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