linux-kernel - Re: [PATCH 0/3] CPU PM notifiers

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Message-Id: <201106142300.42487.rjw@sisk.pl>
Date:	Tue, 14 Jun 2011 23:00:42 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Colin Cross <ccross@...roid.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH 0/3] CPU PM notifiers

On Monday, June 13, 2011, Colin Cross wrote:
> On Mon, Jun 13, 2011 at 11:37 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Monday, June 13, 2011, Colin Cross wrote:
> >> This patch set tries to address Russell's concerns with platform
> >> pm code calling into the driver for every block in the Cortex A9s
> >> during idle, hotplug, and suspend.  The first patch adds cpu pm
> >> notifiers that can be called by platform code, the second uses
> >> the notifier to save and restore the GIC state, and the third
> >> saves the VFP state.
> >>
> >> The notifiers are used for two types of events, CPU PM events and
> >> CPU complex PM events.  CPU PM events are used to save and restore
> >> per-cpu context when a single CPU is preparing to enter or has
> >> just exited a low power state.  For example, the VFP saves the
> >> last thread context, and the GIC saves banked CPU registers.
> >>
> >> CPU complex events are used after all the CPUs in a power domain
> >> have been prepared for the low power state.  The GIC uses these
> >> events to save global register state.
> >>
> >> Platforms that call the cpu_pm APIs must select
> >> CONFIG_ARCH_USES_CPU_PM
> >>
> >> L2 cache is not covered by this patch set, as the determination
> >> of when the L2 is reset and when it is retained is
> >> platform-specific, and most of the APIs necessary are already
> >> present.
> >>
> >>  arch/arm/Kconfig              |    7 ++
> >>  arch/arm/common/gic.c         |  212 +++++++++++++++++++++++++++++++++++++++++
> >>  arch/arm/include/asm/cpu_pm.h |   54 +++++++++++
> >>  arch/arm/kernel/Makefile      |    1 +
> >>  arch/arm/kernel/cpu_pm.c      |  181 +++++++++++++++++++++++++++++++++++
> >
> > Is there any reason why this has to be ARM-specific?  There are other
> > architectures where this kind of feature might make sense (SH and
> > powerpc at least).
> 
> Nothing other than there are currently no adaptations for any drivers
> besides ARM, but I can move it somewhere outside ARM.  Any suggestions
> where?

Well, there is kernel/cpu.c.  It contains mostly CPU hotplug and PM
code at the moment, so it looks like a good place.

> > Besides, is there any overlap between this feature and the CPU hotplug
> > notifiers?
> 
> I don't think so - the hotplug notifiers are used when a CPU is being
> removed from the system, so no saving and restoring is necessary - the
> CPU will be rebooted from scratch.  They are used by systems outside
> the CPU that need to know that a CPU no longer exists.
> 
> CPU PM notifiers are used when a CPU is going through reset in a way
> that should be transparent to most of the system.

Do I guess correctly that you mean cpuidle?

Rafael
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