linux-kernel - Re: [PATCH] imx-mipi-csis: Get the number of active lanes from mbus

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Message-ID: <aJ77VTtZy96JJCHE@valkosipuli.retiisi.eu>
Date: Fri, 15 Aug 2025 09:18:13 +0000
From: Sakari Ailus <sakari.ailus@....fi>
To: Isaac Scott <isaac.scott@...asonboard.com>
Cc: linux-media@...r.kernel.org, rmfrfs@...il.com,
	laurent.pinchart@...asonboard.com, martink@...teo.de,
	kernel@...i.sm, mchehab@...nel.org, shawnguo@...nel.org,
	s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] imx-mipi-csis: Get the number of active lanes from
 mbus_config

Hi Isaac,

Thanks for the patch.

On Thu, Aug 14, 2025 at 12:37:01PM +0100, Isaac Scott wrote:
> Although 4 lanes may be physically available, we may not be using all of
> them. Get the number of configured lanes in the case a driver has
> implemented the get_mbus_config op.
> 
> Signed-off-by: Isaac Scott <isaac.scott@...asonboard.com>
> 
> ---
> 
> Currently, the imx-mipi-csis driver parses the device tree to determine
> the number of configured lanes for the CSI receiver. This may be
> incorrect in the case that the connected device only uses a subset of
> lanes, for example. Allow the drivers for these cameras to create a
> mbus_config to configure the number of lanes that are actually being
> used.
> 
> If the driver does not support the get_mbus_config op, this patch will
> have no functional change.
> 
> Compile tested against media-master (v6.17-rc1)
> ---
>  drivers/media/platform/nxp/imx-mipi-csis.c | 41 ++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/drivers/media/platform/nxp/imx-mipi-csis.c b/drivers/media/platform/nxp/imx-mipi-csis.c
> index 2beb5f43c2c0..efe4e2ad0382 100644
> --- a/drivers/media/platform/nxp/imx-mipi-csis.c
> +++ b/drivers/media/platform/nxp/imx-mipi-csis.c
> @@ -939,6 +939,43 @@ static struct mipi_csis_device *sd_to_mipi_csis_device(struct v4l2_subdev *sdev)
>  	return container_of(sdev, struct mipi_csis_device, sd);
>  }
>  
> +static int mipi_csis_get_active_lanes(struct v4l2_subdev *sd)
> +{
> +	struct mipi_csis_device *csis = sd_to_mipi_csis_device(sd);
> +	struct v4l2_mbus_config mbus_config = { 0 };
> +	int ret;
> +
> +	ret = v4l2_subdev_call(csis->source.sd, pad, get_mbus_config,
> +			       0, &mbus_config);
> +	if (ret == -ENOIOCTLCMD) {
> +		dev_dbg(csis->dev, "No remote mbus configuration available\n");
> +		return 0;
> +	}
> +
> +	if (ret) {
> +		dev_err(csis->dev, "Failed to get remote mbus configuration\n");
> +		return ret;
> +	}
> +
> +	if (mbus_config.type != V4L2_MBUS_CSI2_DPHY) {
> +		dev_err(csis->dev, "Unsupported media bus type %u\n",
> +			mbus_config.type);
> +		return -EINVAL;
> +	}
> +
> +	if (mbus_config.bus.mipi_csi2.num_data_lanes > csis->bus.num_data_lanes) {
> +		dev_err(csis->dev,
> +			"Unsupported mbus config: too many data lanes %u\n",
> +			mbus_config.bus.mipi_csi2.num_data_lanes);
> +		return -EINVAL;
> +	}
> +
> +	csis->bus.num_data_lanes = mbus_config.bus.mipi_csi2.num_data_lanes;
> +	dev_dbg(csis->dev, "Number of lanes: %d\n", csis->bus.num_data_lanes);

None of the above is really specific to this driver. Could you instead
implement a function that parses the information from the fwnode endpoint
and uses mbus configuration on top?

The function could take struct media_pad pointer as an argument, or struct
v4l2_subdev pointer and the pad number.

I wonder if any other parameters could change dynamically but I can't think
of that now, so perhaps just the number of lanes is what the function
should indeed return.

> +
> +	return 0;
> +}
> +
>  static int mipi_csis_s_stream(struct v4l2_subdev *sd, int enable)
>  {
>  	struct mipi_csis_device *csis = sd_to_mipi_csis_device(sd);
> @@ -965,6 +1002,10 @@ static int mipi_csis_s_stream(struct v4l2_subdev *sd, int enable)
>  	format = v4l2_subdev_state_get_format(state, CSIS_PAD_SINK);
>  	csis_fmt = find_csis_format(format->code);
>  
> +	ret = mipi_csis_get_active_lanes(sd);
> +	if (ret < 0)
> +		dev_dbg(csis->dev, "Failed to get active lanes: %d", ret);
> +
>  	ret = mipi_csis_calculate_params(csis, csis_fmt);
>  	if (ret < 0)
>  		goto err_unlock;

-- 
Kind regards,

Sakari Ailus