linux-kernel - Re: oprofile and ARM A9 hardware counter

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Date:	Fri, 17 Feb 2012 00:37:35 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Will Deacon <will.deacon@....com>,
	"eranian@...il.com" <eranian@...il.com>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	David Long <dave.long@...aro.org>,
	"b-cousson@...com" <b-cousson@...com>,
	"mans@...sr.com" <mans@...sr.com>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: oprofile and ARM A9 hardware counter

On Fri, Feb 17, 2012 at 12:19 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Fri, 2012-02-17 at 00:12 +0800, Ming Lei wrote:
>> is triggered: u64 delta = 100 -  1000000 = 18446744073708551716.
>
> on x86 we do:
>
>  int shift = 64 - x86_pmu.cntval_bits;
>  s64 delta;
>
>  delta = (new_raw_count << shift) - (prev_raw_count << shift);
>  delta >>= shift;
>
> This deals with short overflows (on x86 the registers are typically 40
> or 48 bits wide). If the arm register is 32 you can of course also get
> there with some u32 casts.

Good idea, but it may not work if new_raw_count is bigger than prev_raw_count.

thanks
--
Ming Lei
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