linux-kernel - Re: [PATCH] leds: driver for ab5500 high voltage leds

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Date:	Wed, 7 Dec 2011 16:00:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Denis Kuzmenko <linux@...onet.org.ua>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Richard Purdie <rpurdie@...ys.net>,
	linux-kernel@...r.kernel.org, Samuel Ortiz <sameo@...ux.intel.com>,
	Shreshtha Kumar Sahu <shreshthakumar.sahu@...ricsson.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Subject: Re: [PATCH] leds: driver for ab5500 high voltage leds

On Wed, 07 Dec 2011 20:01:46 +0200
Denis Kuzmenko <linux@...onet.org.ua> wrote:

> > The three channels are indeed high-voltage LEDs, they supply
> > up to 20V from a supply voltage of some standard mobile handset
> > battery at say 3.8 V or so. But there is no register to control the
> > voltage or anything like that.
> > 
> > My naive understanding is that you set the current limit and then
> > the HV transformer (I guess this is a buck converter of some kind)
> > will raise the voltage level until it either (A) cannot raise it any more
> > at c:a 20V or (b) the current limit is reached.
> > 
> > I suspect this is because for LEDs of this type you get a
> > specified current but the voltage just has to be "high enough"
> > to break through some diode barrier threshold or so. After
> > that intensity is controlled by limiting the current.
> > 
> > Does this suffice as explanation...?
> > 
> > Yours,
> > Linus Walleij
> 
> Yeah, It does, thank you.
> Does it worth to make additional comment in code to avoid further questions?

Yes, I think that the fact that a reviewer asked a question such as
this is a reliable sign that subsequent readers will be wondering the
same thing, so it is an "oops it needs a comment" signal.

I won't do anything with this patch yet - I'm waiting to see if Mark
stops being surprised ;)

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