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.. SPDX-License-Identifier: GPL-2.0
Writing camera sensor drivers
=============================
CSI-2
-----
Please see what is written on :ref:`MIPI_CSI_2`.
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-clock-rates"
property. 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 smiapp driver (see drivers/media/i2c/smiapp).
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 these controls are lines. The
pixel rate is specified by ``V4L2_CID_PIXEL_RATE`` in the same sub-device. The
unit of that control is Hz.
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
:c:type:`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/smiapp/smiapp-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.
Controls
--------
For camera sensors that are connected to a bus where transmitter and receiver
require common configuration set by drivers, such as CSI-2 or parallel (BT.601
or BT.656) bus, the ``V4L2_CID_LINK_FREQ`` control is mandatory on transmitter
drivers. Receiver drivers can use the ``V4L2_CID_LINK_FREQ`` to query the
frequency used on the bus.
The transmitter drivers should also implement ``V4L2_CID_PIXEL_RATE`` control in
order to tell the maximum pixel rate to the receiver. This is required on raw
camera sensors.
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