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 <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Isaac Scott <[e-mail]>
Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/v4l2-core/v4l2-common.c

···
 	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
 
 /*

include/media/v4l2-common.h

···
 #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,