The documentation being added contains overall description of the LED Flash Class and the related sysfs attributes. There are also chapters devoted specifically to the Flash Manager feature. Signed-off-by: Jacek Anaszewski <j.anaszewski@xxxxxxxxxxx> Acked-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx> Cc: Bryan Wu <cooloney@xxxxxxxxx> Cc: Richard Purdie <rpurdie@xxxxxxxxx> --- Documentation/leds/leds-class-flash.txt | 126 +++++++++++++++++++++++++++++++ 1 file changed, 126 insertions(+) create mode 100644 Documentation/leds/leds-class-flash.txt diff --git a/Documentation/leds/leds-class-flash.txt b/Documentation/leds/leds-class-flash.txt new file mode 100644 index 0000000..0927804 --- /dev/null +++ b/Documentation/leds/leds-class-flash.txt @@ -0,0 +1,126 @@ + +Flash LED handling under Linux +============================== + +Some LED devices support two modes - torch and flash. In order to enable +support for flash LEDs the CONFIG_LEDS_CLASS_FLASH symbol must be defined +in the kernel config. A flash LED driver must register in the LED subsystem +with led_classdev_flash_register to gain flash capabilities. + +Following sysfs attributes are exposed for controlling flash led devices: + + - flash_brightness - flash LED brightness in microamperes (RW) + - max_flash_brightness - maximum available flash LED brightness (RO) + - indicator_brightness - privacy LED brightness in microamperes (RW) + - max_indicator_brightness - maximum privacy LED brightness in + microamperes (RO) + - flash_timeout - flash strobe duration in microseconds (RW) + - max_flash_timeout - maximum available flash strobe duration (RO) + - flash_strobe - flash strobe state (RW) + - external_strobe - some devices expose dedicated hardware pins for + triggering a flash LED - this attribute allows to + set this mode (RW) + - flash_fault - bitmask of flash faults that may have occurred, + possible flags are: + * 0x01 - flash controller voltage to the flash LED has exceeded + the limit specific to the flash controller + * 0x02 - the flash strobe was still on when the timeout set by + the user has expired; not all flash controllers may + set this in all such conditions + * 0x04 - the flash controller has overheated + * 0x08 - the short circuit protection of the flash controller + has been triggered + * 0x10 - current in the LED power supply has exceeded the limit + specific to the flash controller + * 0x40 - flash controller voltage to the flash LED has been + below the minimum limit specific to the flash + * 0x80 - the input voltage of the flash controller is below + the limit under which strobing the flash at full + current will not be possible. The condition persists + until this flag is no longer set + * 0x100 - the temperature of the LED has exceeded its allowed + upper limit + +The LED subsystem driver can be controlled also from the level of VideoForLinux2 +subsystem. In order to enable this the CONFIG_V4L2_FLASH_LED_CLASS symbol has to +be defined in the kernel config. The driver must call v4l2_flash_init function +to get registered in the V4L2 subsystem. On remove v4l2_flash_release function +has to be called (see <media/v4l2-flash.h>). + +After proper initialization V4L2 Flash sub-device is created. The sub-device +exposes a number of V4L2 controls. When the V4L2_CID_FLASH_LED_MODE control +is set to V4L2_FLASH_LED_MODE_TORCH or V4L2_FLASH_LED_MODE_FLASH the +LED subsystem sysfs interface becomes unavailable. The interface can be unlocked +by setting the mode back to V4L2_FLASH_LED_MODE_NONE. + + +Flash Manager +============= + +Flash LED devices often provide two ways of strobing the flash: software +(e.g. by setting a bit in a register) and hardware, by asserting dedicated pin, +which is usually connected to a camera sensor device. There are also flash led +devices which support only hardware strobing - in such cases a multiplexer +device is employed to route the flash strobe signal either from the SoC GPIO or +from the external strobe signal provider, e.g. camera sensor. +Use of multiplexers allows also to change the flash-sensor connection +dynamically if there is more than one flash or external strobe signal provider +available on the board. + +In order to address the aforementioned cases the Flash Manager functionality +has been introduced. Flash Manager is a part of LED Flash Class. It maintains +information about flashes, software and external strobe signal providers and +multiplexers that route strobe signals among them. + +To register a LED Flash Class device in the flash manager the device_node +of a flash device has to be passed as the third argument to the +led_classdev_flash_register function. The device_node is expected to include one +gate-software-strobe subnode and at least one gate-external-strobeN subnode. +Besides that there must defined a flash_muxes node aggregating all the +multiplexers that can be referenced to by the flash led devices. +(for mote details see Documentation/devicetree/bindings/leds/ +leds-flash-manager.txt). + +Flash manager adds following sysfs attributes to the LED Flash Clash +device sysfs interface: + + - strobe_provider - id of the external strobe signal provider associated + with the flash led device. It is created only if + there is more than one external strobe signal + provider defined (RW). + - strobe_providerN - names of the strobe signal providers available + for the flash led device, where N is the + identifier of a strobe signal provider (RO) + - blocking_strobe - informs if enabling either software or external + strobe will block the caller for the expected + time of strobing (1 - true, 0 - false). The call + needs to be blocking if the multiplexers involved + in the strobe signal routing are connected to more + than one flash led device. In such a case flash + manager becomes locked for the expected time of + strobing to prevent reconfigurarion of multiplexers + by the other flash led devices willing to strobe + in the same time (RO). + + +LED Flash Class multiplexers +============================ + +Multiplexers are an indispensable part of the Flash Manager. Flash Manager has +its own led_flash_gpio_mux* helpers for creating, releasing and operating on +the simple gpio driven multiplexers (the ones whose lines are selected by +changing the state of its selector pins) and the user doesn't have to bother +with it. + +It is however possible that a more advanced device will be used for routing +strobe signals. This kind of devices are known to the Flash Manager as +"asynchronous muxes" and can be registered in runtime with use of +led_flash_manager_bind_async_mux API and unregistered with +led_flash_manager_unbind_async_mux. (see Documentation/leds/leds-async-mux.c +for the exemplary implementation of the async mux driver). + +If a LED Flash Class device declares dependency on an async mux, then strobing +the flash, or setting external strobe, will succeed only wnen the async mux +has been bound to the Flash Manager. Async mux module, once bound, can be +removed only after all LED Flash Class devices depending on it are unregistered +from the Flash Manager. -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
