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Date:   Wed, 24 Apr 2019 21:32:51 +0300
From:   Yauhen Kharuzhy <>
To:     Hans de Goede <>
        Jacek Anaszewski <>
Subject: Re: [PATCH v2 0/2] Intel Cherry Trail Whiskey Cove LEDs support

On Sun, Apr 21, 2019 at 09:28:42PM +0200, Hans de Goede wrote:
> Hi Yauhen,

> I had a discussion with Jacek Anaszewski about this in another thread and
> I believe we have come up with a solution for this which should work
> nicely and should allow us to move forward with your driver (after
> it is reworked to match the solution. So the solution we've come up with is:
> 1) After thinking a bit more about the primary use-case for this, I've come
> to the conclusion that putting LED1 / the charging LED in software-controlled
> mode is also the correct thing to do on the GPD win / pocket. The reason for
> this is that ideally the LED would glow while charging and be simply solid
> on when the battery is full, the hw control does not allow this, so the GPD
> win/pocket can benefit from sw-control too.
> 2) To allow the desired behavior we need to define a new
> "<supply-name>-charging-glow-full-solid" trigger in
> drivers/power/supply/power_supply_leds.c; and this must be the default
> trigger for the Intel Cherry Trail Whiskey Cove LED driver so that
> everything will just work. Also we must restore the original hw control
> setting on reboot/shut-down so that this is used on the GPD win/pocket when
> Linux is not running.
> 3) To be able to actually implement this new trigger we first need 2 things
> in the kernel internal LED APIs:
> 3a) An API for triggers to put the LED in glowing mode, we've come up
> with the following prototype for this:
> void led_trigger_glow(struct led_trigger *trigger, unsigned long *cylce_time);
> Where cycle_time is the number of milliseconds for a full glow cycle (from off
> to full-on to off again).  So if cylce_time is set to 1000 then the LED glows
> at 1 Hz, 500, 2 HZ, etc. Note as with led_trigger_blink() the time passed to
> led_trigger_glow is passed by reference as the LED driver may round it to
> match what the hardware can do and the rounded value is returned to the caller
> through the reference.
> 3b) 3a) in turn will require adding a new optional glow_set callback to
> struct led_classdev which will then get called by led_trigger_glow if available.
> We've not discussed yet what to do if led_trigger_glow gets called on
> a led_classdev which does not implement the new glow_set callback, I guess
> the most sensible thing to do then is to fallback to blinking with delay_on
> and delay_off set to cylce_time / 2.
> If you can make some time to work on this solution that would be great. Please
> let me know if you've any questions about the solution outlined below.
> Note that glowing is only exported as in kernel functionality, I see no
> use-case for exporting this to userspace and keeping this in kernel allows
> us to keep things nice and simple.

Unfortunately I am too busy now at fulltime job, so I don't know when I
will return to this. But I still plan to do  :)

Yauhen Kharuzhy

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