new file mode 100644
@@ -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.