linux-kernel - Re: [PATCH 1/4] regulator: core: Support trying to get close to a certain voltage

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Date:	Fri, 12 Dec 2014 12:59:51 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Alexandru Stan <amstan@...omium.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Seungwon Jeon <tgih.jun@...sung.com>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Sonny Rao <sonnyrao@...omium.org>,
	Andrew Bresticker <abrestic@...omium.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] regulator: core: Support trying to get close to a
 certain voltage

On Thu, Dec 11, 2014 at 07:31:43PM -0800, Doug Anderson wrote:
> On Thu, Dec 11, 2014 at 4:24 PM, Mark Brown <broonie@...nel.org> wrote:

> > I mean the latter - what happens if more than one consumer is trying to
> > use the regulator?  This is IIRC why _set_voltage_tol() uses the cheap
> > and nasty implementation it does.

> Ah, I see.  I don't think I've ever encountered a case where there
> were two consumers for a regulator that actually requested voltages...

> ...but isn't regulator_set_voltage_tol() broken there?  If you have
> two clients, A and B and a regulator that can go 1.0V to 5.0V in .05V
> increments:

> A requests 1.8V +/- .1V.  We get 1.8V
> B requests 1.7V +/- .05V.

> The above could be achievable with a voltage of 1.75V but it won't
> work with the current regulator_set_voltage_tol() I think.

Yeah, it's not perfect but it'll work most of the time.

> > There's also the potential
> > performance considerations for the DVS type applications now I think
> > about it.

> Iterating through voltages is really that slow?  If so, perhaps we
> could add some caching to keep track of what voltage we actually got
> last time...  I could also add an optimization to try the exact
> requested voltage right away...

Applications like DVS get pretty performance sensitive and for a
regulator with high resolution if you're trying to hit a voltage at the
top of the range you could be iterating over a hundred or more values.
Perhaps doing something based on the various factorings out of the
voltage mapping would do the trick, add a new op for getting to the
closest voltage?

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