linux-kernel - Re: [PATCH v3 1/3] media: v4l: Add helper to get number of active lanes via a pad

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Message-ID: <20250922071637.GH10540@pendragon.ideasonboard.com>
Date: Mon, 22 Sep 2025 10:16:37 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Sakari Ailus <sakari.ailus@....fi>
Cc: Isaac Scott <isaac.scott@...asonboard.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Rui Miguel Silva <rmfrfs@...il.com>,
	Martin Kepplinger <martink@...teo.de>,
	Purism Kernel Team <kernel@...i.sm>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, imx@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, Frank Li <Frank.Li@....com>
Subject: Re: [PATCH v3 1/3] media: v4l: Add helper to get number of active
 lanes via a pad

On Mon, Sep 22, 2025 at 09:48:39AM +0300, Sakari Ailus wrote:
> On Mon, Sep 15, 2025 at 02:18:33PM +0100, Isaac Scott wrote:
> > Sometimes, users will not use all of the MIPI CSI 2 lanes available when
> > connecting to the MIPI CSI receiver of their device. Add a helper
> > function that checks the mbus_config for the device driver to allow
> > users to define the number of active data lanes through the
> > get_mbus_config op.
> > 
> > If the driver does not implement this op, fall back to using the number
> > of data lanes specified in device tree.
> > 
> > Reviewed-by: Frank Li <Frank.Li@....com>
> > Signed-off-by: Isaac Scott <isaac.scott@...asonboard.com>
> > ---
> >  drivers/media/v4l2-core/v4l2-common.c | 32 ++++++++++++++++++++++++++++++++
> >  include/media/v4l2-common.h           | 21 +++++++++++++++++++++
> >  2 files changed, 53 insertions(+)
> > 
> > diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
> > index 6e585bc76367..2ce8407f1397 100644
> > --- a/drivers/media/v4l2-core/v4l2-common.c
> > +++ b/drivers/media/v4l2-core/v4l2-common.c
> > @@ -571,6 +571,38 @@ s64 __v4l2_get_link_freq_pad(struct media_pad *pad, unsigned int mul,
> >  	return __v4l2_get_link_freq_ctrl(sd->ctrl_handler, mul, div);
> >  }
> >  EXPORT_SYMBOL_GPL(__v4l2_get_link_freq_pad);
> > +
> > +unsigned int v4l2_get_active_data_lanes(const struct media_pad *pad,
> > +					unsigned int max_data_lanes)
> > +{
> > +	struct v4l2_mbus_config mbus_config = {};
> > +	struct v4l2_subdev *sd;
> > +	unsigned int lanes;
> > +	int ret;
> > +
> > +	sd = media_entity_to_v4l2_subdev(pad->entity);
> > +	ret = v4l2_subdev_call(sd, pad, get_mbus_config, pad->index,
> > +			       &mbus_config);
> > +	if (ret < 0 && ret != -ENOIOCTLCMD)
> > +		return ret;
> > +
> > +	/*
> > +	 * This relies on the mbus_config being zeroed at init time.
> > +	 */

This holds on a single line.

> > +	if (!mbus_config.bus.mipi_csi2.num_data_lanes)
> 
> I'd either use the local variable for this (lanes) either all the time, or
> not at all.
> 
> > +		return max_data_lanes;
> > +
> > +	lanes = mbus_config.bus.mipi_csi2.num_data_lanes;
> > +
> > +	if (lanes > max_data_lanes) {
> > +		dev_dbg(sd->dev, "Active data lanes (%u) exceeds max (%u)\n",
> > +			lanes, max_data_lanes);
> > +		return -EINVAL;
> > +	}
> > +
> > +	return lanes;
> > +}
> > +EXPORT_SYMBOL_GPL(v4l2_get_active_data_lanes);
> >  #endif /* CONFIG_MEDIA_CONTROLLER */
> >  
> >  /*
> > diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
> > index 0a43f56578bc..6af0695460ab 100644
> > --- a/include/media/v4l2-common.h
> > +++ b/include/media/v4l2-common.h
> > @@ -584,6 +584,27 @@ int v4l2_fill_pixfmt_mp(struct v4l2_pix_format_mplane *pixfmt, u32 pixelformat,
> >  	(pad, mul, div)
> >  s64 __v4l2_get_link_freq_pad(struct media_pad *pad, unsigned int mul,
> >  			     unsigned int div);
> 
> Is your tree up to date?

We seem to be moving faster than we used to :-)

> > +
> > +/**
> > + * v4l2_get_active_data_lanes - Get number of active data lanes from driver
> > + *
> > + * @pad: The transmitter's media pad.
> > + * @max_data_lanes: The maximum number of active data lanes supported by
> > + * 		    the MIPI CSI link in hardware. This can be configured
> > + * 		    in device tree.
> 
> I'd remove the latter sentence. Alternatively, it needs to be improved:
> there are other sources for this information than DT.
> 
> > + *
> > + * This function is intended for obtaining the number of data lanes that are
> > + * actively being used by the driver for a MIPI CSI-2 device on a given media pad.
> > + * This information is derived from a mbus_config fetched from a device driver
> > + * using the get_mbus_config v4l2_subdev pad op.
> > + *
> > + * Return:
> > + * * >0: Number of active data lanes
> > + * * %-EINVAL: Number of active data lanes is invalid, as it exceeds the maximum
> > + *	       supported data lanes listed in device tree.

Drop "listed in the device tree" here too.

With those small issues fixed,

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

> > + */
> > +unsigned int v4l2_get_active_data_lanes(const struct media_pad *pad,
> > +					unsigned int max_data_lanes);
> >  #else
> >  #define v4l2_get_link_freq(handler, mul, div)		\
> >  	__v4l2_get_link_freq_ctrl(handler, mul, div)

-- 
Regards,

Laurent Pinchart