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Date:   Thu, 15 Jul 2021 14:39:54 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Niklas =?utf-8?Q?S=C3=B6derlund?= <niklas.soderlund@ragnatech.se>
Cc:     Dennis Rachui <drachui@de.adit-jv.com>,
        Steve Longerbeam <slongerbeam@gmail.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Maxime Ripard <mripard@kernel.org>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        linux-media@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] media: rcar-csi2: do not update format while streaming
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Hi Niklas,

On Thu, Jul 15, 2021 at 12:57:37PM +0200, Niklas Söderlund wrote:
> On 2021-07-14 21:40:51 +0300, Laurent Pinchart wrote:
> > On Tue, Jul 13, 2021 at 11:42:17AM +0200, Niklas Söderlund wrote:
> > > On 2021-07-11 01:42:44 +0300, Laurent Pinchart wrote:
> > > > On Fri, Jul 09, 2021 at 04:20:40PM +0200, Niklas Söderlund wrote:
> > > > > On 2021-07-08 15:22:58 +0200, Dennis Rachui wrote:
> > > > > > Verify that streaming is not active before setting the pad format.
> > > > > > 
> > > > > > According to the VIDIOC documentation [1] changes to the active
> > > > > > format of a media pad via the VIDIOC_SUBDEV_S_FMT ioctl are
> > > > > > applied to the underlying hardware.
> > > > > > In rcar-csi2 a format change only applies to hardware, when the
> > > > > > pipeline is started. While the device is not in use, it is therefore
> > > > > > okay to update the format.
> > > > > > 
> > > > > > However, when the pipeline is active, this leads to a format
> > > > > > mismatch between driver and device.
> > > > > > Other applications can query the format with
> > > > > > VIDIOC_SUBDEV_G_FMT at any time and would be reported
> > > > > > a format that does not fit the current stream.
> > > > > > 
> > > > > > This commit prevents format update while streaming is active
> > > > > > and returns -EBUSY to user space, as suggested by [1].
> > > > > > 
> > > > > > [1] Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst
> > > > > 
> > > > > I like that this is addressed, but I wonder is this not something that 
> > > > > should be fixed in the V4L2 core and not in drivers?
> > > > 
> > > > Some drivers may support format changes during streaming (that's allowed
> > > > by the V4L2 API, I'm not sure if it's used anywhere though). While I'd
> > > > favour not duplicating the same logic in different (and differently
> > > > buggy) ways in drivers, I'm not sure how this could be implemented in a
> > > > sane way in the V4L2 core in its current state.
> > > 
> > > I understand it's possible from some devices to support to format 
> > > changes during streaming, but as you point out it's the exception and 
> > > not the rule, if used at all.
> > > 
> > > So my point is if we start to enforce this in drivers we are headed down 
> > > a road where this will be messier to clean up. Would it not make more 
> > > sens to default the V4L2 core to disallow format changes while streaming 
> > > and add a new flag to V4L2_SUBDEV_CAP_ to signal that the subdevice 
> > > supports format changes while streaming?
> > > 
> > > We already have V4L2_SUBDEV_CAP_RO_SUBDEV to signal that a subdevice 
> > > only supports read-only operations so I think it would not be too hard 
> > > to move this functionality into the core?
> > 
> > Yes, that's something we could try. The subdev core will then need to
> > track the streaming state, which may require wrapping the .s_stream()
> > call. Locking should then also likely be handled by the core. Probably
> > nothing impossible, but quite a bit of work. Any volunteer ? :-)
> 
> We already track the stream count in struct media_entity and it's 
> incremented/decremented under the media device lock by 
> media_pipeline_start() and media_pipeline_stop(). So I don't think it's 
> such a hard feature to add.
> 
> The large task IMHO is to figure out if we have any subdevice in tree 
> that allows format changes while streaming and that would need to set 
> this new V4L2_SUBDEV_CAP_ flag.

Many subdevs allow format changes during streaming. The question is
whether any of them do so knowingly, or if they're all buggy :-) I'd be
surprised if there were more than a couple of drivers that actually
support this correctly.

> > > > > > Note: after creation of this commit, it was noticed that Steve
> > > > > > Longerbeam has a very similar solution in his fork.
> > > > > > 
> > > > > > Fixes: 769afd212b16 ("media: rcar-csi2: add Renesas R-Car MIPI CSI-2 receiver driver")
> > > > > > Cc: Steve Longerbeam <slongerbeam@gmail.com>
> > > > > > Signed-off-by: Dennis Rachui <drachui@de.adit-jv.com>
> > > > > > ---
> > > > > >  drivers/media/platform/rcar-vin/rcar-csi2.c | 21 ++++++++++++++++++++-
> > > > > >  1 file changed, 20 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/drivers/media/platform/rcar-vin/rcar-csi2.c b/drivers/media/platform/rcar-vin/rcar-csi2.c
> > > > > > index e28eff0..98152e1 100644
> > > > > > --- a/drivers/media/platform/rcar-vin/rcar-csi2.c
> > > > > > +++ b/drivers/media/platform/rcar-vin/rcar-csi2.c
> > > > > > @@ -724,18 +724,37 @@ static int rcsi2_set_pad_format(struct v4l2_subdev *sd,
> > > > > >  {
> > > > > >  	struct rcar_csi2 *priv = sd_to_csi2(sd);
> > > > > >  	struct v4l2_mbus_framefmt *framefmt;
> > > > > > +	int ret = 0;
> > > > > > +
> > > > > > +	mutex_lock(&priv->lock);
> > > > > >  
> > > > > >  	if (!rcsi2_code_to_fmt(format->format.code))
> > > > > >  		format->format.code = rcar_csi2_formats[0].code;
> > > > > >  
> > > > > >  	if (format->which == V4L2_SUBDEV_FORMAT_ACTIVE) {
> > > > > > +
> > > > > > +		/*
> > > > > > +		 * Do not apply changes to active format while streaming.
> > > > > > +		 *
> > > > > > +		 * Since video streams could be forwarded from sink pad to any
> > > > > > +		 * source pad (depending on CSI-2 channel routing), all
> > > > > > +		 * media pads are effected by this rule.
> > > > > > +		 */
> > > > > > +		if (priv->stream_count > 0) {
> > > > > > +			ret = -EBUSY;
> > > > > > +			goto out;
> > > > > > +		}
> > > > > > +
> > > > > >  		priv->mf = format->format;
> > > > > >  	} else {
> > > > > >  		framefmt = v4l2_subdev_get_try_format(sd, sd_state, 0);
> > > > > >  		*framefmt = format->format;
> > > > > >  	}
> > > > > >  
> > > > > > -	return 0;
> > > > > > +out:
> > > > > > +	mutex_unlock(&priv->lock);
> > > > > > +
> > > > > > +	return ret;
> > > > > >  }
> > > > > >  
> > > > > >  static int rcsi2_get_pad_format(struct v4l2_subdev *sd,

-- 
Regards,

Laurent Pinchart