camera-sensor.rst (6547B)
1.. SPDX-License-Identifier: GPL-2.0 2 3Writing camera sensor drivers 4============================= 5 6CSI-2 and parallel (BT.601 and BT.656) busses 7--------------------------------------------- 8 9Please see :ref:`transmitter-receiver`. 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 30can rely on this frequency being used. 31 32Devicetree 33~~~~~~~~~~ 34 35The currently preferred way to achieve this is using ``assigned-clocks``, 36``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See 37``Documentation/devicetree/bindings/clock/clock-bindings.txt`` for more 38information. The driver then gets the frequency using ``clk_get_rate()``. 39 40This approach has the drawback that there's no guarantee that the frequency 41hasn't been modified directly or indirectly by another driver, or supported by 42the board's clock tree to begin with. Changes to the Common Clock Framework API 43are required to ensure reliability. 44 45Frame size 46---------- 47 48There are two distinct ways to configure the frame size produced by camera 49sensors. 50 51Freely configurable camera sensor drivers 52~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 53 54Freely configurable camera sensor drivers expose the device's internal 55processing pipeline as one or more sub-devices with different cropping and 56scaling configurations. The output size of the device is the result of a series 57of cropping and scaling operations from the device's pixel array's size. 58 59An example of such a driver is the CCS driver (see ``drivers/media/i2c/ccs``). 60 61Register list based drivers 62~~~~~~~~~~~~~~~~~~~~~~~~~~~ 63 64Register list based drivers generally, instead of able to configure the device 65they control based on user requests, are limited to a number of preset 66configurations that combine a number of different parameters that on hardware 67level are independent. How a driver picks such configuration is based on the 68format set on a source pad at the end of the device's internal pipeline. 69 70Most sensor drivers are implemented this way, see e.g. 71``drivers/media/i2c/imx319.c`` for an example. 72 73Frame interval configuration 74---------------------------- 75 76There are two different methods for obtaining possibilities for different frame 77intervals as well as configuring the frame interval. Which one to implement 78depends on the type of the device. 79 80Raw camera sensors 81~~~~~~~~~~~~~~~~~~ 82 83Instead of a high level parameter such as frame interval, the frame interval is 84a result of the configuration of a number of camera sensor implementation 85specific parameters. Luckily, these parameters tend to be the same for more or 86less all modern raw camera sensors. 87 88The frame interval is calculated using the following equation:: 89 90 frame interval = (analogue crop width + horizontal blanking) * 91 (analogue crop height + vertical blanking) / pixel rate 92 93The formula is bus independent and is applicable for raw timing parameters on 94large variety of devices beyond camera sensors. Devices that have no analogue 95crop, use the full source image size, i.e. pixel array size. 96 97Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and 98``V4L2_CID_VBLANK``, respectively. The unit of the ``V4L2_CID_HBLANK`` control 99is pixels and the unit of the ``V4L2_CID_VBLANK`` is lines. The pixel rate in 100the sensor's **pixel array** is specified by ``V4L2_CID_PIXEL_RATE`` in the same 101sub-device. The unit of that control is pixels per second. 102 103Register list based drivers need to implement read-only sub-device nodes for the 104purpose. Devices that are not register list based need these to configure the 105device's internal processing pipeline. 106 107The first entity in the linear pipeline is the pixel array. The pixel array may 108be followed by other entities that are there to allow configuring binning, 109skipping, scaling or digital crop :ref:`v4l2-subdev-selections`. 110 111USB cameras etc. devices 112~~~~~~~~~~~~~~~~~~~~~~~~ 113 114USB video class hardware, as well as many cameras offering a similar higher 115level interface natively, generally use the concept of frame interval (or frame 116rate) on device level in firmware or hardware. This means lower level controls 117implemented by raw cameras may not be used on uAPI (or even kAPI) to control the 118frame interval on these devices. 119 120Power management 121---------------- 122 123Always use runtime PM to manage the power states of your device. Camera sensor 124drivers are in no way special in this respect: they are responsible for 125controlling the power state of the device they otherwise control as well. In 126general, the device must be powered on at least when its registers are being 127accessed and when it is streaming. 128 129Existing camera sensor drivers may rely on the old 130struct v4l2_subdev_core_ops->s_power() callback for bridge or ISP drivers to 131manage their power state. This is however **deprecated**. If you feel you need 132to begin calling an s_power from an ISP or a bridge driver, instead please add 133runtime PM support to the sensor driver you are using. Likewise, new drivers 134should not use s_power. 135 136Please see examples in e.g. ``drivers/media/i2c/ov8856.c`` and 137``drivers/media/i2c/ccs/ccs-core.c``. The two drivers work in both ACPI 138and DT based systems. 139 140Control framework 141~~~~~~~~~~~~~~~~~ 142 143``v4l2_ctrl_handler_setup()`` function may not be used in the device's runtime 144PM ``runtime_resume`` callback, as it has no way to figure out the power state 145of the device. This is because the power state of the device is only changed 146after the power state transition has taken place. The ``s_ctrl`` callback can be 147used to obtain device's power state after the power state transition: 148 149.. c:function:: int pm_runtime_get_if_in_use(struct device *dev); 150 151The function returns a non-zero value if it succeeded getting the power count or 152runtime PM was disabled, in either of which cases the driver may proceed to 153access the device.