Hi Tomasz, On Fri, Nov 23, 2018 at 11:46:43PM +0900, Tomasz Figa wrote: > On Fri, Nov 2, 2018 at 12:03 AM Lars-Peter Clausen wrote: > > On 11/01/2018 03:30 PM, Tomasz Figa wrote: > >> On Thu, Nov 1, 2018 at 11:03 PM Laurent Pinchart wrote: > >>> On Thursday, 18 October 2018 20:28:06 EET Alexandru M Stan wrote: > >>>> On Wed, Oct 17, 2018 at 9:31 PM, Tomasz Figa wrote: > >>>>> On Thu, Oct 18, 2018 at 5:50 AM Laurent Pinchart wrote: > >>>>>> On Wednesday, 17 October 2018 11:28:52 EEST Tomasz Figa wrote: > >>>>>>> On Wed, Oct 17, 2018 at 5:02 PM Laurent Pinchart wrote: > >>>>>>>> On Wednesday, 17 October 2018 10:52:42 EEST Heng-Ruey Hsu wrote: > >>>>>>>>> Android requires camera timestamps to be reported with > >>>>>>>>> CLOCK_BOOTTIME to sync timestamp with other sensor sources. > >>>>>>>> > >>>>>>>> What's the rationale behind this, why can't CLOCK_MONOTONIC work ? If > >>>>>>>> the monotonic clock has shortcomings that make its use impossible for > >>>>>>>> proper synchronization, then we should consider switching to > >>>>>>>> CLOCK_BOOTTIME globally in V4L2, not in selected drivers only. > >>>>>>> > >>>>>>> CLOCK_BOOTTIME includes the time spent in suspend, while > >>>>>>> CLOCK_MONOTONIC doesn't. I can imagine the former being much more > >>>>>>> useful for anything that cares about the actual, long term, time > >>>>>>> tracking. Especially important since suspend is a very common event on > >>>>>>> Android and doesn't stop the time flow there, i.e. applications might > >>>>>>> wake up the device to perform various tasks at necessary times. > >>>>>> > >>>>>> Sure, but this patch mentions timestamp synchronization with other > >>>>>> sensors, and from that point of view, I'd like to know what is wrong with > >>>>>> the monotonic clock if all devices use it. > >>>>> > >>>>> AFAIK the sensors mentioned there are not camera sensors, but rather > >>>>> things we normally put under IIO, e.g. accelerometers, gyroscopes and > >>>>> so on. I'm not sure how IIO deals with timestamps, but Android seems > >>>>> to operate in the CLOCK_BOTTIME domain. Let me add some IIO folks. > >>>>> > >>>>> Gwendal, Alexandru, do you think you could shed some light on how we > >>>>> handle IIO sensors timestamps across the kernel, Chrome OS and > >>>>> Android? > >>>> > >>>> On our devices of interest have a specialized "sensor" that comes via > >>>> IIO (from the EC, cros-ec-ring driver) that can be used to more > >>>> accurately timestamp each frame (since it's recorded with very low > >>>> jitter by a realtime-ish OS). In some high level userspace thing > >>>> (specifically the Android Camera HAL) we try to pick the best > >>>> timestamp from the IIO, whatever's closest to what the V4L stuff gives > >>>> us. > >>>> > >>>> I guess the Android convention is for sensor timestamps to be in > >>>> CLOCK_BOOTTIME (maybe because it likes sleeping so much). There's > >>>> probably no advantage to using one over the other, but the important > >>>> thing is that they have to be the same, otherwise the closest match > >>>> logic would fail. > >>> > >>> That's my understanding too, I don't think CLOCK_BOOTTIME really brings much > >>> benefit in this case, > >> > >> I think it does have a significant benefit. CLOCK_MONOTONIC stops when > >> the device is sleeping, but the sensors can still capture various > >> actions. We would lose the time keeping of those actions if we use > >> CLOCK_MONOTONIC. > >> > >>> but it's important than all timestamps use the same > >>> clock. The question is thus which clock we should select. Mainline mostly uses > >>> CLOCK_MONOTONIC, and Android CLOCK_BOOTTIME. Would you like to submit patches > >>> to switch Android to CLOCK_MONOTONIC ? :-) > >> > >> Is it Android using CLOCK_BOOTTIME or the sensors (IIO?). I have > >> almost zero familiarity with the IIO subsystem and was hoping someone > >> from there could comment on what time domain is used for those > >> sensors. > > > > IIO has the option to choose between BOOTTIME or MONOTONIC (and a few > > others) for the timestamp on a per device basis. > > > > There was a bit of a discussion about this a while back. See > > https://lkml.org/lkml/2018/7/10/432 and the following thread. > > Given that IIO supports BOOTTIME in upstream already and also the > important advantage of using it over MONOTONIC for systems which keep > capturing events during sleep, do you think we could move on with some > way to support it in uvcvideo or preferably V4L2 in general? I'm not opposed to that, but I don't think we should approach that from a UVC point of view. The issue should be addressed in V4L2, and then driver-specific support could be added, if needed. -- Regards, Laurent Pinchart
