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Diffstat (limited to 'Documentation/driver-api/media/camera-sensor.rst')
| -rw-r--r-- | Documentation/driver-api/media/camera-sensor.rst | 205 |
1 files changed, 105 insertions, 100 deletions
diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst index c7d4891bd24e..94bd1dae82d5 100644 --- a/Documentation/driver-api/media/camera-sensor.rst +++ b/Documentation/driver-api/media/camera-sensor.rst @@ -1,10 +1,16 @@ .. SPDX-License-Identifier: GPL-2.0 +.. _media_writing_camera_sensor_drivers: + Writing camera sensor drivers ============================= -CSI-2 and parallel (BT.601 and BT.656) busses ---------------------------------------------- +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`. @@ -13,7 +19,7 @@ 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 +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.** @@ -23,119 +29,81 @@ used in the system. Using another frequency may cause harmful effects elsewhere. Therefore only the pre-determined frequencies are configurable by the user. +The external clock frequency shall be retrieved by obtaining the external clock +using the ``devm_v4l2_sensor_clk_get()`` helper function, and then getting its +frequency with ``clk_get_rate()``. Usage of the helper function guarantees +correct behaviour regardless of whether the sensor is integrated in a DT-based +or ACPI-based system. + ACPI ~~~~ -Read the ``clock-frequency`` _DSD property to denote the frequency. The driver -can rely on this frequency being used. +ACPI-based systems typically don't register the sensor external clock with the +kernel, but specify the external clock frequency in the ``clock-frequency`` +_DSD property. The ``devm_v4l2_sensor_clk_get()`` helper creates and returns a +fixed clock set at that rate. 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()``. +Devicetree-based systems declare the sensor external clock in the device tree +and reference it from the sensor node. The preferred way to select the external +clock frequency is to use the ``assigned-clocks``, ``assigned-clock-parents`` +and ``assigned-clock-rates`` properties in the sensor node to set the clock +rate. See the `clock device tree bindings +<https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_ +for more information. The ``devm_v4l2_sensor_clk_get()`` helper retrieves and +returns that clock. 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. +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 +(``.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 ~~~~~~~~~~~~~~~~~ @@ -151,3 +119,40 @@ used to obtain device's power state after the power state transition: 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) |