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

[Tomasz Figa <tfiga@chromium.org>]

On Thu, Nov 1, 2018 at 11:03 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.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.

Best regards,
Tomasz