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.. SPDX-License-Identifier: GPL-2.0
MIPI CSI-2
==========
CSI-2 is a data bus intended for transferring images from cameras to
the host SoC. It is defined by the `MIPI alliance`_.
.. _`MIPI alliance`: http://www.mipi.org/
Transmitter drivers
-------------------
CSI-2 transmitter, such as a sensor or a TV tuner, drivers need to
provide the CSI-2 receiver with information on the CSI-2 bus
configuration. These include the V4L2_CID_LINK_FREQ and
V4L2_CID_PIXEL_RATE controls and
(:c:type:`v4l2_subdev_video_ops`->s_stream() callback). These
interface elements must be present on the sub-device represents the
CSI-2 transmitter.
The V4L2_CID_LINK_FREQ control is used to tell the receiver driver the
frequency (and not the symbol rate) of the link. The
V4L2_CID_PIXEL_RATE is may be used by the receiver to obtain the pixel
rate the transmitter uses. The
:c:type:`v4l2_subdev_video_ops`->s_stream() callback provides an
ability to start and stop the stream.
The value of the V4L2_CID_PIXEL_RATE is calculated as follows::
pixel_rate = link_freq * 2 * nr_of_lanes / bits_per_sample
where
.. list-table:: variables in pixel rate calculation
:header-rows: 1
* - variable or constant
- description
* - link_freq
- The value of the V4L2_CID_LINK_FREQ integer64 menu item.
* - nr_of_lanes
- Number of data lanes used on the CSI-2 link. This can
be obtained from the OF endpoint configuration.
* - 2
- Two bits are transferred per clock cycle per lane.
* - bits_per_sample
- Number of bits per sample.
The transmitter drivers must, if possible, configure the CSI-2
transmitter to *LP-11 mode* whenever the transmitter is powered on but
not active, and maintain *LP-11 mode* until stream on. Only at stream
on should the transmitter activate the clock on the clock lane and
transition to *HS mode*.
Some transmitters do this automatically but some have to be explicitly
programmed to do so, and some are unable to do so altogether due to
hardware constraints.
Stopping the transmitter
^^^^^^^^^^^^^^^^^^^^^^^^
A transmitter stops sending the stream of images as a result of
calling the ``.s_stream()`` callback. Some transmitters may stop the
stream at a frame boundary whereas others stop immediately,
effectively leaving the current frame unfinished. The receiver driver
should not make assumptions either way, but function properly in both
cases.
Receiver drivers
----------------
Before the receiver driver may enable the CSI-2 transmitter by using
the :c:type:`v4l2_subdev_video_ops`->s_stream(), it must have powered
the transmitter up by using the
:c:type:`v4l2_subdev_core_ops`->s_power() callback. This may take
place either indirectly by using :c:func:`v4l2_pipeline_pm_use` or
directly.
Formats
-------
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.
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