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72 lines
2.7 KiB
Text
72 lines
2.7 KiB
Text
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LED handling under Linux
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========================
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If you're reading this and thinking about keyboard leds, these are
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handled by the input subsystem and the led class is *not* needed.
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In its simplest form, the LED class just allows control of LEDs from
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userspace. LEDs appear in /sys/class/leds/. The brightness file will
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set the brightness of the LED (taking a value 0-255). Most LEDs don't
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have hardware brightness support so will just be turned on for non-zero
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brightness settings.
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The class also introduces the optional concept of an LED trigger. A trigger
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is a kernel based source of led events. Triggers can either be simple or
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complex. A simple trigger isn't configurable and is designed to slot into
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existing subsystems with minimal additional code. Examples are the ide-disk,
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nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
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optimises away.
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Complex triggers whilst available to all LEDs have LED specific
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parameters and work on a per LED basis. The timer trigger is an example.
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You can change triggers in a similar manner to the way an IO scheduler
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is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
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parameters can appear in /sys/class/leds/<device> once a given trigger is
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selected.
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Design Philosophy
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=================
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The underlying design philosophy is simplicity. LEDs are simple devices
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and the aim is to keep a small amount of code giving as much functionality
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as possible. Please keep this in mind when suggesting enhancements.
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LED Device Naming
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=================
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Is currently of the form:
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"devicename:colour"
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There have been calls for LED properties such as colour to be exported as
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individual led class attributes. As a solution which doesn't incur as much
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overhead, I suggest these become part of the device name. The naming scheme
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above leaves scope for further attributes should they be needed.
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Known Issues
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============
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The LED Trigger core cannot be a module as the simple trigger functions
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would cause nightmare dependency issues. I see this as a minor issue
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compared to the benefits the simple trigger functionality brings. The
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rest of the LED subsystem can be modular.
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Some leds can be programmed to flash in hardware. As this isn't a generic
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LED device property, this should be exported as a device specific sysfs
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attribute rather than part of the class if this functionality is required.
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Future Development
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==================
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At the moment, a trigger can't be created specifically for a single LED.
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There are a number of cases where a trigger might only be mappable to a
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particular LED (ACPI?). The addition of triggers provided by the LED driver
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should cover this option and be possible to add without breaking the
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current interface.
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