linux-kernel - Re: [PATCH] arch/arm/kernel: use cpu

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Message-ID: <20101214225126.GD24303@n2100.arm.linux.org.uk>
Date:	Tue, 14 Dec 2010 22:51:26 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jonathan Neuschäfer <j.neuschaefer@....net>
Cc:	linux-kernel@...r.kernel.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Tony Lindgren <tony@...mide.com>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Eric Miao <eric.y.miao@...il.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arch/arm/kernel: use cpu_relax() in halt loops

On Tue, Dec 14, 2010 at 11:48:48PM +0100, Jonathan Neuschäfer wrote:
> Looking throught arch/arm/kernel/ I found some halt loops.
> I just thought we could save some power here using cpu_relax(),
> or am I missing something?

#if __LINUX_ARM_ARCH__ == 6
#define cpu_relax()                     smp_mb()
#else
#define cpu_relax()                     barrier()
#endif

IOW, it's either a memory barrier or compiler barrier on ARM; it
doesn't do any power saving.
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