98 lines
4.0 KiB
Plaintext
98 lines
4.0 KiB
Plaintext
|
|
||
|
|
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 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 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.
|
||
|
|
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.
|
||
|
|
|
||
|
|
|
||
|
|
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 again, use the API function led_brightness_set()
|
||
|
|
as that will not just set the LED brightness but also 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.
|