linux-kernel - Re: [PATCH v4 1/4] lib: Introduce atomic MMIO modify

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Message-Id: <20130828123352.b9beff5237dfde2571a5779e@linux-foundation.org>
Date:	Wed, 28 Aug 2013 12:33:52 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Lior Amsalem <alior@...vell.com>,
	Baruch Siach <baruch@...s.co.il>,
	Will Deacon <will.deacon@....com>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v4 1/4] lib: Introduce atomic MMIO modify

On Wed, 28 Aug 2013 07:24:23 -0300 Ezequiel Garcia <ezequiel.garcia@...e-electrons.com> wrote:

> On Tue, Aug 27, 2013 at 01:37:09PM -0700, Andrew Morton wrote:
> > On Sat, 24 Aug 2013 12:35:29 -0300 Ezequiel Garcia <ezequiel.garcia@...e-electrons.com> wrote:
> > 
> > > Some platforms have MMIO regions that are shared across orthogonal
> > > subsystems. This commit implements a possible solution for the
> > > thread-safe access of such regions through a spinlock-protected API.
> > 
> > Seem sensible.  Perhaps.
> > 
> > It only works if both subsystems agree to use atomic_io_modify().  And
> > if they're both capable of doing that, they are both capable of
> > implementing an agreed-upon internal locking scheme, so why bother?
> > 
> 
> One of the scenarios where this could be helpful and an agreed-upon
> lock seemed difficult to design is this: a watchdog driver that shares
> some control register with *two* different clocksource drivers.
> 
> So, one first solution is to have a function in the two clocksource
> drivers (with matching prototype) and have the watchdog access
> the register through it.
> 
> However, because of multiplatform builds, both these clocksource drivers
> could be built at the same time. Therefore we would have a symbol
> collision, doubly-defined, in each driver.
> 
> How would that work? What other internal locking scheme could we
> implement?

I guess the locking would need to be in a standalone module which the
various driver modules would then depend upon.  I'm not really
advocating doing this - I'm just making noise.

> [..]
> > 
> > I disagree with the presence of the ifndef.  If
> > __HAVE_ARCH_ATOMIC_IO_MODIFY is undefined, the architecture must still
> > implement the identical function signature.  The best way to ensure that
> > is to use the same prototype in both cases.
> > 
> 
> I agree, but how can this be done?

Just remove the ifndefs.  Then remove the identical function prototype
from the arm header.


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