leds: Add support for hardware accelerated LED flashing

Extends the leds subsystem with a blink_set() callback function which can
be optionally implemented by a LED driver. If implemented, the driver can use
the hardware acceleration for blinking a LED.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

1 files changed, 20 insertions, 4 deletions

diff --git a/Documentation/leds-class.txt b/Documentation/leds-class.txt
index 877af6e..56757c7 100644
--- a/Documentation/leds-class.txt
+++ b/Documentation/leds-class.txt
@@ -48,6 +48,26 @@ 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>). If implemeted, triggers can
+attempt to use it before falling back to software timers. The blink_set()
+function should return 0 if the blink setting is supported, or -EINVAL
+otherwise, which means that LED blinking will be handled by software.
+
+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.
+
+Any call to the brightness_set() callback function should cancel the
+previously programmed hardware blinking function so setting the brightness
+to 0 can also cancel the blinking of the LED.
+
+
 Known Issues
 ============
 
@@ -56,10 +76,6 @@ 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
 ==================