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

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Message-ID: <87hblkyrwt.fsf@deeprootsystems.com>
Date:	Thu, 03 Jun 2010 09:58:26 -0700
From:	Kevin Hilman <khilman@...prootsystems.com>
To:	"Gross\, Mark" <mark.gross@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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)

"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.

Kevin
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