linux-kernel - RE: [PATCH] cpufreq: Processor Clocking Control interface driver

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Date:	Wed, 16 Dec 2009 15:31:48 +0000
From:	"Chumbalkar, Nagananda" <Nagananda.Chumbalkar@...com>
To:	Dominik Brodowski <linux@...inikbrodowski.net>
CC:	"davej@...hat.com" <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"mjg@...hat.com" <mjg@...hat.com>, "trenn@...e.de" <trenn@...e.de>,
	"lenb@...nel.org" <lenb@...nel.org>
Subject: RE: [PATCH] cpufreq: Processor Clocking Control interface driver


>> >> +	if (target_freq <= 
>> >(ioread32(&pcch_hdr->minimum_frequency) * 1000)) {
>> >> +		target_freq = 
>> >ioread32(&pcch_hdr->minimum_frequency) * 1000;
>> >> +		dprintk("target: target_freq for cpu %d was 
>> >below limit, "
>> >> +			"converted it to %d\n", cpu, target_freq);
>> >> +	}
>> >
>> >why not do this in the _verify() step? Does 
>pcch_hdr->minimum_frequency
>> >even change "on the fly"?
>> 
>> pcch_hdr->minimum_frequency does not change "on the fly". 
>Also, there is no
>> need for those IO accesses:
>
>target_freq cannot be below policy->min or above policy->max. 
>If it were,
>the whole cpufreq subsystem is broken. So there's no need for 
>these checks,
>AFAICS.
>

I think the "ondemand" governor can ask for a target frequency that is
below policy->min.

Let's say that at a given snapshot, the frequency of a CPU is policy->cur.
At the next sampling, if the "load" on the CPU drops to a level that 
satisifies the equation below, then freq_next requested by "ondemand" 
will be below policy->min:

	load < (threshold - differential) * policy->min / policy->cur

A patch such as below may be needed to sanitize the target frequency 
requested by "ondemand". The "conservative" governor already has this check: 

# diff -bur ./drivers/cpufreq/cpufreq_ondemand.c.orig ./drivers/cpufreq/cpufreq_ondemand.c
--- ./drivers/cpufreq/cpufreq_ondemand.c.orig   2009-12-11 15:06:02.000000000 -0600
+++ ./drivers/cpufreq/cpufreq_ondemand.c        2009-12-15 20:27:57.000000000 -0600
@@ -554,6 +554,9 @@
                                (dbs_tuners_ins.up_threshold -
                                 dbs_tuners_ins.down_differential);

+               if (freq_next < policy->min)
+                       freq_next = policy->min;
+
                if (!dbs_tuners_ins.powersave_bias) {
                        __cpufreq_driver_target(policy, freq_next,
                                        CPUFREQ_RELATION_L);

- naga -

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