linux-kernel - Re: [PATCH 0/3] Add OMAP hardware spinlock misc driver

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Message-ID: <tnxeibnh67q.fsf@e102109-lin.cambridge.arm.com>
Date:	Mon, 18 Oct 2010 16:27:05 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Hari Kanigeri <h-kanigeri2@...com>, Suman Anna <s-anna@...com>,
	Benoit Cousson <b-cousson@...com>,
	Tony Lindgren <tony@...mide.com>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	akpm@...ux-foundation.org, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/3] Add OMAP hardware spinlock misc driver

Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2010-10-18 at 14:35 +0100, Russell King - ARM Linux wrote:
>> In any case, Linux's spinlock API (or more accurately, the ARM exclusive
>> access instructions) relies upon hardware coherency support (a piece of
>> hardware called an exclusive monitor) which isn't present on the M3 nor
>> DSP processors.  So there's no way to ensure that updates from the M3
>> and DSP are atomic wrt the A9 updates.
>
> Right, so the problem is that there simply is no way to do atomic memory
> access from these auxiliary processing units wrt the main CPU? Seeing as
> they operate on the same memory space, wouldn't it make sense to have
> them cache-coherent and thus provide atomicy guarantees through that?

With cache coherency you may get atomicity of writes or reads but
usually not atomic modifications.

-- 
Catalin
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