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: <aNDxR72CmvGofTHy@valkosipuli.retiisi.eu>
Date: Mon, 22 Sep 2025 09:48:39 +0300
From: Sakari Ailus <sakari.ailus@....fi>
To: Isaac Scott <isaac.scott@...asonboard.com>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>,
	Rui Miguel Silva <rmfrfs@...il.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.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

Hi Isaac,

Thanks for the update.

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.
> +	 */
> +	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?

> +
> +/**
> + * 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.
> + */
> +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,

Sakari Ailus