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Flash LED handling under Linux
Some LED devices provide two modes - torch and flash. In the LED subsystem
those modes are supported by LED class (see Documentation/leds/leds-class.rst)
and LED Flash class respectively. The torch mode related features are enabled
by default and the flash ones only if a driver declares it by setting
In order to enable the support for flash LEDs CONFIG_LEDS_CLASS_FLASH symbol
must be defined in the kernel config. A LED Flash class driver must be
registered in the LED subsystem with led_classdev_flash_register function.
Following sysfs attributes are exposed for controlling flash LED devices:
(see Documentation/ABI/testing/sysfs-class-led-flash)
- flash_brightness
- max_flash_brightness
- flash_timeout
- max_flash_timeout
- flash_strobe
- flash_fault
V4L2 flash wrapper for flash LEDs
A LED subsystem driver can be controlled also from the level of VideoForLinux2
subsystem. In order to enable this CONFIG_V4L2_FLASH_LED_CLASS symbol has to
be defined in the kernel config.
The driver must call the v4l2_flash_init function to get registered in the
V4L2 subsystem. The function takes six arguments:
- dev:
flash device, e.g. an I2C device
- of_node:
of_node of the LED, may be NULL if the same as device's
- fled_cdev:
LED flash class device to wrap
- iled_cdev:
LED flash class device representing indicator LED associated with
fled_cdev, may be NULL
- ops:
V4L2 specific ops
* external_strobe_set
defines the source of the flash LED strobe -
V4L2_CID_FLASH_STROBE control or external source, typically
a sensor, which makes it possible to synchronise the flash
strobe start with exposure start,
* intensity_to_led_brightness and led_brightness_to_intensity
enum led_brightness <-> V4L2 intensity conversion in a device
specific manner - they can be used for devices with non-linear
LED current scale.
- config:
configuration for V4L2 Flash sub-device
* dev_name
the name of the media entity, unique in the system,
* flash_faults
bitmask of flash faults that the LED flash class
device can report; corresponding LED_FAULT* bit definitions are
available in <linux/led-class-flash.h>,
* torch_intensity
constraints for the LED in TORCH mode
in microamperes,
* indicator_intensity
constraints for the indicator LED
in microamperes,
* has_external_strobe
determines whether the flash strobe source
can be switched to external,
On remove the v4l2_flash_release function has to be called, which takes one
argument - struct v4l2_flash pointer returned previously by v4l2_flash_init.
This function can be safely called with NULL or error pointer argument.
Please refer to drivers/leds/leds-max77693.c for an exemplary usage of the
v4l2 flash wrapper.
Once the V4L2 sub-device is registered by the driver which created the Media
controller device, the sub-device node acts just as a node of a native V4L2
flash API device would. The calls are simply routed to the LED flash API.
Opening the V4L2 flash sub-device makes the LED subsystem sysfs interface
unavailable. The interface is re-enabled after the V4L2 flash sub-device
is closed.