linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 03 Jun 2010 07:42:37 -0700
From:	Kevin Hilman <khilman@...prootsystems.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Gross\, Mark" <mark.gross@...el.com>,
	Florian Mickler <florian@...kler.org>,
	James Bottomley <James.Bottomley@...e.de>,
	Arve Hjønnevåg <arve@...roid.com>,
	Neil Brown <neilb@...e.de>, "tytso\@mit.edu" <tytso@....edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	"felipe.balbi\@nokia.com" <felipe.balbi@...ia.com>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

Peter Zijlstra <peterz@...radead.org> writes:

> On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote:
>> > [mtg: ] This has been a pain point for the PM_QOS implementation.
>> They change the constrain back and forth at the transaction level of
>> the i2c driver.  The pm_qos code really wasn't made to deal with such
>> hot path use, as each such change triggers a re-computation of what
>> the aggregate qos request is.
>> 
>> That should be trivial in the usual case because 99% of the time you can
>> hot path
>> 
>> 	the QoS entry changing is the latest one
>> 	there have been no other changes
>> 	If it is valid I can use the cached previous aggregate I cunningly
>> 		saved in the top QoS entry when I computed the new one
>> 
>> (ie most of the time from the kernel side you have a QoS stack)
>
> Why would the kernel change the QoS state of a task? Why not have two
> interacting QoS variables, one for the task, one for the subsystem in
> question, and the action depends on their relative value?

Yes, having a QoS parameter per-subsystem (or even per-device) is very
important for SoCs that have independently controlled powerdomains.
If all devices/subsystems in a particular powerdomain have QoS
parameters that permit, the power state of that powerdomain can be
lowered independently from system-wide power state and power states of
other power domains.

Kevin
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