linux-kernel - Re: RGB LED control (was Re: "advanced" LED controllers)

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Date:	Mon, 09 Mar 2015 11:50:56 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Pavel Machek <pavel@....cz>, Alexandre Courbot <gnurou@...il.com>,
	Felipe Balbi <balbi@...com>, Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	"linux-leds\@vger.kernel.org" <linux-leds@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	"linux-gpio\@vger.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: RGB LED control (was Re: "advanced" LED controllers)

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> Hi Pavel,
>
> On Sun, Mar 8, 2015 at 9:57 PM, Pavel Machek <pavel@....cz> wrote:
>> Ok, so I played with RGB LED a bit, and we have quite a gap in
>> documentation: what 50% brightness means is non-trivial and very
>> important in case we want to do smooth blinking and color transitions.
>>
>> Signed-off-by: Pavel Machek <pavel@....cz>
>>
>> diff --git a/Documentation/ABI/testing/sysfs-class-led b/Documentation/ABI/testing/sysfs-class-led
>> index 3646ec8..649d7a6 100644
>> --- a/Documentation/ABI/testing/sysfs-class-led
>> +++ b/Documentation/ABI/testing/sysfs-class-led
>> @@ -8,6 +8,11 @@ Description:
>>                 non-zero brightness settings. The value is between 0 and
>>                 /sys/class/leds/<led>/max_brightness.
>>
>> +               If LED supports continuous brightness settings, 50% brightness
>> +               should correspond to 50% brightness perceived by human, in a similar
>> +               manner pixel brightness on monitor does (not 50% PWM).
>
> How many drivers do it right? How many don't?
>
> For those that don't, perhaps we handle the conversion between perceived and
> pwm in the core, e.g. by adding a new flag to led_classdev.flags to indicate
> the need for conversion?

Some LED controllers do the right thing in hardware, so any adjustment
done in the core needs to be optional.

-- 
Måns Rullgård
mans@...sr.com
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