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 LED handling under Linux) 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 LED_DEV_CAP_FLASH flag.

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
      perform 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.