1 .. SPDX-License-Identifier: GPL-2.0 2 3 .. _transmitter-receiver: 4 5 Pixel data transmitter and receiver drivers 6 =========================================== 7 8 V4L2 supports various devices that transmit and receive pixel data. Examples of 9 these devices include a camera sensor, a TV tuner and a parallel, a BT.656 or a 10 CSI-2 receiver in an SoC. 11 12 Bus types 13 --------- 14 15 The following busses are the most common. This section discusses these two only. 16 17 MIPI CSI-2 18 ^^^^^^^^^^ 19 20 CSI-2 is a data bus intended for transferring images from cameras to 21 the host SoC. It is defined by the `MIPI alliance`_. 22 23 .. _`MIPI alliance`: https://www.mipi.org/ 24 25 Parallel and BT.656 26 ^^^^^^^^^^^^^^^^^^^ 27 28 The parallel and `BT.656`_ buses transport one bit of data on each clock cycle 29 per data line. The parallel bus uses synchronisation and other additional 30 signals whereas BT.656 embeds synchronisation. 31 32 .. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656 33 34 Transmitter drivers 35 ------------------- 36 37 Transmitter drivers generally need to provide the receiver drivers with the 38 configuration of the transmitter. What is required depends on the type of the 39 bus. These are common for both busses. 40 41 Media bus pixel code 42 ^^^^^^^^^^^^^^^^^^^^ 43 44 See :ref:`v4l2-mbus-pixelcode`. 45 46 Link frequency 47 ^^^^^^^^^^^^^^ 48 49 The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the 50 receiver the frequency of the bus (i.e. it is not the same as the symbol rate). 51 52 ``.s_stream()`` callback 53 ^^^^^^^^^^^^^^^^^^^^^^^^ 54 55 The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the 56 receiver driver to control the transmitter driver's streaming state. 57 58 59 CSI-2 transmitter drivers 60 ------------------------- 61 62 Pixel rate 63 ^^^^^^^^^^ 64 65 The pixel rate on the bus is calculated as follows:: 66 67 pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample 68 69 where 70 71 .. list-table:: variables in pixel rate calculation 72 :header-rows: 1 73 74 * - variable or constant 75 - description 76 * - link_freq 77 - The value of the ``V4L2_CID_LINK_FREQ`` integer64 menu item. 78 * - nr_of_lanes 79 - Number of data lanes used on the CSI-2 link. This can 80 be obtained from the OF endpoint configuration. 81 * - 2 82 - Data is transferred on both rising and falling edge of the signal. 83 * - bits_per_sample 84 - Number of bits per sample. 85 * - k 86 - 16 for D-PHY and 7 for C-PHY 87 88 .. note:: 89 90 The pixel rate calculated this way is **not** the same thing as the 91 pixel rate on the camera sensor's pixel array which is indicated by the 92 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control. 93 94 LP-11 and LP-111 states 95 ^^^^^^^^^^^^^^^^^^^^^^^ 96 97 As part of transitioning to high speed mode, a CSI-2 transmitter typically 98 briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period 99 may be as short as 100 µs, during which the receiver observes this state and 100 proceeds its own part of high speed mode transition. 101 102 Most receivers are capable of autonomously handling this once the software has 103 configured them to do so, but there are receivers which require software 104 involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit 105 in software, especially when there is no interrupt telling something is 106 happening. 107 108 One way to address this is to configure the transmitter side explicitly to LP-11 109 or LP-111 state, which requires support from the transmitter hardware. This is 110 not universally available. Many devices return to this state once streaming is 111 stopped while the state after power-on is LP-00 or LP-000. 112 113 The ``.pre_streamon()`` callback may be used to prepare a transmitter for 114 transitioning to streaming state, but not yet start streaming. Similarly, the 115 ``.post_streamoff()`` callback is used to undo what was done by the 116 ``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required 117 to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``. 118 119 In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition 120 the transmitter to the LP-11 or LP-111 state. This also requires powering on the 121 device, so this should be only done when it is needed. 122 123 Receiver drivers that do not need explicit LP-11 or LP-111 state setup are 124 waived from calling the two callbacks. 125 126 Stopping the transmitter 127 ^^^^^^^^^^^^^^^^^^^^^^^^ 128 129 A transmitter stops sending the stream of images as a result of 130 calling the ``.s_stream()`` callback. Some transmitters may stop the 131 stream at a frame boundary whereas others stop immediately, 132 effectively leaving the current frame unfinished. The receiver driver 133 should not make assumptions either way, but function properly in both 134 cases.
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