linux-kernel - Re: cpufreq terminally broken [was Re: community PM requirements/issues and PowerOP]

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Date:	Thu, 14 Sep 2006 07:58:16 -0700
From:	Mark Gross <mgross@...ux.intel.com>
To:	Pavel Machek <pavel@....cz>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	"Eugeny S. Mints" <eugeny.mints@...il.com>,
	Matthew Locke <matt@...adgs.com>, Greg KH <greg@...ah.com>,
	Amit Kucheria <amit.kucheria@...ia.com>,
	pm list <linux-pm@...ts.osdl.org>,
	Preece Scott-PREECE <scott.preece@...orola.com>,
	Igor Stoppa <igor.stoppa@...ia.com>
Subject: Re: cpufreq terminally broken [was Re: community PM requirements/issues and PowerOP]

On Tue, Sep 12, 2006 at 10:33:28AM +0200, Pavel Machek wrote:
> Hi!
> 
> > > > >No, there is reason to keep that in kernel -- so that cpufreq
> > > > >userspace interface can be kept, and so that resulting kernel<->user
> > > > >interface is not ugly.
> > > > Cpuferq defines cpufreq_frequency_table structure in arch independent 
> > > > header while it's arch dependent data structure. A lot of code is built 
> > > > around this invalid basic brick and therefore is invalid: cpufreq tables, 
> > > > interface with cpu freq drivers, etc. Notion of transition latency as it 
> > > > defined by cpufreq is wrong because it's not a function of cpu type but 
> > > > function of current and next operating point. no runtime control on 
> > > > operating points set. it's always the same set of operating points for all 
> > > > system cpus in smp case and no way to define different sets or track any 
> > > > dependencies in case say multi core cpus. insufficient kernel<->user space 
> > > > interface to handle embedded requirements and no way to extend it within 
> > > > current design. Shall I continue?  Why should then anyone want to keep 
> > > > cpufreq userspace interface just due to keep it?
> > > 
> > > Yes, please continue. I do not think we can just rip cpufreq interface
> > > out of kernel, and I do not think it is as broken as you claim it
> > > is. Ripping interface out of kernel takes years.
> > > 
> > > I'm sure cpufreq_frequency_table could be moved to asm/ header if you
> > > felt strongly about that.
> > > 
> > > I believe we need to fix cpufreq if it is broken for embedded
> > > cases.
> > 
> > cpufreq is broken at the cpufreq_driver interface for embedded
> > applications needing control over more than one control variable at a
> > time.
> > 
> > That interface only supports setting target frequencies, and expanding it
> > to set target frequencies and voltages is not possible without something
> > like PowerOP.  Adding the types of parameters to cpufreq would likely
> > make cpufreq a mess.
> 
> Can we at least try adding that, before deciding cpufreq is unsuitable
> and starting new interface? I do not think issues are nearly as big as
> you think.. (at user<->kernel interface level, anyway; you'll need big
> changes under the hood).

We are trying.  The PowerOP patches from Matt and Eugeny start to put
into place some of the kernel mode plumbing for this in a way that
avoids thrashing the existing models, and it addresses the needs of the
operating point PM community.  Which is large in the CE and Embedded
camps.

--mgross
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