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. Best regards, Tomasz
