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

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Date:	Mon, 13 Jun 2011 11:55:40 -0700
From:	Colin Cross <ccross@...roid.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 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?

> 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.  Only drivers within
the CPU itself need a notification.  Note that both ARM drivers I
modified did not have a register_cpu_notifier call.

> Rafael
>
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