Documentation/driver-api/media/camera-sensor.rst - linux - Git at Google

 .. SPDX-License-Identifier: GPL-2.0

 .. _media_writing_camera_sensor_drivers:

 Writing camera sensor drivers
 =============================

 This document covers the in-kernel APIs only. For the best practices on
 userspace API implementation in camera sensor drivers, please see
 :ref:`media_using_camera_sensor_drivers`.

 CSI-2, parallel and BT.656 buses
 --------------------------------

 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 preferred way to achieve this is using ``assigned-clocks``,
 ``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See the
 `clock device tree bindings
 <https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_
 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.

 Power management
 ----------------

 Camera sensors are used in conjunction with other devices to form a camera
 pipeline. They must obey the rules listed herein to ensure coherent power
 management over the pipeline.

 Camera sensor drivers are responsible for controlling the power state of the
 device they otherwise control as well. They shall use runtime PM to manage
 power states. Runtime PM shall be enabled at probe time and disabled at remove
 time. Drivers should enable runtime PM autosuspend. Also see
 :ref:`async sub-device registration <media-registering-async-subdevs>`.

 The runtime PM handlers shall handle clocks, regulators, GPIOs, and other
 system resources required to power the sensor up and down. For drivers that
 don't use any of those resources (such as drivers that support ACPI systems
 only), the runtime PM handlers may be left unimplemented.

 In general, the device shall be powered on at least when its registers are
 being accessed and when it is streaming. Drivers should use
 ``pm_runtime_resume_and_get()`` when starting streaming and
 ``pm_runtime_put()`` or ``pm_runtime_put_autosuspend()`` when stopping
 streaming. They may power the device up at probe time (for example to read
 identification registers), but should not keep it powered unconditionally after
 probe.

 At system suspend time, the whole camera pipeline must stop streaming, and
 restart when the system is resumed. This requires coordination between the
 camera sensor and the rest of the camera pipeline. Bridge drivers are
 responsible for this coordination, and instruct camera sensors to stop and
 restart streaming by calling the appropriate subdev operations
 (``.s_stream()``, ``.enable_streams()`` or ``.disable_streams()``). Camera
 sensor drivers shall therefore **not** keep track of the streaming state to
 stop streaming in the PM suspend handler and restart it in the resume handler.
 Drivers should in general not implement the system PM handlers.

 Camera sensor drivers shall **not** implement the subdev ``.s_power()``
 operation, as it is deprecated. While this operation is implemented in some
 existing drivers as they predate the deprecation, new drivers shall use runtime
 PM instead. If you feel you need to begin calling ``.s_power()`` from an ISP or
 a bridge driver, instead add runtime PM support to the sensor driver you are
 using and drop its ``.s_power()`` handler.

 Please also see :ref:`examples <media-camera-sensor-examples>`.

 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
 ----------------------------------

 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.

 .. _media-camera-sensor-examples:

 Example drivers
 ---------------

 Features implemented by sensor drivers vary, and depending on the set of
 supported features and other qualities, particular sensor drivers better serve
 the purpose of an example. The following drivers are known to be good examples:

 .. flat-table:: Example sensor drivers
     :header-rows: 0
     :widths:      1 1 1 2

     * - Driver name
       - File(s)
       - Driver type
       - Example topic
     * - CCS
       - ``drivers/media/i2c/ccs/``
       - Freely configurable
       - Power management (ACPI and DT), UAPI
     * - imx219
       - ``drivers/media/i2c/imx219.c``
       - Register list based
       - Power management (DT), UAPI, mode selection
     * - imx319
       - ``drivers/media/i2c/imx319.c``
       - Register list based
       - Power management (ACPI and DT)
	.. SPDX-License-Identifier: GPL-2.0

	.. _media_writing_camera_sensor_drivers:

	Writing camera sensor drivers
	=============================

	This document covers the in-kernel APIs only. For the best practices on
	userspace API implementation in camera sensor drivers, please see
	:ref:`media_using_camera_sensor_drivers`.

	CSI-2, parallel and BT.656 buses
	--------------------------------

	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 preferred way to achieve this is using ``assigned-clocks``,
	``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See the
	`clock device tree bindings
	<https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_
	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.

	Power management
	----------------

	Camera sensors are used in conjunction with other devices to form a camera
	pipeline. They must obey the rules listed herein to ensure coherent power
	management over the pipeline.

	Camera sensor drivers are responsible for controlling the power state of the
	device they otherwise control as well. They shall use runtime PM to manage
	power states. Runtime PM shall be enabled at probe time and disabled at remove
	time. Drivers should enable runtime PM autosuspend. Also see
	:ref:`async sub-device registration <media-registering-async-subdevs>`.

	The runtime PM handlers shall handle clocks, regulators, GPIOs, and other
	system resources required to power the sensor up and down. For drivers that
	don't use any of those resources (such as drivers that support ACPI systems
	only), the runtime PM handlers may be left unimplemented.

	In general, the device shall be powered on at least when its registers are
	being accessed and when it is streaming. Drivers should use
	``pm_runtime_resume_and_get()`` when starting streaming and
	``pm_runtime_put()`` or ``pm_runtime_put_autosuspend()`` when stopping
	streaming. They may power the device up at probe time (for example to read
	identification registers), but should not keep it powered unconditionally after
	probe.

	At system suspend time, the whole camera pipeline must stop streaming, and
	restart when the system is resumed. This requires coordination between the
	camera sensor and the rest of the camera pipeline. Bridge drivers are
	responsible for this coordination, and instruct camera sensors to stop and
	restart streaming by calling the appropriate subdev operations
	(``.s_stream()``, ``.enable_streams()`` or ``.disable_streams()``). Camera
	sensor drivers shall therefore not keep track of the streaming state to
	stop streaming in the PM suspend handler and restart it in the resume handler.
	Drivers should in general not implement the system PM handlers.

	Camera sensor drivers shall not implement the subdev ``.s_power()``
	operation, as it is deprecated. While this operation is implemented in some
	existing drivers as they predate the deprecation, new drivers shall use runtime
	PM instead. If you feel you need to begin calling ``.s_power()`` from an ISP or
	a bridge driver, instead add runtime PM support to the sensor driver you are
	using and drop its ``.s_power()`` handler.

	Please also see :ref:`examples <media-camera-sensor-examples>`.

	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
	----------------------------------

	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.

	.. _media-camera-sensor-examples:

	Example drivers
	---------------

	Features implemented by sensor drivers vary, and depending on the set of
	supported features and other qualities, particular sensor drivers better serve
	the purpose of an example. The following drivers are known to be good examples:

	.. flat-table:: Example sensor drivers
	:header-rows: 0
	:widths: 1 1 1 2

	* - Driver name
	- File(s)
	- Driver type
	- Example topic
	* - CCS
	- ``drivers/media/i2c/ccs/``
	- Freely configurable
	- Power management (ACPI and DT), UAPI
	* - imx219
	- ``drivers/media/i2c/imx219.c``
	- Register list based
	- Power management (DT), UAPI, mode selection
	* - imx319
	- ``drivers/media/i2c/imx319.c``
	- Register list based
	- Power management (ACPI and DT)