1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 .. _transmitter-receiver: 3 .. _transmitter-receiver:
4 4
5 Pixel data transmitter and receiver drivers 5 Pixel data transmitter and receiver drivers
6 =========================================== 6 ===========================================
7 7
8 V4L2 supports various devices that transmit an 8 V4L2 supports various devices that transmit and receive pixel data. Examples of
9 these devices include a camera sensor, a TV tu 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. 10 CSI-2 receiver in an SoC.
11 11
12 Bus types 12 Bus types
13 --------- 13 ---------
14 14
15 The following busses are the most common. This 15 The following busses are the most common. This section discusses these two only.
16 16
17 MIPI CSI-2 17 MIPI CSI-2
18 ^^^^^^^^^^ 18 ^^^^^^^^^^
19 19
20 CSI-2 is a data bus intended for transferring 20 CSI-2 is a data bus intended for transferring images from cameras to
21 the host SoC. It is defined by the `MIPI allia 21 the host SoC. It is defined by the `MIPI alliance`_.
22 22
23 .. _`MIPI alliance`: https://www.mipi.org/ 23 .. _`MIPI alliance`: https://www.mipi.org/
24 24
25 Parallel and BT.656 25 Parallel and BT.656
26 ^^^^^^^^^^^^^^^^^^^ 26 ^^^^^^^^^^^^^^^^^^^
27 27
28 The parallel and `BT.656`_ buses transport one 28 The parallel and `BT.656`_ buses transport one bit of data on each clock cycle
29 per data line. The parallel bus uses synchroni 29 per data line. The parallel bus uses synchronisation and other additional
30 signals whereas BT.656 embeds synchronisation. 30 signals whereas BT.656 embeds synchronisation.
31 31
32 .. _`BT.656`: https://en.wikipedia.org/wiki/IT 32 .. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656
33 33
34 Transmitter drivers 34 Transmitter drivers
35 ------------------- 35 -------------------
36 36
37 Transmitter drivers generally need to provide 37 Transmitter drivers generally need to provide the receiver drivers with the
38 configuration of the transmitter. What is requ 38 configuration of the transmitter. What is required depends on the type of the
39 bus. These are common for both busses. 39 bus. These are common for both busses.
40 40
41 Media bus pixel code 41 Media bus pixel code
42 ^^^^^^^^^^^^^^^^^^^^ 42 ^^^^^^^^^^^^^^^^^^^^
43 43
44 See :ref:`v4l2-mbus-pixelcode`. 44 See :ref:`v4l2-mbus-pixelcode`.
45 45
46 Link frequency 46 Link frequency
47 ^^^^^^^^^^^^^^ 47 ^^^^^^^^^^^^^^
48 48
49 The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-fr 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 50 receiver the frequency of the bus (i.e. it is not the same as the symbol rate).
51 51
52 ``.s_stream()`` callback 52 ``.s_stream()`` callback
53 ^^^^^^^^^^^^^^^^^^^^^^^^ 53 ^^^^^^^^^^^^^^^^^^^^^^^^
54 54
55 The struct struct v4l2_subdev_video_ops->s_str 55 The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
56 receiver driver to control the transmitter dri 56 receiver driver to control the transmitter driver's streaming state.
57 57
58 58
59 CSI-2 transmitter drivers 59 CSI-2 transmitter drivers
60 ------------------------- 60 -------------------------
61 61
62 Pixel rate 62 Pixel rate
63 ^^^^^^^^^^ 63 ^^^^^^^^^^
64 64
65 The pixel rate on the bus is calculated as fol 65 The pixel rate on the bus is calculated as follows::
66 66
67 pixel_rate = link_freq * 2 * nr_of_lan 67 pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample
68 68
69 where 69 where
70 70
71 .. list-table:: variables in pixel rate calcul 71 .. list-table:: variables in pixel rate calculation
72 :header-rows: 1 72 :header-rows: 1
73 73
74 * - variable or constant 74 * - variable or constant
75 - description 75 - description
76 * - link_freq 76 * - link_freq
77 - The value of the ``V4L2_CID_LINK_FREQ`` 77 - The value of the ``V4L2_CID_LINK_FREQ`` integer64 menu item.
78 * - nr_of_lanes 78 * - nr_of_lanes
79 - Number of data lanes used on the CSI-2 79 - Number of data lanes used on the CSI-2 link. This can
80 be obtained from the OF endpoint config 80 be obtained from the OF endpoint configuration.
81 * - 2 81 * - 2
82 - Data is transferred on both rising and 82 - Data is transferred on both rising and falling edge of the signal.
83 * - bits_per_sample 83 * - bits_per_sample
84 - Number of bits per sample. 84 - Number of bits per sample.
85 * - k 85 * - k
86 - 16 for D-PHY and 7 for C-PHY 86 - 16 for D-PHY and 7 for C-PHY
87 87
88 .. note:: 88 .. note::
89 89
90 The pixel rate calculated this way is 90 The pixel rate calculated this way is **not** the same thing as the
91 pixel rate on the camera sensor's pixe 91 pixel rate on the camera sensor's pixel array which is indicated by the
92 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pi 92 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control.
93 93
94 LP-11 and LP-111 states 94 LP-11 and LP-111 states
95 ^^^^^^^^^^^^^^^^^^^^^^^ 95 ^^^^^^^^^^^^^^^^^^^^^^^
96 96
97 As part of transitioning to high speed mode, a 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, 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 r 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 trans 100 proceeds its own part of high speed mode transition.
101 101
102 Most receivers are capable of autonomously han 102 Most receivers are capable of autonomously handling this once the software has
103 configured them to do so, but there are receiv 103 configured them to do so, but there are receivers which require software
104 involvement in observing LP-11 or LP-111 state 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 inter 105 in software, especially when there is no interrupt telling something is
106 happening. 106 happening.
107 107
108 One way to address this is to configure the tr 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 t 109 or LP-111 state, which requires support from the transmitter hardware. This is
110 not universally available. Many devices return 110 not universally available. Many devices return to this state once streaming is
111 stopped while the state after power-on is LP-0 111 stopped while the state after power-on is LP-00 or LP-000.
112 112
113 The ``.pre_streamon()`` callback may be used t 113 The ``.pre_streamon()`` callback may be used to prepare a transmitter for
114 transitioning to streaming state, but not yet 114 transitioning to streaming state, but not yet start streaming. Similarly, the
115 ``.post_streamoff()`` callback is used to undo 115 ``.post_streamoff()`` callback is used to undo what was done by the
116 ``.pre_streamon()`` callback. The caller of `` 116 ``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required
117 to call ``.post_streamoff()`` for each success 117 to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``.
118 118
119 In the context of CSI-2, the ``.pre_streamon() 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. 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 121 device, so this should be only done when it is needed.
122 122
123 Receiver drivers that do not need explicit LP- 123 Receiver drivers that do not need explicit LP-11 or LP-111 state setup are
124 waived from calling the two callbacks. 124 waived from calling the two callbacks.
125 125
126 Stopping the transmitter 126 Stopping the transmitter
127 ^^^^^^^^^^^^^^^^^^^^^^^^ 127 ^^^^^^^^^^^^^^^^^^^^^^^^
128 128
129 A transmitter stops sending the stream of imag 129 A transmitter stops sending the stream of images as a result of
130 calling the ``.s_stream()`` callback. Some tra 130 calling the ``.s_stream()`` callback. Some transmitters may stop the
131 stream at a frame boundary whereas others stop 131 stream at a frame boundary whereas others stop immediately,
132 effectively leaving the current frame unfinish 132 effectively leaving the current frame unfinished. The receiver driver
133 should not make assumptions either way, but fu 133 should not make assumptions either way, but function properly in both
134 cases. 134 cases.
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