linux-kernel - Re: [GIT PULL] LED updates

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Date:	Sun, 10 Feb 2008 12:52:30 +0100
From:	Németh Márton <nm127@...email.hu>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
CC:	Richard Purdie <rpurdie@...ys.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] LED updates

Henrique de Moraes Holschuh wrote:
> On Fri, 08 Feb 2008, Németh Márton wrote:
>> Richard Purdie wrote:
>>>>>       leds: Add support for hardware accelerated LED flashing
>>>> This one has a loose end: when you call brightness_set on a led with
>>>> hardware flash acceleration, you will leave the trigger armed, BUT the led
>>>> won't blink anymore.  That's just wrong.
>>> Agreed.
>> My only question is that do you know any LED hardware which can blink _and_
>> can set the brightness independently? If there would be such a LED I could
> 
> Several, but none on laptops or other stuff that runs Linux.  That behaviour
> is not common on indicator LEDs.  I have seen standby LEDs on laptops which
> "blink" by slowly fading from full to off, and then back to full, though.
> 
>> imagine that the brightness can be changed while the LED remains blinking at
>> some low frequency. For example a simple LED with brightness set possibility and
>> blinking directed by software is an example where the blinking and the brightness
>> setting are completely independent.
> 
> Sure, it is perfectly possible.  I am not sure it is *desireable*, though.
> The way we have triggers work make what you describe impossible right now,
> the software triggers are LED_OFF:LED_FULL, not LED_OFF:old-brightness.
> 
> And so are the common hardware triggers on laptops, for that matter.
> 
> If we go and fix every trigger to use the current brightness (as long as it
> is non-zero) as the "turn LED on" trigger event, then the documentation has
> to be changed accordingly to do what you said above, and we would stop the
> trigger only by setting brightness to zero or by explicitly removing it.
> 
> I don't think it is worth the hassle, though.  But we better decide that
> *now*, because all this changing of the LED class ABI (even if it is, IMHO,
> a big improvement) is not a good idea.  We better do it all during the
> 2.6.25 cycle.

I investigated what would have to be changed if we decide that the
brightness and the blinking parameters can be set independently. There
are not much too change I think, please have a look at my next mail.

	Márton Németh
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