linux-kernel - Re: [PATCH v2] ARM: perf: save/restore pmu registers in pm notifier

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Date:	Tue, 15 Apr 2014 09:47:50 +0100
From:	Will Deacon <will.deacon@....com>
To:	Neil Zhang <zhangwm@...vell.com>
Cc:	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sudeep Holla <Sudeep.Holla@....com>
Subject: Re: [PATCH v2] ARM: perf: save/restore pmu registers in pm notifier

Hello,

On Mon, Apr 14, 2014 at 02:42:22AM +0100, Neil Zhang wrote:
> From: Sudeep KarkadaNagesha <sudeep.karkadanagesha@....com>
> 
> This adds core support for saving and restoring CPU PMU registers
> for suspend/resume support i.e. deeper C-states in cpuidle terms.
> This patch adds support only to ARMv7 PMU registers save/restore.
> It needs to be extended to xscale and ARMv6 if needed.
> 
> [Neil] We found that DS-5 not work on our CA7 based SoCs.
> After debuging, found PMU registers were lost because of core power down.
> Then i found Sudeep had a patch to fix it about two years ago but not in
> the mainline, just port it.

What I don't like about this patch is that we're introducing significant
overhead for SoCs that don't require save/restore of the PMU state. I'd much
rather see core power down disabled whilst the PMU is in use but, if that's
not possible, then I think we need to:

 (1) Make this conditional for cores that really need it

 (2) Only save/restore if the PMU is in use (even better, just save/restore
     the live registers, but that's probably not worth the effort
     initially).

 (3) Ensure we ->reset the PMU before doing the restore

Will
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