linux-kernel - Re: [PATCH 2/2] ARM: perf: Add support for Scorpion PMUs

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Date:	Wed, 11 Feb 2015 10:27:17 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
Cc:	Will Deacon <will.deacon@....com>,
	Devicetree List <devicetree@...r.kernel.org>,
	Ashwin Chaugule <ashwinc@...eaurora.org>,
	linux-arm-msm@...r.kernel.org,
	Neil Leeder <nleeder@...eaurora.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] ARM: perf: Add support for Scorpion PMUs

On 02/10, Ashwin Chaugule wrote:
> Hi Stephen,
> 
> On 10 February 2015 at 20:05, Stephen Boyd <sboyd@...eaurora.org> wrote:
> > Scorpion supports a set of local performance monitor event
> > selection registers (LPM) sitting behind a cp15 based interface
> > that extend the architected PMU events to include Scorpion CPU
> > and Venum VFP specific events. To use these events the user is
> > expected to program the lpm register with the event code shifted
> > into the group they care about and then point the PMNx event at
> > that region+group combo by writing a LPMn_GROUPx event. Add
> > support for this hardware.
> >
> > Note: the raw event number is a pure software construct that
> > allows us to map the multi-dimensional number space of regions,
> > groups, and event codes into a flat event number space suitable
> > for use by the perf framework.
> >
> > This is based on code originally written by Ashwin Chaugule and
> > Neil Leeder [1] massed to become similar to the Krait PMU support
> > code.
> 
> Thanks for taking this up!
> Overall this series looks good to me, but from what I faintly
> recollect, doesn't this (and the Krait pmu code) get affected by
> powercollapse issues anymore?
> e.g.
> https://www.codeaurora.org/cgit/quic/la/kernel/msm/commit/arch/arm/kernel/perf_event_msm.c?h=msm-3.4&id=b5ca687960f0fea2f4735e83ca5c9543474c19de
> 

Right now there isn't any power collapse support in mainline so
there's no immediate problem. Once we add power collapse support
(i.e. cpuidle) to the Scorpion and Krait platforms we'll need to
do something in the perf event code to properly maintain the
counts across idle. I imagine it would be done by registering for
cpu_pm notifications and then doing the save/restore on
CPU_PM_ENTER and CPU_PM_EXIT. At least, that's what you started
doing in this patch[1]. And then it seems the patch you mention
came after that and actually did the save/restore of the counts.

[1] https://www.codeaurora.org/cgit/quic/la/kernel/msm/commit/?h=msm-3.4&id=464983a7e991a484cac0bc0885cee4fee318d659

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