Bug when using both "set" and "blink" functions

[Craig McQueen <craig.mcqueen@innerrange.com.au>]

I'd like to control a LED to possibly be any of:

* Off
* On
* Blinking

I've had this working fine in 3.14.x kernel.

But when upgrading to 4.4.x, I found that the transition from "blinking" to "on" didn't work. Namely, if it's blinking and then I call led_set_brightness(led_cdev, LED_FULL), then it wouldn't work (I can't remember if it turned off, or remained blinking; it wasn't on anyway). I worked around it by calling led_set_brightness(led_cdev, LED_OFF) just before led_set_brightness(led_cdev, LED_FULL).

Now I have upgraded to 4.9.x, and found that the transition from "blinking" to "on" again isn't working. The LED ends up being off instead of on.

Examining the code of led_set_brightness():

* Behaviour is different depending on whether the brightness is LED_OFF (it schedules the blink to be turned off "soon") or other (it alters the brightness of subsequent blinks).
* It looks as though there are race conditions in the transition from blinking to steady off -- it schedules the blink to be turned off "soon", so it's difficult to guarantee a reliable transition from blinking to on/off.

The combination of the above two points makes it seem difficult to robustly go from blinking to off/on.

So, my questions are:

* What is the correct way to use the API to reliably control an LED for a combination of off/on/blinking?
* Is my above analysis of the code correct, so that there are indeed race conditions going from blinking to off, leading to undefined behaviour? Can that be fixed?

-- 
Craig McQueen