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