linux-kernel - Re: [PATCH v4 2/4] ARM: Add atomic_io

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Date:	Wed, 28 Aug 2013 12:01:22 +0200
From:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
To:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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>
Subject: Re: [PATCH v4 2/4] ARM: Add atomic_io_modify optimized routines

Dear Ezequiel Garcia,

On Wed, 28 Aug 2013 06:49:08 -0300, Ezequiel Garcia wrote:

> > Is this any different from the generic one introduced in patch 1/4? I
> > would rather just use the generic definition.
> 
> Well, according to Will Deacon (and as documented in the commit log)
> we can optimize in ARM by using readl_relaxed instead of readl.
> 
> Now, I'm sure you now better than me if that results (or not) in any
> significant optimization.
> 
> > Similarly, a generic
> > atomic_io_modify_relaxed() but guarded with something like
> > __HAVE_ARCH_RELAXED_IO.
> > 
> 
> No, that's not possible. As far as I understand, there's no guarantee
> of _relaxed variants to be available architecture-wide.

I think what Catalin was suggesting is that atomic_io_modify() should
use readl() and writel() (i.e *not* the relaxed variants), and that a
separate atomic_io_modify_relaxed() could be added on architectures
that define __HAVE_ARCH_RELAXED_IO.

I think you misread Catalin's comment when you say there's no guarantee
of _relaxed variants to be available architecture-wide, since Catalin
precisely suggested to guard that with __HAVE_ARCH_RELAXED_IO, which
indicates that _relaxed variants are available.

Thanks,

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com
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