linux-kernel - Re: [PATCH v10 4/6] media: uvcvideo: Allow hw clock updates with buffers not full

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Date: Wed, 12 Jun 2024 12:28:56 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Ricardo Ribalda <ribalda@...omium.org>, Mauro Carvalho Chehab <mchehab@...nel.org>, 
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org, 
	"hn.chen" <hn.chen@...plusit.com>, Hans Verkuil <hverkuil@...all.nl>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v10 4/6] media: uvcvideo: Allow hw clock updates with
 buffers not full

On Mon, Jun 10, 2024 at 8:43 PM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> On Wed, May 29, 2024 at 05:03:08PM +0900, Tomasz Figa wrote:
> > On Sat, Mar 23, 2024 at 10:48:05AM +0000, Ricardo Ribalda wrote:
> > > With UVC 1.5 we get as little as one clock sample per frame. Which means
> > > that it takes 32 frames to move from the software timestamp to the
> > > hardware timestamp method.
> > >
> > > This results in abrupt changes in the timestamping after 32 frames (~1
> > > second), resulting in noticeable artifacts when used for encoding.
> > >
> > > With this patch we modify the update algorithm to work with whatever
> > > amount of values are available.
> > >
> > > Tested-by: HungNien Chen <hn.chen@...plusit.com>
> > > Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> > > Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > ---
> > >  drivers/media/usb/uvc/uvc_video.c | 16 ++++++++++++++--
> > >  1 file changed, 14 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > > index d6ca383f643e3..af25b9f1b53fe 100644
> > > --- a/drivers/media/usb/uvc/uvc_video.c
> > > +++ b/drivers/media/usb/uvc/uvc_video.c
> > > @@ -768,10 +768,10 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
> > >
> > >     spin_lock_irqsave(&clock->lock, flags);
> > >
> > > -   if (clock->count < clock->size)
> > > +   if (clock->count < 2)
> > >             goto done;
> > >
> > > -   first = &clock->samples[clock->head];
> > > +   first = &clock->samples[(clock->head - clock->count + clock->size) % clock->size];
> > >     last = &clock->samples[(clock->head - 1 + clock->size) % clock->size];
> > >
> > >     /* First step, PTS to SOF conversion. */
> > > @@ -786,6 +786,18 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
> > >     if (y2 < y1)
> > >             y2 += 2048 << 16;
> > >
> > > +   /*
> > > +    * Have at least 1/4 of a second of timestamps before we
> > > +    * try to do any calculation. Otherwise we do not have enough
> > > +    * precision. This value was determined by running Android CTS
> > > +    * on different devices.
> > > +    *
> > > +    * dev_sof runs at 1KHz, and we have a fixed point precision of
> > > +    * 16 bits.
> > > +    */
> > > +   if ((y2 - y1) < ((1000 / 4) << 16))
> > > +           goto done;
> >
> > Not a comment for this patch directly, but...
> >
> > This kind of makes me wonder if we don't want to have some documentation
> > that specifies what the userspace can expect from the timestamps, so
> > that this isn't changed randomly in the future breaking what was fixed
> > by this patch.
>
> I think timestamp handling should really be moved to userspace. It will
> be easier to handle with floating-point arithmetic there. That would
> have been difficult to manage for applications a while ago, but now that
> we have libcamera, we could implement it there. This isn't high on my
> todo list though.

While indeed that would probably be a better way to handle the complex
logic if we designed the driver today, we already have userspace that
expects the timestamps to be handled correctly in the kernel. I
suspect moving it to the userspace would require some core V4L2
changes to define a new timestamp handling mode, where multiple raw
hardware timestamps are exposed to the userspace, instead of the high
level system monotonic one.

Best regards,
Tomasz