On 11/01/2018 03:30 PM, Tomasz Figa wrote: > On Thu, Nov 1, 2018 at 11:03 PM Laurent Pinchart > <laurent.pinchart@xxxxxxxxxxxxxxxx> 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.
