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