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

[Tomasz Figa <tfiga@chromium.org>]

On Tue, Aug 6, 2019 at 5:34 PM Kieran Bingham
<kieran.bingham@ideasonboard.com> wrote:
>
> Hi Tomasz,
>
> On 06/08/2019 05:15, Tomasz Figa wrote:
> > On Wed, Mar 13, 2019 at 11:38 AM Tomasz Figa <tfiga@chromium.org> wrote:
> >>
> >> On Wed, Mar 13, 2019 at 10:25 AM Laurent Pinchart
> >> <laurent.pinchart@ideasonboard.com> wrote:
> >>>
> >>> 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.
>
> That's an important distinction. If there are operations that can run
> while the main host is in 'suspend' and still maintain "relative"
> timestamps in any form - then time must continue during suspend.
>
>
> >>>>>>> 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.
>
> Agreed, this is a V4L2 topic - not a UVC specific topic.
>
>
> >> Yes, fully agreed. The purpose of sending this patch was just to start
> >> the discussion on how to support this.
> >>
> >> Do you think something like a buffer flag called
> >> V4L2_BUF_FLAG_TIMESTAMP_BOOTTIME that could be set by the userspace at
> >> QBUF could work here? (That would change the timestamp flags
> >> semantics, because it used to be just the information from the driver,
> >> but shouldn't have any compatibility implications.) I suppose we would
> >> also need some capability flag for querying purposes, possibly added
> >> to the capability flags returned by REQBUFS/CREATE_BUFS?
>
> What kind of 'compatibility' do we actually need to maintain here?

The existing applications would expect the timestamps to come from
CLOCK_MONOTONIC, so I believe that we can't make CLOCK_BOOTTIME the
default.

> IMO -
> CLOCK_BOOTTIME makes much more sense globally for video, because it's
> more representative of the temporal difference between frames captured
> if a system goes into suspend.
>
> If frames are captured:
>
> A B         C D
>    <suspend>
>
> Then I believe it would be correct for the timestamp delta between B-C
> to be large <representative of the suspend duration/real time>
>'

Indeed.

>
> > Any thoughts?
>
> Aha, there might be some gotchas around non-live sources operating
> across suspend-resume boundaries .. so perhaps there are certainly
> use-cases where both _MONOTONIC and _BOOTTIME have their relevance ...
>

What would be an example of such a non-live source?

>
> > Adding Hans and Kieran for more insight.
>
> I think if we're talking about core-V4L2, Hans' opinion takes more
> weight than my mumblings :-) - but overall - supporting _BOOTTIME in
> some form sounds beneficial to me.
>

Your input is very valuable. Thanks a lot! :)

>
> > Best regards,
> > Tomasz
> >
>
> --
> Regards
> --
> Kieran