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

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Message-ID: <20230829082635.GC4698@pendragon.ideasonboard.com>
Date:   Tue, 29 Aug 2023 11:26:35 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Alain Volmat <alain.volmat@...s.st.com>
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        devicetree@...r.kernel.org,
        Hugues Fruchet <hugues.fruchet@...s.st.com>,
        linux-kernel@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        Rob Herring <robh+dt@...nel.org>,
        Dan Scally <dan.scally@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [Linux-stm32] [PATCH v1 3/5] media: stm32-dcmipp: STM32 DCMIPP
 camera interface driver

On Thu, Aug 24, 2023 at 06:05:06PM +0200, Alain Volmat wrote:
> Hi Laurent,
> 
> On Thu, Aug 24, 2023 at 04:04:32PM +0300, Laurent Pinchart wrote:
> > 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 ?
> 
> This is basically to allow reducing the framerate of the
> captured stream when this is not possible at the producer
> (sensor) level and we need to lower down the stress on elements down the
> pipeline.

I wonder if the frame interval API is a good fit for this. The driver
accepts frame rates of 1, 2, 4 and 8 fps and maps them to the
corresponding decimation factor, these values are not the actual frame
rate.

-- 
Regards,

Laurent Pinchart