media: uapi: Clarify MBUS color component order for serial buses

The subdev format documentation has a subsection describing how to use
the media bus pixel codes for serial buses. While it describes the
sampling part well, it doesn't really describe the current convention
used for the components order.

Let's improve that.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>

1 files changed, 12 insertions, 8 deletions

diff --git a/Documentation/userspace-api/media/v4l/subdev-formats.rst b/Documentation/userspace-api/media/v4l/subdev-formats.rst
index 896177c5334f..c9999b929773 100644
--- a/Documentation/userspace-api/media/v4l/subdev-formats.rst
+++ b/Documentation/userspace-api/media/v4l/subdev-formats.rst
@@ -159,14 +159,18 @@ formats in memory (a raw Bayer image won't be magically converted to
 JPEG just by storing it to memory), there is no one-to-one
 correspondence between them.
 
-The media bus pixel codes document parallel formats. Should the pixel data be
-transported over a serial bus, the media bus pixel code that describes a
-parallel format that transfers a sample on a single clock cycle is used. For
-instance, both MEDIA_BUS_FMT_BGR888_1X24 and MEDIA_BUS_FMT_BGR888_3X8 are used
-on parallel busses for transferring an 8 bits per sample BGR data, whereas on
-serial busses the data in this format is only referred to using
-MEDIA_BUS_FMT_BGR888_1X24. This is because there is effectively only a single
-way to transport that format on the serial busses.
+While the media bus pixel codes are named based on how pixels are
+transmitted on parallel buses, serial buses do not define separate
+codes. By convention, they use the codes that transfer a sample on a
+single clock cycle, and whose bit orders from LSB to MSB correspond to
+the order in which colour components are transmitted on the serial bus.
+For instance, the MIPI CSI-2 24-bit RGB (RGB888) format uses the
+MEDIA_BUS_FMT_RGB888_1X24 media bus code because CSI-2 transmits the
+blue colour component first, followed by green and red, and
+MEDIA_BUS_FMT_RGB888_1X24 defines the first bit of blue at bit 0.
+While used for 24-bit RGB data on parallel buses, the
+MEDIA_BUS_FMT_RGB888_3X8 or MEDIA_BUS_FMT_BGR888_1X24 codes must not be
+used for CSI-2.
 
 Packed RGB Formats
 ^^^^^^^^^^^^^^^^^^