Hello, On Wed, Nov 08, 2023 at 03:32:23PM -0500, nicolas@xxxxxxxxxxxx wrote: > Le mercredi 08 novembre 2023 à 08:04 +0100, Ricardo Ribalda a écrit : > > On Wed, 8 Nov 2023 at 07:54, Esker Wong wrote: > > > > > > Hi Nicholas and Sakari, > > > > > > We need it as precise as possible. Currently the earliest time of a > > > frame we can have in userspace is the dqbuf. > > > > > > And for UVC timestamp, it is somewhat awkward for us to use. Since > > > other functions in our stacks do not necessarily contain such > > > timestamps. So we want some event to be trigger and we can get the > > > system time directly. > > The fact that you interpret the time from FRAME_SYNC to DQBUF (well the > READ IO notification) as the actual latency is yours of course. It > assumes that the camera on the other end does not introduce other > source of latency (or that these are negligible). You are also going to > introduce a lot of jitter, since it relies on when the OS decides to > wake up your process. > > I think my opinion resides in if you can accurately *enough* implement > what the spec says for FRAME_SYNC then do it, otherwise just don't lie. > I think for ISO, "after the first chunk" i a small lie, but acceptable. > But for BULK, the way it was explained is that it will be always very > close to DQBUF time. and it should not emit FRAME_SYNC for this type of > UVC device. If it fits other events fine of course, I'm just making a > judgment on if its fits V4L2_EVENT_FRAME_SYNC or not. I agree. V4L2_EVENT_FRAME_SYNC should be fine for isoc-based devices as it should be "close enough" to the start of frame. For bulk it woul dbe too much of a lie, so I would not emit it for bulk-based devices. > In term of accuracy, if timestamp was passed with the FRAME_SYNC event, > it would not matter how fast your process the event anymore and greatly > improve accuracy. > > > Not to mention that the UVC timestamping requires a bit of love. > > > > @Laurent Pinchart, @Kieran Bingham any progress reviewing :P : > > https://patchwork.linuxtv.org/project/linux-media/list/?series=10083 > > Thanks for working on this by the way, hope someone will find the time > to review this. The timestamps should in theory provide a jitter free > measurement of the delay Esker is trying to measure, and if it wasn't > of bugs (and crazy complexity) it would in the worst case match the > transfer time. Assuming the device firmware isn't too buggy, the UVC timestamps should indeed provide much better accuracy than when V4L2_EVENT_FRAME_SYNC could give. I think the biggest problem will be to figure out if a particular device can be trusted. > > > If the V4L2_EVENT_FRAME_SYNC will be earlier then V4L2_EVENT_VSYNC, > > > then it has value. We would want to know the delay of a frame being > > > captured to the time it is displayed. > > > > > > I'm not sure for bulk is the V4L2_EVENT_VSYNC more accurate? > > > > V4L2_EVENT_VSYNC wont be more accurate than V4L2_EVENT_FRAME_SYNC. > > > > My understanding is that Sakari thinks that the description of > > V4L2_EVENT_FRAME_SYNC > > https://www.kernel.org/doc/html/v4.9/media/uapi/v4l/vidioc-dqevent.html#description > > does not match the current implementation, and suggests using > > V4L2_EVENT_VSYNC instead. > > > > > On Wed, Nov 8, 2023 at 3:27 AM <nicolas@xxxxxxxxxxxx> wrote: > > > > Le mardi 07 novembre 2023 à 13:06 +0800, Esker Wong a écrit : > > > > > [send again in text mode] > > > > > Hi Sakari, > > > > > > > > > > Sequence number is important to us. We need it to measure the latency > > > > > from this event to the time we display the frame. > > > > > > > > how much precision do you expect, because as described, this number > > > > will be completely false for bulk. > > > > > > > > Aren't UVC timestamp support to allow measuring latency properly ? > > > > > > > > > On Mon, Nov 6, 2023 at 7:06 PM Sakari Ailus wrote: > > > > > > On Mon, Nov 06, 2023 at 10:52:27AM +0000, Ricardo Ribalda wrote: > > > > > > > Add support for the frame_sync event, so user-space can become aware > > > > > > > earlier of new frames. > > > > > > > > > > > > > > Suggested-by: Esker Wong <esker@xxxxxxxxxxxx> > > > > > > > Tested-by: Esker Wong <esker@xxxxxxxxxxxx> > > > > > > > Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx> > > > > > > > --- > > > > > > > We have measured a latency of around 30msecs between frame sync > > > > > > > and dqbuf. > > > > > > > --- > > > > > > > Changes in v2: > > > > > > > - Suggested by Laurent. Split sequence++ and event init. > > > > > > > - Link to v1: https://lore.kernel.org/r/20231020-uvc-event-v1-1-3baa0e9f6952@xxxxxxxxxxxx > > > > > > > --- > > > > > > > drivers/media/usb/uvc/uvc_v4l2.c | 2 ++ > > > > > > > drivers/media/usb/uvc/uvc_video.c | 7 +++++++ > > > > > > > 2 files changed, 9 insertions(+) > > > > > > > > > > > > > > diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c > > > > > > > index f4988f03640a..9f3fb5fd2375 100644 > > > > > > > --- a/drivers/media/usb/uvc/uvc_v4l2.c > > > > > > > +++ b/drivers/media/usb/uvc/uvc_v4l2.c > > > > > > > @@ -1352,6 +1352,8 @@ static int uvc_ioctl_subscribe_event(struct v4l2_fh *fh, > > > > > > > switch (sub->type) { > > > > > > > case V4L2_EVENT_CTRL: > > > > > > > return v4l2_event_subscribe(fh, sub, 0, &uvc_ctrl_sub_ev_ops); > > > > > > > + case V4L2_EVENT_FRAME_SYNC: > > > > > > > + return v4l2_event_subscribe(fh, sub, 0, NULL); > > > > > > > default: > > > > > > > return -EINVAL; > > > > > > > } > > > > > > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c > > > > > > > index 28dde08ec6c5..4f3a510ca4fe 100644 > > > > > > > --- a/drivers/media/usb/uvc/uvc_video.c > > > > > > > +++ b/drivers/media/usb/uvc/uvc_video.c > > > > > > > @@ -1073,9 +1073,16 @@ static int uvc_video_decode_start(struct uvc_streaming *stream, > > > > > > > * that discontinuous sequence numbers always indicate lost frames. > > > > > > > */ > > > > > > > if (stream->last_fid != fid) { > > > > > > > + struct v4l2_event event = { > > > > > > > + .type = V4L2_EVENT_FRAME_SYNC, > > > > > > > + }; > > > > > > > + > > > > > > > stream->sequence++; > > > > > > > if (stream->sequence) > > > > > > > uvc_video_stats_update(stream); > > > > > > > + > > > > > > > + event.u.frame_sync.frame_sequence = stream->sequence, > > > > > > > + v4l2_event_queue(&stream->vdev, &event); > > > > > > > > > > > > uvc_video_decode_start() is called when the reception of the entire frame > > > > > > has been completed. However, the documentation for V4L2_EVENT_FRAME_SYNC > > > > > > says that the event is "Triggered immediately when the reception of a frame > > > > > > has begun.". The functionality here doesn't seem to fit to this patch. > > > > > > > > > > > > Wouldn't V4L2_EVENT_VSYNC be a better fit, even if we don't really have a > > > > > > concept of vertical sync in the case of USB? That event doesn't have the > > > > > > sequence though but I guess it's not an issue at least if your case. > > > > > > > > > > > > Another technically correct option could be to create a new event for this > > > > > > but I'm not sure it's worth it. > > > > > > > > > > > > > } > > > > > > > > > > > > > > uvc_video_clock_decode(stream, buf, data, len); > > > > > > > -- Regards, Laurent Pinchart
