linux-kernel - Re: [PATCH 1/2] leds: Add driver for Qualcomm LPG

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Message-ID: <20170407125440.GD4756@amd>
Date:   Fri, 7 Apr 2017 14:54:40 +0200
From:   Pavel Machek <pavel@....cz>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Richard Purdie <rpurdie@...ys.net>,
        linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 1/2] leds: Add driver for Qualcomm LPG

Hi!

> > Second, there are more issues than just patterns with the RGB
> > LED. Most important is ability to set particular colors. You want to
> > set the RGB LED to "white", but that does not mean you can set
> > red=green=blue=1.0. You want color to look the same on LCD and on the
> > LED, which means coefficients for white and some kind of function for
> > brightness-to-PWM conversion.
> 
> Shouldn't we leave that entirely to the userspace? Can we come up
> with coefficients that will guarantee the same result on all existing
> LCD devices?

I don't think we should. We want (red = 70%, green = 80%, blue = 20%)
to look approximately the same on all the hardware. That's currently
not the case; even (red = green = blue = 100%) is not white.

I believe easiest solution is "the kernel does the work", as it does
for LCD screens.

[Now... as long as userspace has enough information to display white
and specific colors, I don't care much -- having kernel present
coefficients for userspace would work, too.]

								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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