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: <20250904083305.GB1207681@ragnatech.se>
Date: Thu, 4 Sep 2025 10:33:05 +0200
From: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
To: Isaac Scott <isaac.scott@...asonboard.com>
Cc: laurent.pinchart@...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,
	sakari.ailus@...ux.intel.com, tomi.valkeinen@...asonboard.com,
	jonas@...boo.se, dan.scally+renesas@...asonboard.com,
	m.szyprowski@...sung.com, mehdi.djait@...ux.intel.com
Subject: Re: [PATCH v2 1/3] media: v4l: Add helper to get number of active
 lanes via a pad

Hi Issac,

Thanks for your work.

On 2025-09-03 11:22:40 +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)
> +{
> +	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;

The function prototype is 'unsigned int' and here you return a signed 
value here.

> +

Maybe add a comment here that this check depends on mbus_config being 
zeroed at init if the call above pass with -ENOIOCTLCMD? It's not 
immediately clear what is going on here even tho the code is clever ;-)

> +	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)
> +		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);
> +unsigned int v4l2_get_active_data_lanes(const struct media_pad *pad, unsigned int dt_lanes);
>  #else
>  #define v4l2_get_link_freq(handler, mul, div)		\
>  	__v4l2_get_link_freq_ctrl(handler, mul, div)
> -- 
> 2.43.0
> 

-- 
Kind Regards,
Niklas Söderlund