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 .. SPDX-License-Identifier: GPL-2.0
 
 Writing camera sensor drivers
 =============================
 
 CSI-2 and parallel (BT.601 and BT.656) busses
 ---------------------------------------------
 
 Please see :ref:`transmitter-receiver`.
 
 Handling clocks
 ---------------
 
 Camera sensors have an internal clock tree including a PLL and a number of
 divisors. The clock tree is generally configured by the driver based on a few
 input parameters that are specific to the hardware:: the external clock frequency
 and the link frequency. The two parameters generally are obtained from system
 firmware. **No other frequencies should be used in any circumstances.**
 
 The reason why the clock frequencies are so important is that the clock signals
 come out of the SoC, and in many cases a specific frequency is designed to be
 used in the system. Using another frequency may cause harmful effects
 elsewhere. Therefore only the pre-determined frequencies are configurable by the
 user.
 
 ACPI
 ~~~~
 
 Read the ``clock-frequency`` _DSD property to denote the frequency. The driver
 can rely on this frequency being used.
 
 Devicetree
 ~~~~~~~~~~
 
 The currently preferred way to achieve this is using ``assigned-clocks``,
 ``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See
 ``Documentation/devicetree/bindings/clock/clock-bindings.txt`` for more
 information. The driver then gets the frequency using ``clk_get_rate()``.
 
 This approach has the drawback that there's no guarantee that the frequency
 hasn't been modified directly or indirectly by another driver, or supported by
 the board's clock tree to begin with. Changes to the Common Clock Framework API
 are required to ensure reliability.
 
 Frame size
 ----------
 
 There are two distinct ways to configure the frame size produced by camera
 sensors.
 
 Freely configurable camera sensor drivers
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Freely configurable camera sensor drivers expose the device's internal
 processing pipeline as one or more sub-devices with different cropping and
 scaling configurations. The output size of the device is the result of a series
 of cropping and scaling operations from the device's pixel array's size.
 
 An example of such a driver is the CCS driver (see ``drivers/media/i2c/ccs``).
 
 Register list based drivers
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Register list based drivers generally, instead of able to configure the device
 they control based on user requests, are limited to a number of preset
 configurations that combine a number of different parameters that on hardware
 level are independent. How a driver picks such configuration is based on the
 format set on a source pad at the end of the device's internal pipeline.
 
 Most sensor drivers are implemented this way, see e.g.
 ``drivers/media/i2c/imx319.c`` for an example.
 
 Frame interval configuration
 ----------------------------
 
 There are two different methods for obtaining possibilities for different frame
 intervals as well as configuring the frame interval. Which one to implement
 depends on the type of the device.
 
 Raw camera sensors
 ~~~~~~~~~~~~~~~~~~
 
 Instead of a high level parameter such as frame interval, the frame interval is
 a result of the configuration of a number of camera sensor implementation
 specific parameters. Luckily, these parameters tend to be the same for more or
 less all modern raw camera sensors.
 
 The frame interval is calculated using the following equation::
 
         frame interval = (analogue crop width + horizontal blanking) *
                          (analogue crop height + vertical blanking) / pixel rate
 
 The formula is bus independent and is applicable for raw timing parameters on
 large variety of devices beyond camera sensors. Devices that have no analogue
 crop, use the full source image size, i.e. pixel array size.
 
 Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and
 ``V4L2_CID_VBLANK``, respectively. The unit of the ``V4L2_CID_HBLANK`` control
 is pixels and the unit of the ``V4L2_CID_VBLANK`` is lines. The pixel rate in
 the sensor's **pixel array** is specified by ``V4L2_CID_PIXEL_RATE`` in the same
 sub-device. The unit of that control is pixels per second.
 
 Register list based drivers need to implement read-only sub-device nodes for the
 purpose. Devices that are not register list based need these to configure the
 device's internal processing pipeline.
 
 The first entity in the linear pipeline is the pixel array. The pixel array may
 be followed by other entities that are there to allow configuring binning,
 skipping, scaling or digital crop :ref:`v4l2-subdev-selections`.
 
 USB cameras etc. devices
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 USB video class hardware, as well as many cameras offering a similar higher
 level interface natively, generally use the concept of frame interval (or frame
 rate) on device level in firmware or hardware. This means lower level controls
 implemented by raw cameras may not be used on uAPI (or even kAPI) to control the
 frame interval on these devices.
 
 Power management
 ----------------
 
 Always use runtime PM to manage the power states of your device. Camera sensor
 drivers are in no way special in this respect: they are responsible for
 controlling the power state of the device they otherwise control as well. In
 general, the device must be powered on at least when its registers are being
 accessed and when it is streaming.
 
 Existing camera sensor drivers may rely on the old
 struct v4l2_subdev_core_ops->s_power() callback for bridge or ISP drivers to
 manage their power state. This is however **deprecated**. If you feel you need
 to begin calling an s_power from an ISP or a bridge driver, instead please add
 runtime PM support to the sensor driver you are using. Likewise, new drivers
 should not use s_power.
 
 Please see examples in e.g. ``drivers/media/i2c/ov8856.c`` and
 ``drivers/media/i2c/ccs/ccs-core.c``. The two drivers work in both ACPI
 and DT based systems.
 
 Control framework
 ~~~~~~~~~~~~~~~~~
 
 ``v4l2_ctrl_handler_setup()`` function may not be used in the device's runtime
 PM ``runtime_resume`` callback, as it has no way to figure out the power state
 of the device. This is because the power state of the device is only changed
 after the power state transition has taken place. The ``s_ctrl`` callback can be
 used to obtain device's power state after the power state transition:
 
 .. c:function:: int pm_runtime_get_if_in_use(struct device *dev);
 
 The function returns a non-zero value if it succeeded getting the power count or
 runtime PM was disabled, in either of which cases the driver may proceed to
 access the device.

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