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Date: Fri, 17 Feb 2012 00:37:35 +0800
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Subject: Re: oprofile and ARM A9 hardware counter
From: Ming Lei <ming.lei@canonical.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Will Deacon <will.deacon@arm.com>, "eranian@gmail.com" <eranian@gmail.com>,
        "Shilimkar, Santosh" <santosh.shilimkar@ti.com>,
        David Long <dave.long@linaro.org>,
        "b-cousson@ti.com" <b-cousson@ti.com>,
        "mans@mansr.com" <mans@mansr.com>,
        linux-arm <linux-arm-kernel@lists.infradead.org>,
        Ingo Molnar <mingo@elte.hu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Feb 17, 2012 at 12:19 AM, Peter Zijlstra <a.p.zijlstra@chello.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