linux-kernel - Re: [PATCH v3 1/2] hwmon: (lm90) Add power control

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Date:	Tue, 10 Sep 2013 11:07:47 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Mark Brown <broonie@...nel.org>, Wei Ni <wni@...dia.com>,
	"khali@...ux-fr.org" <khali@...ux-fr.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] hwmon: (lm90) Add power control

On Tue, Sep 10, 2013 at 11:44:05AM -0600, Stephen Warren wrote:
> On 09/10/2013 11:04 AM, Mark Brown wrote:
> > On Tue, Sep 10, 2013 at 09:07:43AM -0600, Stephen Warren wrote:
> >> On 09/10/2013 04:09 AM, Mark Brown wrote:
> > 
> >>> No.  There are a couple of issues here.  One is that we don't want
> >>> to litter all drivers with conditional code to check if they
> >>> actually got the regulator and so on, that's just pointless make
> >>> work on the part of consumers.
> > 
> >> So that's exactly the difference between (a) and (b) above.
> > 
> > Right, but the idea is that we just only ignore a failure to get a
> > supply if we can usefully run without that supply being present and
> > there's more code there than simply ignoring the error - if the driver
> > can genuinely just ignore all errors and otherwise not do anything
> > different then requesting the regulator in the first place is clearly a
> > waste of time and enabling it would be a waste of power.
> > 
> > A driver should only be carrying code for a missing regulator if it can
> > usefully work without it, like the cases where devices can use an
> > internal reference if one is not available.
> 
> OK, so I believe you're saying that the case of a chip with just a
> single power source, which absolutely must be present in HW for the chip
> to be powered, isn't appropriate for regulator_get_optional(). Something
> must always define a regulator for that power source, even if there is
> no external SW control over that power source.
> 
I think you are supposed to use a dummy regulator in that case.

Guenter

> If so, how does a driver (or binding) that's been written without any
> support for a regulator (since so far all boards have had no SW control
> over that power source; it's always on) get enhanced to support boards
> where there is SW control over the power source?
> 
> We either allow the regulator to be optional (since SW control over the
> regulator is optional), or go back to every board file and DT and add a
> dummy regulator in (which then breaks DT ABI, and even ignoring that is
> a pain).
> 
> And note that when I say "optional" at the start of the previous
> paragraph, I'm talking about probe-time regulator_get() operations and
> DT content. Clearly as far as the rest of the driver is concerned,
> something can always provide a dummy regulator so that e.g.
> regulator_enable/disable() elsewhere  always have something to operate
> on. However, probe() either needs to call an API that automatically
> provides such a dummy regulator, or open-code that itself. I'm still not
> clear which option you think should be used.
> 
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