Re: [RFC PATCH 1/4] media: rcar-vin: use v4l2_get_link_freq() to calculate phypll frequency

["Niklas Söderlund" <niklas.soderlund@ragnatech.se>]

Hi Sakari,

On 2021-03-23 15:10:41 +0200, Sakari Ailus wrote:
> Hi Niklas,
> 
> On Mon, Mar 08, 2021 at 02:46:52PM +0100, Niklas Söderlund wrote:
> > Hi Andrey,
> > 
> > Thanks for your patch.
> > 
> > On 2021-03-03 21:08:14 +0300, Andrey Konovalov wrote:
> > > To get the link frequency value, or to calculate a parameter depending on
> > > it the receiver driver should use V4L2_CID_LINK_FREQ. If V4L2_CID_LINK_FREQ
> > > control is not implemented in the remote subdevice, the link frequency
> > > can be calculated from V4L2_CID_PIXEL_RATE control value. But the latter
> > > may not give the correct link frequency, and should only be used as the
> > > last resort. v4l2_get_link_freq() does exactly that, so use it instead
> > > of reading V4L2_CID_PIXEL_RATE directly.
> > 
> > I like the direction this patch is taking, but I'm a bit concerned about 
> > that V4L2_CID_LINK_FREQ is not able to replace V4L2_CID_PIXEL_RATE as it 
> > is designed today. Maybe my concern is unfounded and only reflects my 
> > own misunderstanding of the API.
> > 
> > When I wrote this code I tried to first do it using V4L2_CID_LINK_FREQ 
> > but I found no way to be able to express the wide rang of values needed 
> > for my use-case given that V4L2_CID_LINK_FREQ is a menu control. I had 
> 
> I think we could make it alternatively a 64-bit integer control if that
> helps. The helper needs to be adjusted accordingly.

That would solve my concern.

> 
> > to use V4L2_CID_PIXEL_RATE as it allowed me to at runtime calculate and 
> > report the link speed based on input formats. The Use-cases I need to 
> > address are where CSI-2 transmitter themself are a bridge in the video 
> > pipeline, for example
> 
> Is the actual bus frequency changed based on this?

Yes

> 
> Depending on the system where this chip is being used, only certain
> frequencies may be allowed on that bus. It would be most straightforward to
> use only those, but on the other hand, if any frequency can be used and
> that is certain, then I have no objections to allowing that either. We
> simply would make the link-frequencies property optional.

The transmitter is a ADV748x and depending on the video input source 
(HDMI or CVBS) the output frequency changes. Failing to negotiate this 
of course results in the CSI-2 receiver never detecting LP-11.

> 
> > 
> > * Case 1 - HDMI video source
> > 
> > HDMI source -> ADV748x (HDMI-to-CSI-2) ->[CSI-2 bus]-> R-Car CSI-2 receiver
> > 
> > The R-Car CSI-2 receiver needs to know the CSI-2 link frequency and 
> > queries the ADV748x using V4L2_CID_PIXEL_RATE. The ADV748x reports the 
> > pixel rate based on the HDMI format detected on its sink pad.
> > 
> > This could be done using V4L2_CID_LINK_FREQ, but as it's a menu control 
> > it becomes rather tricky to populate it with all possible values, but I 
> > guess it could be doable?
> > 
> > * Case 2 - Multiple video streams over a CSI-2 bus (GMSL)
> > 
> > Camera 1 -|
> > Camera 2 -|
> > Camera 3 -|---> MAX9286 (GMSL-to CSI-2) ->[CSI-2 bus]-> R-Car CSI-2 receiver
> > Camera 4 -|
> > 
> > The MAX9286 has 4 sink pads each connected to an independent camera and 
> > a single CSI-2 source pad. When streaming starts the MAX9286 computes 
> > the total CSI-2 link speed as V4L2_CID_PIXEL_RATE based on the format on 
> > each of it's 4 sink pads.
> > 
> > As in case 1 this could be reported by V4L2_CID_LINK_FREQ but I don't 
> > see it as feasible to populate the menu control with all possible 
> > frequencies before hand.
> > 
> > Hopefully this is all easily solvable and I have only misunderstood how 
> > menu controls work. If not I think this needs to be considered as part 
> > of this series as otherwise it could leave the CSI-2 bridge drivers 
> > without a path forward.
> > 
> > > 
> > > Signed-off-by: Andrey Konovalov <andrey.konovalov@linaro.org>
> > 
> > I tested this and it works as expected. Also as expected it prints lots 
> > of warnings about the usage of V4L2_CID_PIXEL_RATE :-) Once I understand 
> > how I can fix the CSI-2 transmitters used as bridges in the R-Car boards 
> > I will be happy to add my tag to this series as well as fix the bridge 
> > drivers.
> > 
> > > ---
> > >  drivers/media/platform/rcar-vin/rcar-csi2.c | 18 +++++++-----------
> > >  1 file changed, 7 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/drivers/media/platform/rcar-vin/rcar-csi2.c b/drivers/media/platform/rcar-vin/rcar-csi2.c
> > > index e06cd512aba2..eec8f9dd9060 100644
> > > --- a/drivers/media/platform/rcar-vin/rcar-csi2.c
> > > +++ b/drivers/media/platform/rcar-vin/rcar-csi2.c
> > > @@ -455,29 +455,25 @@ static int rcsi2_calc_mbps(struct rcar_csi2 *priv, unsigned int bpp,
> > >  			   unsigned int lanes)
> > >  {
> > >  	struct v4l2_subdev *source;
> > > -	struct v4l2_ctrl *ctrl;
> > > -	u64 mbps;
> > > +	s64 mbps;
> > >  
> > >  	if (!priv->remote)
> > >  		return -ENODEV;
> > >  
> > >  	source = priv->remote;
> > >  
> > > -	/* Read the pixel rate control from remote. */
> > > -	ctrl = v4l2_ctrl_find(source->ctrl_handler, V4L2_CID_PIXEL_RATE);
> > > -	if (!ctrl) {
> > > -		dev_err(priv->dev, "no pixel rate control in subdev %s\n",
> > > +	/* Read the link frequency from the remote subdev. */
> > > +	mbps = v4l2_get_link_freq(source->ctrl_handler, bpp, 2 * lanes);
> > > +	if (mbps < 0) {
> > > +		dev_err(priv->dev, "failed to get link rate from subdev %s\n",
> > >  			source->name);
> > > -		return -EINVAL;
> > > +		return mbps;
> > >  	}
> > > -
> > >  	/*
> > >  	 * Calculate the phypll in mbps.
> > > -	 * link_freq = (pixel_rate * bits_per_sample) / (2 * nr_of_lanes)
> > >  	 * bps = link_freq * 2
> > >  	 */
> > > -	mbps = v4l2_ctrl_g_ctrl_int64(ctrl) * bpp;
> > > -	do_div(mbps, lanes * 1000000);
> > > +	do_div(mbps, 1000000 / 2);
> > >  
> > >  	return mbps;
> > >  }
> > > -- 
> > > 2.17.1
> > > 
> 
> -- 
> Kind regards,
> 
> Sakari Ailus

-- 
Regards,
Niklas Söderlund