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 an
9 these devices include a camera sensor, a TV tu
10 CSI-2 receiver in an SoC.
11
12 Bus types
13 ---------
14
15 The following busses are the most common. This
16
17 MIPI CSI-2
18 ^^^^^^^^^^
19
20 CSI-2 is a data bus intended for transferring
21 the host SoC. It is defined by the `MIPI allia
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
29 per data line. The parallel bus uses synchroni
30 signals whereas BT.656 embeds synchronisation.
31
32 .. _`BT.656`: https://en.wikipedia.org/wiki/IT
33
34 Transmitter drivers
35 -------------------
36
37 Transmitter drivers generally need to provide
38 configuration of the transmitter. What is requ
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-fr
50 receiver the frequency of the bus (i.e. it is
51
52 ``.s_stream()`` callback
53 ^^^^^^^^^^^^^^^^^^^^^^^^
54
55 The struct struct v4l2_subdev_video_ops->s_str
56 receiver driver to control the transmitter dri
57
58
59 CSI-2 transmitter drivers
60 -------------------------
61
62 Pixel rate
63 ^^^^^^^^^^
64
65 The pixel rate on the bus is calculated as fol
66
67 pixel_rate = link_freq * 2 * nr_of_lan
68
69 where
70
71 .. list-table:: variables in pixel rate calcul
72 :header-rows: 1
73
74 * - variable or constant
75 - description
76 * - link_freq
77 - The value of the ``V4L2_CID_LINK_FREQ``
78 * - nr_of_lanes
79 - Number of data lanes used on the CSI-2
80 be obtained from the OF endpoint config
81 * - 2
82 - Data is transferred on both rising and
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
91 pixel rate on the camera sensor's pixe
92 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pi
93
94 LP-11 and LP-111 states
95 ^^^^^^^^^^^^^^^^^^^^^^^
96
97 As part of transitioning to high speed mode, a
98 briefly sets the bus to LP-11 or LP-111 state,
99 may be as short as 100 µs, during which the r
100 proceeds its own part of high speed mode trans
101
102 Most receivers are capable of autonomously han
103 configured them to do so, but there are receiv
104 involvement in observing LP-11 or LP-111 state
105 in software, especially when there is no inter
106 happening.
107
108 One way to address this is to configure the tr
109 or LP-111 state, which requires support from t
110 not universally available. Many devices return
111 stopped while the state after power-on is LP-0
112
113 The ``.pre_streamon()`` callback may be used t
114 transitioning to streaming state, but not yet
115 ``.post_streamoff()`` callback is used to undo
116 ``.pre_streamon()`` callback. The caller of ``
117 to call ``.post_streamoff()`` for each success
118
119 In the context of CSI-2, the ``.pre_streamon()
120 the transmitter to the LP-11 or LP-111 state.
121 device, so this should be only done when it is
122
123 Receiver drivers that do not need explicit LP-
124 waived from calling the two callbacks.
125
126 Stopping the transmitter
127 ^^^^^^^^^^^^^^^^^^^^^^^^
128
129 A transmitter stops sending the stream of imag
130 calling the ``.s_stream()`` callback. Some tra
131 stream at a frame boundary whereas others stop
132 effectively leaving the current frame unfinish
133 should not make assumptions either way, but fu
134 cases.
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