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 buses 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 buses. 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 52Drivers that do not have user-configurable link frequency should report it 53through the ``.get_mbus_config()`` subdev pad operation, in the ``link_freq`` 54field of struct v4l2_mbus_config, instead of through controls. 55 56Receiver drivers should use :c:func:`v4l2_get_link_freq` helper to obtain the 57link frequency from the transmitter sub-device. 58 59``.enable_streams()`` and ``.disable_streams()`` callbacks 60^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 61 62The struct v4l2_subdev_pad_ops->enable_streams() and struct 63v4l2_subdev_pad_ops->disable_streams() callbacks are used by the receiver driver 64to control the transmitter driver's streaming state. These callbacks may not be 65called directly, but by using ``v4l2_subdev_enable_streams()`` and 66``v4l2_subdev_disable_streams()``. 67 68Stopping the transmitter 69^^^^^^^^^^^^^^^^^^^^^^^^ 70 71A transmitter stops sending the stream of images as a result of 72calling the ``.disable_streams()`` callback. Some transmitters may stop the 73stream at a frame boundary whereas others stop immediately, 74effectively leaving the current frame unfinished. The receiver driver 75should not make assumptions either way, but function properly in both 76cases. 77 78CSI-2 transmitter drivers 79------------------------- 80 81Pixel rate 82^^^^^^^^^^ 83 84The pixel rate on the bus is calculated as follows:: 85 86 pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample 87 88where 89 90.. list-table:: variables in pixel rate calculation 91 :header-rows: 1 92 93 * - variable or constant 94 - description 95 * - link_freq 96 - The value of the :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` integer64 97 menu item. 98 * - nr_of_lanes 99 - Number of data lanes used on the CSI-2 link. 100 * - 2 101 - Data is transferred on both rising and falling edge of the signal. 102 * - bits_per_sample 103 - Number of bits per sample. 104 * - k 105 - 16 for D-PHY and 7 for C-PHY. 106 107Information on whether D-PHY or C-PHY is used, and the value of ``nr_of_lanes``, can be obtained from the OF endpoint configuration. 108 109.. note:: 110 111 The pixel rate calculated this way is **not** the same thing as the 112 pixel rate on the camera sensor's pixel array which is indicated by the 113 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control. 114 115LP-11 and LP-111 states 116^^^^^^^^^^^^^^^^^^^^^^^ 117 118As part of transitioning to high speed mode, a CSI-2 transmitter typically 119briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period 120may be as short as 100 µs, during which the receiver observes this state and 121proceeds its own part of high speed mode transition. 122 123Most receivers are capable of autonomously handling this once the software has 124configured them to do so, but there are receivers which require software 125involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit 126in software, especially when there is no interrupt telling something is 127happening. 128 129One way to address this is to configure the transmitter side explicitly to LP-11 130or LP-111 state, which requires support from the transmitter hardware. This is 131not universally available. Many devices return to this state once streaming is 132stopped while the state after power-on is LP-00 or LP-000. 133 134The ``.pre_streamon()`` callback may be used to prepare a transmitter for 135transitioning to streaming state, but not yet start streaming. Similarly, the 136``.post_streamoff()`` callback is used to undo what was done by the 137``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required 138to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``. 139 140In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition 141the transmitter to the LP-11 or LP-111 state. This also requires powering on the 142device, so this should be only done when it is needed. 143 144Receiver drivers that do not need explicit LP-11 or LP-111 state setup are 145waived from calling the two callbacks. 146