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

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Date: Mon, 10 Jun 2024 14:43:06 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Tomasz Figa <tfiga@...omium.org>
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 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.

> Anyway:
> 
> Reviewed-by: Tomasz Figa <tfiga@...omium.org>

-- 
Regards,

Laurent Pinchart