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- LED handling under Linux
- ========================
- If you're reading this and thinking about keyboard leds, these are
- handled by the input subsystem and the led class is *not* needed.
- In its simplest form, the LED class just allows control of LEDs from
- userspace. LEDs appear in /sys/class/leds/. The brightness file will
- set the brightness of the LED (taking a value 0-255). 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 ide-disk,
- 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.
- 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"
- 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.
- 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.
- Some leds can be programmed to flash in hardware. As this isn't a generic
- LED device property, this should be exported as a device specific sysfs
- attribute rather than part of the class if this functionality is required.
- 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.
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