1.. SPDX-License-Identifier: GPL-2.0 2 3.. _transmitter-receiver: 4 5Pixel data transmitter and receiver drivers 6=========================================== 7 8V4L2 supports various devices that transmit and receive pixel data. Examples of 9these devices include a camera sensor, a TV tuner and a parallel, a BT.656 or a 10CSI-2 receiver in an SoC. 11 12Bus types 13--------- 14 15The following busses are the most common. This section discusses these two only. 16 17MIPI CSI-2 18^^^^^^^^^^ 19 20CSI-2 is a data bus intended for transferring images from cameras to 21the host SoC. It is defined by the `MIPI alliance`_. 22 23.. _`MIPI alliance`: https://www.mipi.org/ 24 25Parallel and BT.656 26^^^^^^^^^^^^^^^^^^^ 27 28The parallel and `BT.656`_ buses transport one bit of data on each clock cycle 29per data line. The parallel bus uses synchronisation and other additional 30signals whereas BT.656 embeds synchronisation. 31 32.. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656 33 34Transmitter drivers 35------------------- 36 37Transmitter drivers generally need to provide the receiver drivers with the 38configuration of the transmitter. What is required depends on the type of the 39bus. These are common for both busses. 40 41Media bus pixel code 42^^^^^^^^^^^^^^^^^^^^ 43 44See :ref:`v4l2-mbus-pixelcode`. 45 46Link frequency 47^^^^^^^^^^^^^^ 48 49The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the 50receiver the frequency of the bus (i.e. it is not the same as the symbol rate). 51 52``.enable_streams()`` and ``.disable_streams()`` callbacks 53^^^^^^^^^^^^^^^^^^^^^^^^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 54 55The struct v4l2_subdev_pad_ops->enable_streams() and struct 56v4l2_subdev_pad_ops->disable_streams() callbacks are used by the receiver driver 57to control the transmitter driver's streaming state. These callbacks may not be 58called directly, but by using ``v4l2_subdev_enable_streams()`` and 59``v4l2_subdev_disable_streams()``. 60 61 62CSI-2 transmitter drivers 63------------------------- 64 65Pixel rate 66^^^^^^^^^^ 67 68The pixel rate on the bus is calculated as follows:: 69 70 pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample 71 72where 73 74.. list-table:: variables in pixel rate calculation 75 :header-rows: 1 76 77 * - variable or constant 78 - description 79 * - link_freq 80 - The value of the ``V4L2_CID_LINK_FREQ`` integer64 menu item. 81 * - nr_of_lanes 82 - Number of data lanes used on the CSI-2 link. This can 83 be obtained from the OF endpoint configuration. 84 * - 2 85 - Data is transferred on both rising and falling edge of the signal. 86 * - bits_per_sample 87 - Number of bits per sample. 88 * - k 89 - 16 for D-PHY and 7 for C-PHY 90 91.. note:: 92 93 The pixel rate calculated this way is **not** the same thing as the 94 pixel rate on the camera sensor's pixel array which is indicated by the 95 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control. 96 97LP-11 and LP-111 states 98^^^^^^^^^^^^^^^^^^^^^^^ 99 100As part of transitioning to high speed mode, a CSI-2 transmitter typically 101briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period 102may be as short as 100 µs, during which the receiver observes this state and 103proceeds its own part of high speed mode transition. 104 105Most receivers are capable of autonomously handling this once the software has 106configured them to do so, but there are receivers which require software 107involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit 108in software, especially when there is no interrupt telling something is 109happening. 110 111One way to address this is to configure the transmitter side explicitly to LP-11 112or LP-111 state, which requires support from the transmitter hardware. This is 113not universally available. Many devices return to this state once streaming is 114stopped while the state after power-on is LP-00 or LP-000. 115 116The ``.pre_streamon()`` callback may be used to prepare a transmitter for 117transitioning to streaming state, but not yet start streaming. Similarly, the 118``.post_streamoff()`` callback is used to undo what was done by the 119``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required 120to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``. 121 122In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition 123the transmitter to the LP-11 or LP-111 state. This also requires powering on the 124device, so this should be only done when it is needed. 125 126Receiver drivers that do not need explicit LP-11 or LP-111 state setup are 127waived from calling the two callbacks. 128 129Stopping the transmitter 130^^^^^^^^^^^^^^^^^^^^^^^^ 131 132A transmitter stops sending the stream of images as a result of 133calling the ``.disable_streams()`` callback. Some transmitters may stop the 134stream at a frame boundary whereas others stop immediately, 135effectively leaving the current frame unfinished. The receiver driver 136should not make assumptions either way, but function properly in both 137cases. 138