linux-kernel - Re: [Update 2x][RFC][PATCH 2/2] PM / Domains: Add preliminary cpuidle support

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Message-ID: <87sjeqzge3.fsf@ti.com>
Date:	Wed, 23 May 2012 15:23:00 -0700
From:	Kevin Hilman <khilman@...com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Colin Cross <ccross@...roid.com>,
	Magnus Damm <magnus.damm@...il.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Santosh Shilimkar <santosh.shilimkar@...com>
Subject: Re: [Update 2x][RFC][PATCH 2/2] PM / Domains: Add preliminary cpuidle support

Hi Rafael,

"Rafael J. Wysocki" <rjw@...k.pl> writes:

> From: Rafael J. Wysocki <rjw@...k.pl>
>
> On some systems there are CPU cores located in the same power
> domains as I/O devices.  Then, power can only be removed from the
> domain if all I/O devices in it are not in use and the CPU core
> is idle.  Add preliminary support for that to the generic PM domains
> framework.
>
> First, the platform is expected to provide a cpuidle driver with one
> extra state designated for use with the generic PM domains code.
> This state should be initially disabled and its exit_latency value
> should be set to whatever time is needed to bring up the CPU core
> itself after restoring power to it, not including the domain's
> power on latency.  Its .enter() callback should point to a procedure
> that will remove power from the domain containing the CPU core at
> the end of the CPU power transition.
>
> The remaining characteristics of the extra cpuidle state, referred to
> as the "domain" cpuidle state below, (e.g. power usage, target
> residency) should be populated in accordance with the properties of
> the hardware.
>
> Next, the platform should execute genpd_attach_cpuidle() on the PM
> domain containing the CPU core.  That will cause the generic PM
> domains framework to treat that domain in a special way such that:
>
>  * When all devices in the domain have been suspended and it is about
>    to be turned off, the states of the devices will be saved, but
>    power will not be removed from the domain.  Instead, the "domain"
>    cpuidle state will be enabled so that power can be removed from
>    the domain when the CPU core is idle and the state has been chosen
>    as the target by the cpuidle governor.
>
>  * When the first I/O device in the domain is resumed and
>    __pm_genpd_poweron(() is called for the first time after
>    power has been removed from the domain, the "domain" cpuidle
>    state will be disabled to avoid subsequent surprise power removals
>    via cpuidle.

This looks like a good approach.  I like that it keeps a pretty clean
separation between CPUidle and PM domains.

My only comment would be that the recalc of the exit_latency should be
described a bit more.  Specifically, I'm not sure why it's adjused at
every genpd poweron.  At first I thought it was just supposed to be
adjusted upon attach, then adjusted back on detatch, but with the recalc
also in every poweron, I'm a little confused.  Care to clarify?

Thanks,

Kevin
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