media: v4l2- mediabus: Add usage note for V4L2_MBUS_*

With the removal of the legacy g_mbus_config and s_mbus_config video
operations, the sole users of V4L2_MBUS_* flags are now the newly
introduced get_mbus_config and set_mbus_config pad operations.

As the semantic of the new operations differs from the semantic of
the legacy ones, add a usage note in the v4l2-mediabus.h header to
specify how to use the flags.

Also add a TODO note to record that we intend to replace the existing
flags with fields, to prevent users from mixing conflicting values
in a single operation call.
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

include/media/v4l2-mediabus.h

@@ -11,9 +11,34 @@
 #include <linux/v4l2-mediabus.h>
 #include <linux/bitops.h>
 
+/*
+ * How to use the V4L2_MBUS_* flags:
+ * Flags are defined for each of the possible states and values of a media
+ * bus configuration parameter. One and only one bit of each group of flags
+ * shall be set by the users of the v4l2_subdev_pad_ops.get_mbus_config and
+ * v4l2_subdev_pad_ops.set_mbus_config operations to ensure that no
+ * conflicting settings are specified when reporting and setting the media bus
+ * configuration with the two operations respectively. For example, it is
+ * invalid to set or clear both the V4L2_MBUS_HSYNC_ACTIVE_HIGH and the
+ * V4L2_MBUS_HSYNC_ACTIVE_LOW flag at the same time. Instead either flag
+ * V4L2_MBUS_HSYNC_ACTIVE_HIGH or flag V4L2_MBUS_HSYNC_ACTIVE_LOW shall be
+ * set. The same is true for the V4L2_MBUS_CSI2_1/2/3/4_LANE flags group: only
+ * one of these four bits shall be set.
+ *
+ * TODO: replace the existing V4L2_MBUS_* flags with structures of fields
+ * to avoid conflicting settings.
+ *
+ * In example:
+ *     #define V4L2_MBUS_HSYNC_ACTIVE_HIGH             BIT(2)
+ *     #define V4L2_MBUS_HSYNC_ACTIVE_LOW              BIT(3)
+ * will be replaced by a field whose value reports the intended active state of
+ * the signal:
+ *     unsigned int v4l2_mbus_hsync_active : 1;
+ */
+
 /* Parallel flags */
 /*
- * Can the client run in master or in slave mode. By "Master mode" an operation
+ * The client runs in master or in slave mode. By "Master mode" an operation
  * mode is meant, when the client (e.g., a camera sensor) is producing
  * horizontal and vertical synchronisation. In "Slave mode" the host is
  * providing these signals to the slave.
@@ -45,17 +70,17 @@
 #define V4L2_MBUS_DATA_ENABLE_LOW		BIT(15)
 
 /* Serial flags */
-/* How many lanes the client can use */
+/* CSI-2 D-PHY number of data lanes. */
 #define V4L2_MBUS_CSI2_1_LANE			BIT(0)
 #define V4L2_MBUS_CSI2_2_LANE			BIT(1)
 #define V4L2_MBUS_CSI2_3_LANE			BIT(2)
 #define V4L2_MBUS_CSI2_4_LANE			BIT(3)
-/* On which channels it can send video data */
+/* CSI-2 Virtual Channel identifiers. */
 #define V4L2_MBUS_CSI2_CHANNEL_0		BIT(4)
 #define V4L2_MBUS_CSI2_CHANNEL_1		BIT(5)
 #define V4L2_MBUS_CSI2_CHANNEL_2		BIT(6)
 #define V4L2_MBUS_CSI2_CHANNEL_3		BIT(7)
-/* Does it support only continuous or also non-continuous clock mode */
+/* Clock non-continuous mode support. */
 #define V4L2_MBUS_CSI2_CONTINUOUS_CLOCK		BIT(8)
 #define V4L2_MBUS_CSI2_NONCONTINUOUS_CLOCK	BIT(9)