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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>
This commit is contained in:
Maxime Ripard 2026-03-09 16:07:40 +01:00 committed by Hans Verkuil
parent 40c6da8a9c
commit 7b0288bc05
1 changed files with 12 additions and 8 deletions
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20
Documentation/userspace-api/media/v4l/subdev-formats.rst
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@ -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
^^^^^^^^^^^^^^^^^^