xref: /openbmc/linux/Documentation/driver-api/media/camera-sensor.rst (revision 2612e3bbc0386368a850140a6c9b990cd496a5ec)

.. 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.

Rotation, orientation and flipping
----------------------------------

Some systems have the camera sensor mounted upside down compared to its natural
mounting rotation. In such cases, drivers shall expose the information to
userspace with the :ref:`V4L2_CID_CAMERA_SENSOR_ROTATION
<v4l2-camera-sensor-rotation>` control.

Sensor drivers shall also report the sensor's mounting orientation with the
:ref:`V4L2_CID_CAMERA_SENSOR_ORIENTATION <v4l2-camera-sensor-orientation>`.

Use ``v4l2_fwnode_device_parse()`` to obtain rotation and orientation
information from system firmware and ``v4l2_ctrl_new_fwnode_properties()`` to
register the appropriate controls.

Sensor drivers that have any vertical or horizontal flips embedded in the
register programming sequences shall initialize the V4L2_CID_HFLIP and
V4L2_CID_VFLIP controls with the values programmed by the register sequences.
The default values of these controls shall be 0 (disabled). Especially these
controls shall not be inverted, independently of the sensor's mounting
rotation.