linux-kernel - Re: [PATCH] regulator: core: Fix enable GPIO reference counting

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Date:	Tue, 3 Mar 2015 14:23:11 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Doug Anderson <dianders@...omium.org>
Cc:	milo.kim@...com, Axel Lin <axel.lin@...ics.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Olof Johansson <olof@...om.net>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Paul Stewart <pstew@...omium.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] regulator: core: Fix enable GPIO reference counting

On Mon, Mar 02, 2015 at 01:13:56PM -0800, Doug Anderson wrote:
> On Mon, Mar 2, 2015 at 10:47 AM, Mark Brown <broonie@...nel.org> wrote:

> > Looking at the code it seems that you're adding checks to skip calls in
> > the standard enable and disable paths but not touching other paths,
> > based on this patch by itself I can't tell if this is a good idea or
> > not.  It certainly doesn't feel robust - if we're missing reference
> > counting skipping operations seems likely to lead to other bugs popping
> > up elsewhere when the other user that isn't doing a disable currently
> > decides to start doing so.

> I guess it depends on whether _regulator_do_enable() on an
> already-enabled rdev is supposed to be a noop or not.  My assumption
> was that it was supposed to be a noop with reference counting handled
> by _regulator_enable().

Yes, that's the point.

> My assumption is that regulator drivers themselves shouldn't do
> reference counting.  That is: if you call
> rdev->desc->ops->enable(rdev) twice you should not have to call
> rdev->desc->ops->disable(rdev) twice to disable.  Right?  That means
> my fix is making the "ena_pin" symmetric to how normal regulator
> drivers work.

> The refcounting being skipped by my patch is refcounting that's used
> only when the same GPIO is shared by more than one regulator.  That
> is, if "vcc_a" uses GPIO1 and "vcc_b" also uses "GPIO1" we need
> refcounting.  GPIO1 will be in the "on" state if either vcc_a or vcc_b
> is on.  The problem came in because _regulator_do_enable() was
> incrementing the shared refcount every time it was called even if the
> specific regulator was already on.

This is all analysis which should have been in the changelog...
possibly not quite so verbosely but it should be there.

> Anyway, I looked at Javier's patch and it's also fine / reasonable.
> ...and in fact I would argue that possibly we could take both patches.
> Javier's patch eliminates the one known place where
> _regulator_do_enable() is called for an already-enabled regulator and
> my patch means that if someone else adds a new call we won't end up
> back in this same subtle bug.  I'm happy to update the CL desc to make
> it more obvious if you'd like.

Yes, the changelog definitely needs to be *much* clearer.  Especially
for things like locking and reference counting the changelog needs to
explain what the fix is and why it's safe, without that working it is a
lot harder to do a review as the reviewer needs to go back and check
that everything has been thought through properly.

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