Re: [PATCH] media: uvcvideo: Add boottime clock support

[Laurent Pinchart]

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