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

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Date:	Thu, 03 Jun 2010 12:01:33 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	"Gross, Mark" <mark.gross@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Florian Mickler <florian@...kler.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Neil Brown <neilb@...e.de>, "tytso@....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@...ia.com" <felipe.balbi@...ia.com>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Thu, 2010-06-03 at 09:58 -0700, Kevin Hilman wrote:
> "Gross, Mark" <mark.gross@...el.com> writes:
> 
> >>-----Original Message-----
> >>From: Kevin Hilman [mailto:khilman@...prootsystems.com]
> >>Sent: Thursday, June 03, 2010 7:43 AM
> >>To: Peter Zijlstra
> >>Cc: Alan Cox; Gross, Mark; Florian Mickler; James Bottomley; Arve
> >>Hjønnevåg; Neil Brown; tytso@....edu; LKML; Thomas Gleixner; Linux OMAP
> >>Mailing List; Linux PM; 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.
> >>
> > This seems similar to that pm_qos generalization into bus drivers we where 
> > waving our hands at during the collab summit in April?  We never did get 
> > into meaningful detail at that time.
> 
> The hand-waving was around how to generalize it into the driver-model,
> or PM QoS.  We're already doing this for OMAP, but in an OMAP-specific
> way, but it's become clear that this is something useful to
> generalize.

Do you have a pointer to the source and description?  It might be useful
to look at to do a reality check on what we're talking about.

James


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