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Date:   Wed, 6 Mar 2019 15:09:54 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Alexandru Stan <amstan@...omium.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Gwendal Grignou <gwendal@...omium.org>,
        Heng-Ruey Hsu <henryhsu@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ricky Liang <jcliang@...omium.org>, linux-iio@...r.kernel.org,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>
Subject: Re: [PATCH] media: uvcvideo: Add boottime clock support

On Fri, Nov 23, 2018 at 11:46 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> Hi Laurent,
>
> On Fri, Nov 2, 2018 at 12:03 AM Lars-Peter Clausen <lars@...afoo.de> wrote:
> >
> > On 11/01/2018 03:30 PM, Tomasz Figa wrote:
> > > On Thu, Nov 1, 2018 at 11:03 PM Laurent Pinchart
> > > <laurent.pinchart@...asonboard.com> wrote:
> > >>
> > >> Hi Alexandru,
> > >>
> > >> 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?

Gentle ping.

Best regards,
Tomasz

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