diff options
Diffstat (limited to '')
| -rw-r--r-- | Documentation/leds/leds-class.txt | 122 |
1 files changed, 122 insertions, 0 deletions
diff --git a/Documentation/leds/leds-class.txt b/Documentation/leds/leds-class.txt new file mode 100644 index 000000000..836cb16d6 --- /dev/null +++ b/Documentation/leds/leds-class.txt @@ -0,0 +1,122 @@ + +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 whilst 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:colour:function" + +There have been calls for LED properties such as colour 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. |