (if I've > understood this correctly, and there is a pts sent over the wire), There's a PTS sent over the wire, yes. > and > > * the value of system monotonic clock at the point when the frame > was received by the kernel. > > Is there any reasonable way to get this information out from the > driver? The system monotonic clock timestamp is what the driver provides (with the above patch at least). We however have no field in the v4l2_buffer structure at the moment to provide the PTS. > We could then do estimation of the camera's epoch and clock drift in the > application. Unless I'm mistaken, you can only do that if you get the SCR/PTS values in your application, and they're currently not provided. How do you plan to solve that ? > The raw pts is the most important piece of information. What do you want to use it for by the way ? > Second, I'd like to try to provide some logs to help track down the > bug. To reproduce, I'm using the example program at > https://gist.github.com/maxlapshin/1253534, modified to print out > camera timestamp and gettimeofday for each frame. Log attached as > time-2.log. Thank you. I have a device I can use to reproduce the problem, but haven't had time to fix it yet :-/ Performing the timestamp translation in userspace would allow for more precise calculation, so I'm not advocating for a kernel-only solution. However, I don't want every application to implement timestamp translation. A common implementation in libv4l2 could be a good solution. > I also enabled tracing of the clock translation logic using > > echo 4096 > /sys/module/uvcvideo/parameters/trace > > The corresponding kernel log messages are attached as trace-2.log. > > In time-2.log (i.e., the application log), I see that camera > timestamps move backwards in time, > > TIMESTAMP_MONOTONIC > cam: 2321521.085372 > sys: 1477913910.983620 > TIMESTAMP_MONOTONIC > cam: 2321520.879272 > sys: 1477913911.051628 > > In trace-2.log (i.e., kernel log messages) I see > > uvcvideo: Logitech Webcam C930e: PTS 219483992 y 4084.798004 SOF > 4084.798004 (x1 2064310082 x2 2148397132 y1 218759168 y2 268238848 SOF > offset 170) > uvcvideo: Logitech Webcam C930e: SOF 4084.798004 y 3105900702 ts > 2321520.879272 buf ts 2321521.153372 (x1 218759168/1546/1290 x2 > 274071552/1878/2045 y1 1000000000 y2 3380001263) > uvcvideo: Logitech Webcam C930e: PTS 221480532 y 4156.709564 SOF > 4156.709564 (x1 2079524156 x2 2148397450 y1 256376832 y2 272629760 SOF > offset 170) > uvcvideo: Logitech Webcam C930e: SOF 4156.709564 y 2453257742 ts > 2321520.378627 buf ts 2321521.217373 (x1 262275072/1698/1864 x2 > 278265856/1942/64 y1 1000000000 y2 3292003672) > uvcvideo: Logitech Webcam C930e: PTS 223477044 y 4223.428085 SOF > 4223.428085 (x1 2081269216 x2 2148397122 y1 264568832 y2 276955136 SOF > offset 170) > uvcvideo: Logitech Webcam C930e: SOF 2175.428085 y 2158773894 ts > 2321520.208143 buf ts 2321521.285373 (x1 136183808/1822/1989 x2 > 148504576/2010/130 y1 1000000000 y2 3236003012) > > I don't know the details of the usb protocol, but it looks like the > "SOF" value is usually increasing. But close to the bogus output > timestamp of 2321520.879272, it goes through some kind of wraparound, > with the sequence of values > > 4156.709564 > 4223.428085 > 2175.428085 # 2048 less than previous value > 2243.169921 > > I hope the attached logs provide enough information to analyze where > uvc_video_clock_update gets this wrong. -- 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
