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

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Date:	Wed, 28 Aug 2013 09:16:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <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, Aug 28, 2013 at 3:37 AM, Ezequiel Garcia
<ezequiel.garcia@...e-electrons.com> wrote:
> Linus,
>
> Andrew suggested you might have opinions on this, so I'm cc'ing you.
> Since you'll probably want some better context, here it is:
>
> http://lwn.net/Articles/564709/
>
> On Sat, Aug 24, 2013 at 12:35:29PM -0300, Ezequiel Garcia 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.
>>
>> Concurrent access is protected with a single spinlock for the
>> entire MMIO address space. While this protects shared-registers,
>> it also serializes access to unrelated/unshared registers.

I have nothing against this, except that
"__HAVE_ARCH_ATOMIC_IO_MODIFY" needs to die, along with the #ifdef.

It should be a CONFIG_xyz option that gets set by the architectures
that have it, and then instead of an #ifdef in code, the Makefile
should make the generic target be conditional.

So add a generic

    bool GENERIC_ATOMIC_MMIO_MODIFY
        depends on !ARCH_ATOMIC_MMIO_MODIFY
        default y

to the lib/Kconfig file, and then lib/Makefile just does

    obj-$(CONFIG_GENERIC_ATOMIC_MMIO_MODIFY) += atomic_io.o

After that, ARM can then implement some architecture-optimized
version, and in its own Kconfig file just do "select
ARCH_ATOMIC_MMIO_MODIFY" to let the generic code know that it should
disable that generic version.

                 Linus
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