tx-rx.rst (4512B)
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 or a CSI-2 10receiver 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 26^^^^^^^^ 27 28`BT.601`_ and `BT.656`_ are the most common parallel busses. 29 30.. _`BT.601`: https://en.wikipedia.org/wiki/Rec._601 31.. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656 32 33Transmitter drivers 34------------------- 35 36Transmitter drivers generally need to provide the receiver drivers with the 37configuration of the transmitter. What is required depends on the type of the 38bus. These are common for both busses. 39 40Media bus pixel code 41^^^^^^^^^^^^^^^^^^^^ 42 43See :ref:`v4l2-mbus-pixelcode`. 44 45Link frequency 46^^^^^^^^^^^^^^ 47 48The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the 49receiver the frequency of the bus (i.e. it is not the same as the symbol rate). 50 51``.s_stream()`` callback 52^^^^^^^^^^^^^^^^^^^^^^^^ 53 54The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the 55receiver driver to control the transmitter driver's streaming state. 56 57 58CSI-2 transmitter drivers 59------------------------- 60 61Pixel rate 62^^^^^^^^^^ 63 64The pixel rate on the bus is calculated as follows:: 65 66 pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample 67 68where 69 70.. list-table:: variables in pixel rate calculation 71 :header-rows: 1 72 73 * - variable or constant 74 - description 75 * - link_freq 76 - The value of the ``V4L2_CID_LINK_FREQ`` integer64 menu item. 77 * - nr_of_lanes 78 - Number of data lanes used on the CSI-2 link. This can 79 be obtained from the OF endpoint configuration. 80 * - 2 81 - Data is transferred on both rising and falling edge of the signal. 82 * - bits_per_sample 83 - Number of bits per sample. 84 * - k 85 - 16 for D-PHY and 7 for C-PHY 86 87.. note:: 88 89 The pixel rate calculated this way is **not** the same thing as the 90 pixel rate on the camera sensor's pixel array which is indicated by the 91 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control. 92 93LP-11 and LP-111 modes 94^^^^^^^^^^^^^^^^^^^^^^ 95 96As part of transitioning to high speed mode, a CSI-2 transmitter typically 97briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period 98may be as short as 100 µs, during which the receiver observes this state and 99proceeds its own part of high speed mode transition. 100 101Most receivers are capable of autonomously handling this once the software has 102configured them to do so, but there are receivers which require software 103involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit 104in software, especially when there is no interrupt telling something is 105happening. 106 107One way to address this is to configure the transmitter side explicitly to LP-11 108or LP-111 mode, which requires support from the transmitter hardware. This is 109not universally available. Many devices return to this state once streaming is 110stopped while the state after power-on is LP-00 or LP-000. 111 112The ``.pre_streamon()`` callback may be used to prepare a transmitter for 113transitioning to streaming state, but not yet start streaming. Similarly, the 114``.post_streamoff()`` callback is used to undo what was done by the 115``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required 116to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``. 117 118In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition 119the transmitter to the LP-11 or LP-111 mode. This also requires powering on the 120device, so this should be only done when it is needed. 121 122Receiver drivers that do not need explicit LP-11 or LP-111 mode setup are waived 123from calling the two callbacks. 124 125Stopping the transmitter 126^^^^^^^^^^^^^^^^^^^^^^^^ 127 128A transmitter stops sending the stream of images as a result of 129calling the ``.s_stream()`` callback. Some transmitters may stop the 130stream at a frame boundary whereas others stop immediately, 131effectively leaving the current frame unfinished. The receiver driver 132should not make assumptions either way, but function properly in both 133cases.