8. 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 Using camera sensor drivers.
8.1. CSI-2, parallel and BT.656 buses¶
Please see Pixel data transmitter and receiver drivers.
8.2. 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.
8.2.1. ACPI¶
Read the clock-frequency _DSD property to denote the frequency. The driver
can rely on this frequency being used.
8.2.2. 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
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.
8.3. 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 async sub-device registration.
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 examples.
8.3.1. 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:
-
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.
8.4. 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.
8.5. 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:
Driver name |
File(s) |
Driver type |
Example topic |
CCS |
|
Freely configurable |
Power management (ACPI and DT), UAPI |
imx219 |
|
Register list based |
Power management (DT), UAPI, mode selection |
imx319 |
|
Register list based |
Power management (ACPI and DT) |