1.. SPDX-License-Identifier: GPL-2.0
2
3Writing camera sensor drivers
4=============================
5
6CSI-2
7-----
8
9Please see what is written on :ref:`MIPI_CSI_2`.
10
11Handling clocks
12---------------
13
14Camera sensors have an internal clock tree including a PLL and a number of
15divisors. The clock tree is generally configured by the driver based on a few
16input parameters that are specific to the hardware:: the external clock frequency
17and the link frequency. The two parameters generally are obtained from system
18firmware. **No other frequencies should be used in any circumstances.**
19
20The reason why the clock frequencies are so important is that the clock signals
21come out of the SoC, and in many cases a specific frequency is designed to be
22used in the system. Using another frequency may cause harmful effects
23elsewhere. Therefore only the pre-determined frequencies are configurable by the
24user.
25
26ACPI
27~~~~
28
29Read the "clock-frequency" _DSD property to denote the frequency. The driver can
30rely on this frequency being used.
31
32Devicetree
33~~~~~~~~~~
34
35The currently preferred way to achieve this is using "assigned-clock-rates"
36property. See Documentation/devicetree/bindings/clock/clock-bindings.txt for
37more information. The driver then gets the frequency using clk_get_rate().
38
39This approach has the drawback that there's no guarantee that the frequency
40hasn't been modified directly or indirectly by another driver, or supported by
41the board's clock tree to begin with. Changes to the Common Clock Framework API
42are required to ensure reliability.
43
44Frame size
45----------
46
47There are two distinct ways to configure the frame size produced by camera
48sensors.
49
50Freely configurable camera sensor drivers
51~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
52
53Freely configurable camera sensor drivers expose the device's internal
54processing pipeline as one or more sub-devices with different cropping and
55scaling configurations. The output size of the device is the result of a series
56of cropping and scaling operations from the device's pixel array's size.
57
58An example of such a driver is the smiapp driver (see drivers/media/i2c/smiapp).
59
60Register list based drivers
61~~~~~~~~~~~~~~~~~~~~~~~~~~~
62
63Register list based drivers generally, instead of able to configure the device
64they control based on user requests, are limited to a number of preset
65configurations that combine a number of different parameters that on hardware
66level are independent. How a driver picks such configuration is based on the
67format set on a source pad at the end of the device's internal pipeline.
68
69Most sensor drivers are implemented this way, see e.g.
70drivers/media/i2c/imx319.c for an example.
71
72Frame interval configuration
73----------------------------
74
75There are two different methods for obtaining possibilities for different frame
76intervals as well as configuring the frame interval. Which one to implement
77depends on the type of the device.
78
79Raw camera sensors
80~~~~~~~~~~~~~~~~~~
81
82Instead of a high level parameter such as frame interval, the frame interval is
83a result of the configuration of a number of camera sensor implementation
84specific parameters. Luckily, these parameters tend to be the same for more or
85less all modern raw camera sensors.
86
87The frame interval is calculated using the following equation::
88
89	frame interval = (analogue crop width + horizontal blanking) *
90			 (analogue crop height + vertical blanking) / pixel rate
91
92The formula is bus independent and is applicable for raw timing parameters on
93large variety of devices beyond camera sensors. Devices that have no analogue
94crop, use the full source image size, i.e. pixel array size.
95
96Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and
97``V4L2_CID_VBLANK``, respectively. The unit of these controls are lines. The
98pixel rate is specified by ``V4L2_CID_PIXEL_RATE`` in the same sub-device. The
99unit of that control is Hz.
100
101Register list based drivers need to implement read-only sub-device nodes for the
102purpose. Devices that are not register list based need these to configure the
103device's internal processing pipeline.
104
105The first entity in the linear pipeline is the pixel array. The pixel array may
106be followed by other entities that are there to allow configuring binning,
107skipping, scaling or digital crop :ref:`v4l2-subdev-selections`.
108
109USB cameras etc. devices
110~~~~~~~~~~~~~~~~~~~~~~~~
111
112USB video class hardware, as well as many cameras offering a similar higher
113level interface natively, generally use the concept of frame interval (or frame
114rate) on device level in firmware or hardware. This means lower level controls
115implemented by raw cameras may not be used on uAPI (or even kAPI) to control the
116frame interval on these devices.
117
118Power management
119----------------
120
121Always use runtime PM to manage the power states of your device. Camera sensor
122drivers are in no way special in this respect: they are responsible for
123controlling the power state of the device they otherwise control as well. In
124general, the device must be powered on at least when its registers are being
125accessed and when it is streaming.
126
127Existing camera sensor drivers may rely on the old
128:c:type:`v4l2_subdev_core_ops`->s_power() callback for bridge or ISP drivers to
129manage their power state. This is however **deprecated**. If you feel you need
130to begin calling an s_power from an ISP or a bridge driver, instead please add
131runtime PM support to the sensor driver you are using. Likewise, new drivers
132should not use s_power.
133
134Please see examples in e.g. ``drivers/media/i2c/ov8856.c`` and
135``drivers/media/i2c/smiapp/smiapp-core.c``. The two drivers work in both ACPI
136and DT based systems.
137
138Control framework
139~~~~~~~~~~~~~~~~~
140
141``v4l2_ctrl_handler_setup()`` function may not be used in the device's runtime
142PM ``runtime_resume`` callback, as it has no way to figure out the power state
143of the device. This is because the power state of the device is only changed
144after the power state transition has taken place. The ``s_ctrl`` callback can be
145used to obtain device's power state after the power state transition:
146
147.. c:function:: int pm_runtime_get_if_in_use(struct device *dev);
148
149The function returns a non-zero value if it succeeded getting the power count or
150runtime PM was disabled, in either of which cases the driver may proceed to
151access the device.
152
153Controls
154--------
155
156For camera sensors that are connected to a bus where transmitter and receiver
157require common configuration set by drivers, such as CSI-2 or parallel (BT.601
158or BT.656) bus, the ``V4L2_CID_LINK_FREQ`` control is mandatory on transmitter
159drivers. Receiver drivers can use the ``V4L2_CID_LINK_FREQ`` to query the
160frequency used on the bus.
161
162The transmitter drivers should also implement ``V4L2_CID_PIXEL_RATE`` control in
163order to tell the maximum pixel rate to the receiver. This is required on raw
164camera sensors.
165