linux-kernel - Re: [patch 2.6.29-rc3-git 1/2] regulator: twl4030 regulators

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Message-Id: <200902081604.29905.david-b@pacbell.net>
Date:	Sun, 8 Feb 2009 16:04:29 -0800
From:	David Brownell <david-b@...bell.net>
To:	Mark Brown <broonie@...ena.org.uk>
Cc:	Liam Girdwood <lrg@...mlogic.co.uk>,
	lkml <linux-kernel@...r.kernel.org>,
	OMAP <linux-omap@...r.kernel.org>
Subject: Re: [patch 2.6.29-rc3-git 1/2] regulator: twl4030 regulators

On Sunday 08 February 2009, Mark Brown wrote:
> > +     /* Constrain board-specific capabilities according to what
> > +      * this driver and the chip itself can actually do.
> > +      */
> > +     c = &initdata->constraints;
> > +     if (!c->min_uV || c->min_uV < min_uV)
> > +             c->min_uV = min_uV;
> 
> If we're going to do this I think it'd be better to push it into the
> core and let the regulators pass in a set of constraints so that the
> behaviour will be consistent between drivers.

Maybe, but there is no such mechanism right yet.
When it's created, then this could switch over.

> I'd also expect to have the registration fail or at least issue a
> warning if the code kicks in since that indicates that the board
> constraints have been set up incorrectly.

A warning might make sense in some cases ... that's
something I would expect to see from the regulator
core, though.  Example, I see no "max < min" checks
triggering registration errors.

> There's a reasonable chance 
> that the fixed up constraints will still need to be changed for the
> board to be configured properly and things may end up being driven out
> of spec, potentially causing damage.

I don't see that happening ... all that code does is
tighten existing constraints to match what the hardware
can do.  The result is just a subset of the range the
board already said was OK.  If no valid subset exists,
that's a board design bug ... albeit one the regulator
core could detect.  (E.g. those "max < min" checks that
don't currently exist.)

I can easily imagine having things partially set up, and
not really caring whether, on a particular board, those
initial constraints are really the most specific ones
applicable.  One component tolerates a range of 1V8..3V6
maybe, but on any given board it can be hooked up to any
supply that's even partially in-range.

If I hook such a component up to a supply supporting 1V2
through 2V5, it's really no worry that the 1V2..1V8 part
of that range won't be used; or if 2V5..3V6 could also
work.  Those options really don't matter at all.  The
only significant part is that only the 1V8..2V5 will be
software-accessible on that board.

- Dave

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