180 строки
7.2 KiB
ReStructuredText
180 строки
7.2 KiB
ReStructuredText
========================
|
|
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 while 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:color:function"
|
|
|
|
- devicename:
|
|
it should refer to a unique identifier created by the kernel,
|
|
like e.g. phyN for network devices or inputN for input devices, rather
|
|
than to the hardware; the information related to the product and the bus
|
|
to which given device is hooked is available in sysfs and can be
|
|
retrieved using get_led_device_info.sh script from tools/leds; generally
|
|
this section is expected mostly for LEDs that are somehow associated with
|
|
other devices.
|
|
|
|
- color:
|
|
one of LED_COLOR_ID_* definitions from the header
|
|
include/dt-bindings/leds/common.h.
|
|
|
|
- function:
|
|
one of LED_FUNCTION_* definitions from the header
|
|
include/dt-bindings/leds/common.h.
|
|
|
|
If required color or function is missing, please submit a patch
|
|
to linux-leds@vger.kernel.org.
|
|
|
|
It is possible that more than one LED with the same color and function will
|
|
be required for given platform, differing only with an ordinal number.
|
|
In this case it is preferable to just concatenate the predefined LED_FUNCTION_*
|
|
name with required "-N" suffix in the driver. fwnode based drivers can use
|
|
function-enumerator property for that and then the concatenation will be handled
|
|
automatically by the LED core upon LED class device registration.
|
|
|
|
LED subsystem has also a protection against name clash, that may occur
|
|
when LED class device is created by a driver of hot-pluggable device and
|
|
it doesn't provide unique devicename section. In this case numerical
|
|
suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class
|
|
device name.
|
|
|
|
There might be still LED class drivers around using vendor or product name
|
|
for devicename, but this approach is now deprecated as it doesn't convey
|
|
any added value. Product information can be found in other places in sysfs
|
|
(see tools/leds/get_led_device_info.sh).
|
|
|
|
Examples of proper LED names:
|
|
|
|
- "red:disk"
|
|
- "white:flash"
|
|
- "red:indicator"
|
|
- "phy1:green:wlan"
|
|
- "phy3::wlan"
|
|
- ":kbd_backlight"
|
|
- "input5::kbd_backlight"
|
|
- "input3::numlock"
|
|
- "input3::scrolllock"
|
|
- "input3::capslock"
|
|
- "mmc1::status"
|
|
- "white:status"
|
|
|
|
get_led_device_info.sh script can be used for verifying if the LED name
|
|
meets the requirements pointed out here. It performs validation of the LED class
|
|
devicename sections and gives hints on expected value for a section in case
|
|
the validation fails for it. So far the script supports validation
|
|
of associations between LEDs and following types of devices:
|
|
|
|
- input devices
|
|
- ieee80211 compliant USB devices
|
|
|
|
The script is open to extensions.
|
|
|
|
There have been calls for LED properties such as color 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.
|