linux-kernel - [PATCH v6 1/4] media: v4l: Add helper to get number of active lanes via a pad

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Message-ID: <20251029180321.153680-2-isaac.scott@ideasonboard.com>
Date: Wed, 29 Oct 2025 18:03:18 +0000
From: Isaac Scott <isaac.scott@...asonboard.com>
To: mchehab@...nel.org
Cc: rmfrfs@...il.com,
	laurent.pinchart@...asonboard.com,
	martink@...teo.de,
	kernel@...i.sm,
	shawnguo@...nel.org,
	s.hauer@...gutronix.de,
	kernel@...gutronix.de,
	festevam@...il.com,
	sakari.ailus@...ux.intel.com,
	linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	imx@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	Frank.Li@....com,
	Isaac Scott <isaac.scott@...asonboard.com>
Subject: [PATCH v6 1/4] 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 maximum
number of lanes available.

Reviewed-by: Frank Li <Frank.Li@....com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Signed-off-by: Isaac Scott <isaac.scott@...asonboard.com>
---
 drivers/media/v4l2-core/v4l2-common.c | 29 +++++++++++++++++++++++++++
 include/media/v4l2-common.h           | 20 ++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
index b367d479d6b3..554c591e1113 100644
--- a/drivers/media/v4l2-core/v4l2-common.c
+++ b/drivers/media/v4l2-core/v4l2-common.c
@@ -573,6 +573,35 @@ s64 v4l2_get_link_freq(const 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);
+
+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 */
+	lanes = mbus_config.bus.mipi_csi2.num_data_lanes;
+	if (!lanes)
+		return max_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
 
 /*
diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
index 5c0a7f6b5bb6..f8b1faced79c 100644
--- a/include/media/v4l2-common.h
+++ b/include/media/v4l2-common.h
@@ -581,6 +581,26 @@ int v4l2_fill_pixfmt_mp(struct v4l2_pix_format_mplane *pixfmt, u32 pixelformat,
 #ifdef CONFIG_MEDIA_CONTROLLER
 s64 v4l2_get_link_freq(const 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 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.
+ */
+int v4l2_get_active_data_lanes(const struct media_pad *pad,
+			       unsigned int max_data_lanes);
 #endif
 
 void v4l2_simplify_fraction(u32 *numerator, u32 *denominator,
-- 
2.43.0