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

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Message-ID: <20060912001701.GC14234@linux.intel.com>
Date:	Mon, 11 Sep 2006 17:17:01 -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 12:56:17AM +0200, Pavel Machek wrote:
> Hi!
> 
> Just for the record... this goes out to the lkml. This discussion was
> internal for way too long. (for interested lkml readers, I'm sure
> linux-pm mailing list has public archive somewhere).
>

This was rude.

> On Tue 2006-09-12 02:05:26, Eugeny S. Mints wrote:
> > Pavel Machek wrote:
> > >>>>- PowerOP is only one layer (towards the bottom) in a power management 
> > >>>>solution.
> > >>>>- PowerOP does *not* replace cpufreq
> > >>>PowerOP provides userland interface for changing processor
> > >>>frequency. That's bad -- duplicate interface.
> > >>Basically the biggest problem with cpufreq interface is that cpufreq has 
> > >>"chose
> > >>predefined closest to a given frequency" functionality implemented in the
> > >>kernel while there is _no_ any reason to have this functionality 
> > >>implemented in
> > >>the kernel if we have sysfs interface exported by PowerOP in place - you 
> > >>just
> > >
> > >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.  I think we would be better off with something that
coexists with cpufreq, like the powerop patch from Eugeny.

God help you if you try to use cpufreq on a complex non-PC platform with
multiple power and clock domains that need to be tweaked to squeeze out
competitive battery life.

Because of the existing user base of cpufreq removing cpufreq will never
happen.  No one supporting the PowerOP patch has never recommended
such a thing.  However; holding back innovation because of an existing
solution that doesn't support a large class of users seems dumb.

--mgross
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