linux-kernel - Re: [PATCH v2 1/3] media: v4l: Add helper to get number of active lanes via a pad

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Message-ID: <20250904150723.GC6174@pendragon.ideasonboard.com>
Date: Thu, 4 Sep 2025 17:07:23 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: Isaac Scott <isaac.scott@...asonboard.com>, rmfrfs@...il.com,
	martink@...teo.de, kernel@...i.sm, mchehab@...nel.org,
	shawnguo@...nel.org, s.hauer@...gutronix.de, kernel@...gutronix.de,
	festevam@...il.com, linux-media@...r.kernel.org,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, hverkuil@...nel.org,
	nicolas.dufresne@...labora.com, tomi.valkeinen@...asonboard.com,
	jonas@...boo.se, dan.scally+renesas@...asonboard.com,
	m.szyprowski@...sung.com, mehdi.djait@...ux.intel.com,
	niklas.soderlund+renesas@...natech.se
Subject: Re: [PATCH v2 1/3] media: v4l: Add helper to get number of active
 lanes via a pad

On Thu, Sep 04, 2025 at 11:44:49AM +0300, Sakari Ailus wrote:
> On Wed, Sep 03, 2025 at 11:22:40AM +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.
> > 
> > Signed-off-by: Isaac Scott <isaac.scott@...asonboard.com>
> > ---
> >  drivers/media/v4l2-core/v4l2-common.c | 25 +++++++++++++++++++++++++
> >  include/media/v4l2-common.h           |  1 +
> >  2 files changed, 26 insertions(+)
> > 
> > diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
> > index 6e585bc76367..8683107b3704 100644
> > --- a/drivers/media/v4l2-core/v4l2-common.c
> > +++ b/drivers/media/v4l2-core/v4l2-common.c
> > @@ -571,6 +571,31 @@ 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 dt_lanes)
> 
> This line would benefit from being wrapped.

I'd also rename the dt_lanes parameter to max_data_lanes. "dt" sounds
like data type.

> > +{
> > +	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;
> > +
> > +	if (!mbus_config.bus.mipi_csi2.num_data_lanes)
> > +		return dt_lanes;
> > +
> > +	lanes = mbus_config.bus.mipi_csi2.num_data_lanes;
> > +
> > +	if (lanes < 0 || lanes > dt_lanes)
> 
> lanes is unsigned int so no need to check for less than 0.
> 
> I might just not use a local variable for this, up to you.
> 

A debug message would also be nice.

> > +		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..3f8937260c76 100644
> > --- a/include/media/v4l2-common.h
> > +++ b/include/media/v4l2-common.h
> > @@ -584,6 +584,7 @@ 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);
> 
> Some kernel-doc documentation would be nice.
> 
> > +unsigned int v4l2_get_active_data_lanes(const struct media_pad *pad, unsigned int dt_lanes);
> 
> Please wrap this, too.
> 
> >  #else
> >  #define v4l2_get_link_freq(handler, mul, div)		\
> >  	__v4l2_get_link_freq_ctrl(handler, mul, div)

-- 
Regards,

Laurent Pinchart