linux-kernel - Re: [Update 3x][PATCH 2/2] PM / Domains: Add preliminary support for cpuidle

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Message-Id: <201205312102.05358.rjw@sisk.pl>
Date:	Thu, 31 May 2012 21:02:05 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kevin Hilman <khilman@...com>
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 3x][PATCH 2/2] PM / Domains: Add preliminary support for cpuidle

On Thursday, May 31, 2012, Kevin Hilman wrote:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
> > On Friday, May 25, 2012, Kevin Hilman wrote:
> >> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> >> 
> >> > On Thursday, May 24, 2012, Kevin Hilman wrote:
> >> >> 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?
> >> >
> >> > The problem is that the PM domains code measures the time it takes to
> >> > power off a domain and updates its power on latency parameter if the
> >> > measured time is greater.  This is done for PM QoS to operate on realistic
> >> > numbers (most of the time at least).
> >> 
> >> OK, I see.  Maybe clarifying that in the changelog would help make that
> >> clearer.
> >
> > Sure.  I hope the updated changelog below is better.
> 
> Perfect, thanks.
> 
> Reviewed-by: Kevin Hilman <khilman@...com>

Cool, thanks!
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