========================

  LED handling under Linux
  ========================

  In its simplest form, the LED class just allows control of LEDs from

  userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
  LED is defined in max_brightness file. The brightness file will set the brightness
  of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
  brightness support so will just be turned on for non-zero brightness settings.

  
  The class also introduces the optional concept of an LED trigger. A trigger
  is a kernel based source of led events. Triggers can either be simple or
  complex. A simple trigger isn't configurable and is designed to slot into

  existing subsystems with minimal additional code. Examples are the disk-activity,

  nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
  optimises away.

  Complex triggers while available to all LEDs have LED specific

  parameters and work on a per LED basis. The timer trigger is an example.

  The timer trigger will periodically change the LED brightness between
  LED_OFF and the current brightness setting. The "on" and "off" time can
  be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
  You can change the brightness value of a LED independently of the timer
  trigger. However, if you set the brightness value to LED_OFF it will
  also disable the timer trigger.

  
  You can change triggers in a similar manner to the way an IO scheduler
  is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
  parameters can appear in /sys/class/leds/<device> once a given trigger is
  selected.
  
  
  Design Philosophy
  =================
  
  The underlying design philosophy is simplicity. LEDs are simple devices
  and the aim is to keep a small amount of code giving as much functionality
  as possible.  Please keep this in mind when suggesting enhancements.
  
  
  LED Device Naming
  =================
  
  Is currently of the form:

  	"devicename:color:function"

  - devicename:
          it should refer to a unique identifier created by the kernel,
          like e.g. phyN for network devices or inputN for input devices, rather
          than to the hardware; the information related to the product and the bus
          to which given device is hooked is available in sysfs and can be
          retrieved using get_led_device_info.sh script from tools/leds; generally
          this section is expected mostly for LEDs that are somehow associated with
          other devices.
  
  - color:
          one of LED_COLOR_ID_* definitions from the header
          include/dt-bindings/leds/common.h.
  
  - function:
          one of LED_FUNCTION_* definitions from the header
          include/dt-bindings/leds/common.h.
  
  If required color or function is missing, please submit a patch
  to linux-leds@vger.kernel.org.
  
  It is possible that more than one LED with the same color and function will
  be required for given platform, differing only with an ordinal number.
  In this case it is preferable to just concatenate the predefined LED_FUNCTION_*
  name with required "-N" suffix in the driver. fwnode based drivers can use
  function-enumerator property for that and then the concatenation will be handled
  automatically by the LED core upon LED class device registration.
  
  LED subsystem has also a protection against name clash, that may occur
  when LED class device is created by a driver of hot-pluggable device and
  it doesn't provide unique devicename section. In this case numerical
  suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class
  device name.
  
  There might be still LED class drivers around using vendor or product name
  for devicename, but this approach is now deprecated as it doesn't convey
  any added value. Product information can be found in other places in sysfs
  (see tools/leds/get_led_device_info.sh).
  
  Examples of proper LED names:
  
    - "red:disk"
    - "white:flash"
    - "red:indicator"
    - "phy1:green:wlan"
    - "phy3::wlan"
    - ":kbd_backlight"
    - "input5::kbd_backlight"
    - "input3::numlock"
    - "input3::scrolllock"
    - "input3::capslock"
    - "mmc1::status"
    - "white:status"
  
  get_led_device_info.sh script can be used for verifying if the LED name
  meets the requirements pointed out here. It performs validation of the LED class
  devicename sections and gives hints on expected value for a section in case
  the validation fails for it. So far the script supports validation
  of associations between LEDs and following types of devices:
  
          - input devices
          - ieee80211 compliant USB devices
  
  The script is open to extensions.
  
  There have been calls for LED properties such as color to be exported as

  individual led class attributes. As a solution which doesn't incur as much
  overhead, I suggest these become part of the device name. The naming scheme

  above leaves scope for further attributes should they be needed. If sections
  of the name don't apply, just leave that section blank.

  Brightness setting API
  ======================
  
  LED subsystem core exposes following API for setting brightness:

      - led_set_brightness:
  		it is guaranteed not to sleep, passing LED_OFF stops

  		blinking,

  
      - led_set_brightness_sync:
  		for use cases when immediate effect is desired -

  		it can block the caller for the time required for accessing
  		device registers and can sleep, passing LED_OFF stops hardware
  		blinking, returns -EBUSY if software blink fallback is enabled.

  LED registration API
  ====================
  
  A driver wanting to register a LED classdev for use by other drivers /
  userspace needs to allocate and fill a led_classdev struct and then call

  `[devm_]led_classdev_register`. If the non devm version is used the driver

  must call led_classdev_unregister from its remove function before
  free-ing the led_classdev struct.
  
  If the driver can detect hardware initiated brightness changes and thus
  wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
  flag must be set in flags before registering. Calling
  led_classdev_notify_brightness_hw_changed on a classdev not registered with
  the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.

  Hardware accelerated blink of LEDs
  ==================================
  
  Some LEDs can be programmed to blink without any CPU interaction. To
  support this feature, a LED driver can optionally implement the

  blink_set() function (see <linux/leds.h>). To set an LED to blinking,

  however, it is better to use the API function led_blink_set(), as it
  will check and implement software fallback if necessary.

  To turn off blinking, use the API function led_brightness_set()
  with brightness value LED_OFF, which should stop any software

  timers that may have been required for blinking.
  
  The blink_set() function should choose a user friendly blinking value

  if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this

  case the driver should give back the chosen value through delay_on and
  delay_off parameters to the leds subsystem.

  Setting the brightness to zero with brightness_set() callback function
  should completely turn off the LED and cancel the previously programmed
  hardware blinking function, if any.

  Known Issues
  ============
  
  The LED Trigger core cannot be a module as the simple trigger functions
  would cause nightmare dependency issues. I see this as a minor issue
  compared to the benefits the simple trigger functionality brings. The
  rest of the LED subsystem can be modular.

  
  Future Development
  ==================
  
  At the moment, a trigger can't be created specifically for a single LED.
  There are a number of cases where a trigger might only be mappable to a
  particular LED (ACPI?). The addition of triggers provided by the LED driver
  should cover this option and be possible to add without breaking the
  current interface.