ia64/linux-2.6.18-xen.hg

view Documentation/leds-class.txt @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 LED handling under Linux
2 ========================
4 If you're reading this and thinking about keyboard leds, these are
5 handled by the input subsystem and the led class is *not* needed.
7 In its simplest form, the LED class just allows control of LEDs from
8 userspace. LEDs appear in /sys/class/leds/. The brightness file will
9 set the brightness of the LED (taking a value 0-255). Most LEDs don't
10 have hardware brightness support so will just be turned on for non-zero
11 brightness settings.
13 The class also introduces the optional concept of an LED trigger. A trigger
14 is a kernel based source of led events. Triggers can either be simple or
15 complex. A simple trigger isn't configurable and is designed to slot into
16 existing subsystems with minimal additional code. Examples are the ide-disk,
17 nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
18 optimises away.
20 Complex triggers whilst available to all LEDs have LED specific
21 parameters and work on a per LED basis. The timer trigger is an example.
23 You can change triggers in a similar manner to the way an IO scheduler
24 is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
25 parameters can appear in /sys/class/leds/<device> once a given trigger is
26 selected.
29 Design Philosophy
30 =================
32 The underlying design philosophy is simplicity. LEDs are simple devices
33 and the aim is to keep a small amount of code giving as much functionality
34 as possible. Please keep this in mind when suggesting enhancements.
37 LED Device Naming
38 =================
40 Is currently of the form:
42 "devicename:colour"
44 There have been calls for LED properties such as colour to be exported as
45 individual led class attributes. As a solution which doesn't incur as much
46 overhead, I suggest these become part of the device name. The naming scheme
47 above leaves scope for further attributes should they be needed.
50 Known Issues
51 ============
53 The LED Trigger core cannot be a module as the simple trigger functions
54 would cause nightmare dependency issues. I see this as a minor issue
55 compared to the benefits the simple trigger functionality brings. The
56 rest of the LED subsystem can be modular.
58 Some leds can be programmed to flash in hardware. As this isn't a generic
59 LED device property, this should be exported as a device specific sysfs
60 attribute rather than part of the class if this functionality is required.
63 Future Development
64 ==================
66 At the moment, a trigger can't be created specifically for a single LED.
67 There are a number of cases where a trigger might only be mappable to a
68 particular LED (ACPI?). The addition of triggers provided by the LED driver
69 should cover this option and be possible to add without breaking the
70 current interface.