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``.s_stream()`` callback
53^^^^^^^^^^^^^^^^^^^^^^^^
54
55The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
56receiver driver to control the transmitter driver's streaming state.
57
58
59CSI-2 transmitter drivers
60-------------------------
61
62Pixel rate
63^^^^^^^^^^
64
65The 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
69where
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
94LP-11 and LP-111 states
95^^^^^^^^^^^^^^^^^^^^^^^
96
97As part of transitioning to high speed mode, a CSI-2 transmitter typically
98briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period
99may be as short as 100 µs, during which the receiver observes this state and
100proceeds its own part of high speed mode transition.
101
102Most receivers are capable of autonomously handling this once the software has
103configured them to do so, but there are receivers which require software
104involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit
105in software, especially when there is no interrupt telling something is
106happening.
107
108One way to address this is to configure the transmitter side explicitly to LP-11
109or LP-111 state, which requires support from the transmitter hardware. This is
110not universally available. Many devices return to this state once streaming is
111stopped while the state after power-on is LP-00 or LP-000.
112
113The ``.pre_streamon()`` callback may be used to prepare a transmitter for
114transitioning 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
117to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``.
118
119In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition
120the transmitter to the LP-11 or LP-111 state. This also requires powering on the
121device, so this should be only done when it is needed.
122
123Receiver drivers that do not need explicit LP-11 or LP-111 state setup are
124waived from calling the two callbacks.
125
126Stopping the transmitter
127^^^^^^^^^^^^^^^^^^^^^^^^
128
129A transmitter stops sending the stream of images as a result of
130calling the ``.s_stream()`` callback. Some transmitters may stop the
131stream at a frame boundary whereas others stop immediately,
132effectively leaving the current frame unfinished. The receiver driver
133should not make assumptions either way, but function properly in both
134cases.
135