linux-kernel - Re: [PATCH 1/2] perf tools: allow building for ARM

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Date:	Fri, 11 Dec 2009 13:08:38 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Will Deacon <will.deacon@....com>,
	Jamie Iles <jamie.iles@...ochip.com>,
	linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] perf tools: allow building for ARM

On Fri, Dec 11, 2009 at 1:02 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Will Deacon <will.deacon@....com> wrote:
>
>> > From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-owner@...r.kernel.org]
>> > On Behalf Of Jamie Iles
>> > Sent: 11 December 2009 10:31
>> >
>> > The implementation of the barriers depend on the CPU arch revision
>> > which is defined in the kernel config. As the perf tools don't use
>> > the kernel config, we don't know here what arch revision we're
>> > building for. Perhaps we need a LINUX_ARM_ARCH parameter when
>> > building for ARM so we can pick the correct one.
>>
>> Hi Jamie, Ingo,
>>
>> Surely a better way to proceed with this would be to build the perf
>> tool as a side effect of building the kernel? That way the relevant
>> definitions from system.h could be included directly and there would
>> be no need to duplicate the architectural conditionals in perf.h.
>
> Might make sense.
>
>> I'm also working on perf-events for ARM and am using:
>>
>> #define rmb()           asm volatile("mcr p15, 0, %0, c7, c10, 5" :: "r" (0) : "memory")
>>
>> This will work on v6 and v7 [although the dmb instruction is preferred
>> here] cores.
>
> Note that the codepath where it's used isnt very performance sensitive
> (we call it about once per batch of event processing), so we could use
> the broadest instruction that works on as many cores as possible - to
> keep things simple.

How plausible is it to reuse the bits in
arch/arm/include/asm/cputype.h and do an version of rmb() that has
if-else for the v6 and v7 cases?
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