linux-kernel - Re: [PATCH v1 3/5] media: stm32-dcmipp: STM32 DCMIPP camera interface driver

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Message-ID: <20230824130432.GB27092@pendragon.ideasonboard.com>
Date:   Thu, 24 Aug 2023 16:04:32 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Hugues Fruchet <hugues.fruchet@...s.st.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        Philippe CORNU <philippe.cornu@...s.st.com>,
        Dan Scally <dan.scally@...asonboard.com>
Subject: Re: [PATCH v1 3/5] media: stm32-dcmipp: STM32 DCMIPP camera
 interface driver

On Thu, Aug 24, 2023 at 12:26:42PM +0000, Sakari Ailus wrote:
> On Thu, Aug 24, 2023 at 01:09:34PM +0200, Alain Volmat wrote:
> > Hi Sakari,
> > 
> > thanks a lot for the review.  I've already taken care of the comments I got
> > from Dan and will also add fixes for your comments as well before
> > pushing the v2.  Before going into that I thought I'd better clarify the
> > framerate part which seems the most tricky part.
> > 
> > On Mon, Aug 07, 2023 at 09:29:55AM +0000, Sakari Ailus wrote:
> 
> ...
> 
> > > > +static int dcmipp_byteproc_g_frame_interval(struct v4l2_subdev *sd,
> > > > +					    struct v4l2_subdev_frame_interval *fi)
> > > > +{
> > > > +	struct dcmipp_byteproc_device *byteproc = v4l2_get_subdevdata(sd);
> > > > +
> > > > +	if (IS_SINK(fi->pad))
> > > > +		fi->interval = byteproc->sink_interval;
> > > > +	else
> > > > +		fi->interval = byteproc->src_interval;
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static int dcmipp_byteproc_s_frame_interval(struct v4l2_subdev *sd,
> > > > +					    struct v4l2_subdev_frame_interval *fi)
> > > > +{
> > > > +	struct dcmipp_byteproc_device *byteproc = v4l2_get_subdevdata(sd);
> > > > +
> > > > +	mutex_lock(&byteproc->lock);
> > > > +
> > > > +	if (byteproc->streaming) {
> > > > +		mutex_unlock(&byteproc->lock);
> > > > +		return -EBUSY;
> > > > +	}
> > > > +
> > > > +	if (fi->interval.numerator == 0 || fi->interval.denominator == 0)
> > > > +		fi->interval = byteproc->sink_interval;
> > > > +
> > > > +	if (IS_SINK(fi->pad)) {
> > > > +		/*
> > > > +		 * Setting sink frame interval resets frame skipping.
> > > > +		 * Sink frame interval is propagated to src.
> > > > +		 */
> > > > +		byteproc->frate = 0;
> > > > +		byteproc->sink_interval = fi->interval;
> > > > +		byteproc->src_interval = byteproc->sink_interval;
> > > 
> > > Is this used for anything else than configure skipping?
> > > 
> > > I think I'd just have a control for it in that case.
> > > 
> > > I don't think exposing frame interval configuration is necessarily even
> > > meaningful for a device that just processes data but does not produce it.
> > 
> > The DCMIPP is able to perform frame drop, 1/2, 1/4, 1/8 basically.
> > As Dan pointed me out, indeed setting frame interval as we did on both
> > sink and source pad isn't a defined behavior.  I first thought that
> > using the frame interval was the proper way to do that but that is
> > indeed only used on producers such as sensors ....
> > Which ctrl would you propose in such case ?
> 
> We don't have one, AFAIK, and I think it may be unlikely this will be
> needed elsewhere. So I'd use a private control.
> 
> I wonder what others think. Cc Laurent as well.

What are the use cases for this feature ?

-- 
Regards,

Laurent Pinchart