linux-kernel - Re: [PATCH v4 2/4] mmc: core: Add mmc_regulator_set

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Message-ID: <20150320112828.GF2869@sirena.org.uk>
Date:	Fri, 20 Mar 2015 11:28:28 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Doug Anderson <dianders@...omium.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Seungwon Jeon <tgih.jun@...sung.com>,
	Alexandru Stan <amstan@...omium.org>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Sonny Rao <sonnyrao@...omium.org>,
	Andrew Bresticker <abrestic@...omium.org>,
	Addy Ke <addy.ke@...k-chips.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Chris Ball <chris@...ntf.net>,
	Johan Rudholm <johan.rudholm@...s.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Tim Kryger <tim.kryger@...il.com>,
	Andrew Gabbasov <andrew_gabbasov@...tor.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/4] mmc: core: Add mmc_regulator_set_vqmmc()

On Fri, Mar 20, 2015 at 11:55:50AM +0100, Ulf Hansson wrote:
> On 19 March 2015 at 12:36, Mark Brown <broonie@...nel.org> wrote:

> > The implementation *should* do that anyway, it's just not trivial to
> > implement in an efficient fashion with the current information we have
> > from drivers.

> The APIs regulator_count_voltages() and regulator_list_voltage(), are
> currently used from the mmc core to find out which voltages that is
> supported (with 0.1V granularity). Then that information can be used
> when trying to set a new voltage.

> But I guess such a wrapper API is out of the question?

> Anyway, I get the feeling that we will need to do the same for this case.

As previously discussed the problem is that there can be a *lot* of
voltages on a modern regulator with fine grained voltage steps and
tolerances are also used for things like cpufreq where we care about
performance.  We need something that doesn't require a linear scan of
possible values.

> >> would be good to allow both upper and lower limits to be zero.

> > The lower limit can be zero already though it isn't clear to me that
> > this is useful.  Setting an upper limit of zero seems nonsensical, an
> > upper limit that is lower than the lower limit isn't terribly obvious
> > and removing the upper limit isn't safe - it means that we'll happily
> > oversupply things which is a road to physical damage.

> I am not sure I follow here. In the regulator_set_voltage_tol() you
> can only specifiy one limit (tolerance?). What Dough proposed was to
> add a new API which can have both a low tolerance value and a high
> tolerance value.

Tolerances are not limits - when you say "limit" that sounds like a hard
value.  I can't see any reason why the code would prevent anyone setting
a tolerance of zero, though it should be rare that this is actually the
best thing to do.

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