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

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Message-Id: <20250915-mbus-config-active-lanes-v3-1-97a1282a410b@ideasonboard.com>
Date: Mon, 15 Sep 2025 14:18:33 +0100
From: Isaac Scott <isaac.scott@...asonboard.com>
To: 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>
Cc: 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>, Isaac Scott <isaac.scott@...asonboard.com>
Subject: [PATCH v3 1/3] media: v4l: Add helper to get number of active
 lanes via a pad

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)
+		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);
+
+/**
+ * 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.
+ *
+ * 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)

-- 
2.43.0