134 lines
4.4 KiB
ReStructuredText
134 lines
4.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. _transmitter-receiver:
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Pixel data transmitter and receiver drivers
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===========================================
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V4L2 supports various devices that transmit and receive pixel data. Examples of
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these devices include a camera sensor, a TV tuner and a parallel or a CSI-2
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receiver in an SoC.
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Bus types
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---------
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The following busses are the most common. This section discusses these two only.
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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`: https://www.mipi.org/
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Parallel
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^^^^^^^^
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`BT.601`_ and `BT.656`_ are the most common parallel busses.
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.. _`BT.601`: https://en.wikipedia.org/wiki/Rec._601
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.. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656
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Transmitter drivers
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-------------------
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Transmitter drivers generally need to provide the receiver drivers with the
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configuration of the transmitter. What is required depends on the type of the
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bus. These are common for both busses.
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Media bus pixel code
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^^^^^^^^^^^^^^^^^^^^
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See :ref:`v4l2-mbus-pixelcode`.
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Link frequency
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^^^^^^^^^^^^^^
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The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the
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receiver the frequency of the bus (i.e. it is not the same as the symbol rate).
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``.s_stream()`` callback
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^^^^^^^^^^^^^^^^^^^^^^^^
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The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
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receiver driver to control the transmitter driver's streaming state.
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CSI-2 transmitter drivers
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-------------------------
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Pixel rate
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^^^^^^^^^^
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The pixel rate on the bus is calculated as follows::
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pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / 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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- Data is transferred on both rising and falling edge of the signal.
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* - bits_per_sample
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- Number of bits per sample.
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* - k
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- 16 for D-PHY and 7 for C-PHY
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.. note::
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The pixel rate calculated this way is **not** the same thing as the
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pixel rate on the camera sensor's pixel array which is indicated by the
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:ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control.
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LP-11 and LP-111 modes
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^^^^^^^^^^^^^^^^^^^^^^
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As part of transitioning to high speed mode, a CSI-2 transmitter typically
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briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period
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may be as short as 100 µs, during which the receiver observes this state and
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proceeds its own part of high speed mode transition.
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Most receivers are capable of autonomously handling this once the software has
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configured them to do so, but there are receivers which require software
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involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit
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in software, especially when there is no interrupt telling something is
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happening.
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One way to address this is to configure the transmitter side explicitly to LP-11
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or LP-111 mode, which requires support from the transmitter hardware. This is
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not universally available. Many devices return to this state once streaming is
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stopped while the state after power-on is LP-00 or LP-000.
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The ``.pre_streamon()`` callback may be used to prepare a transmitter for
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transitioning to streaming state, but not yet start streaming. Similarly, the
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``.post_streamoff()`` callback is used to undo what was done by the
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``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required
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to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``.
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In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition
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the transmitter to the LP-11 or LP-111 mode. This also requires powering on the
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device, so this should be only done when it is needed.
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Receiver drivers that do not need explicit LP-11 or LP-111 mode setup are waived
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from calling the two callbacks.
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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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