Query: CPU Freq Framework

Query: CPU Freq Framework

[Deepak Sikri]

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Hi,
The System timer (Clock Source & ClockEvent) for our board is sharing the
clock with CPU.On changing the CPU frequency using the CPU freq governors,
we are changing the clock being fed to CPU and timers. This is affecting the
kernel time keeping as the timer frequecy has been changed. What is the
mechanism for managing this through the CPU freq/some other Linux Framework
?

Regards
Deepak

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Re: Query: CPU Freq Framework

[Dominik Brodowski]

Hey,

On Tue, Jan 12, 2010 at 12:14:29PM +0530, Deepak Sikri wrote:
> The System timer (Clock Source & ClockEvent) for our board is sharing the
> clock with CPU.On changing the CPU frequency using the CPU freq governors,
> we are changing the clock being fed to CPU and timers. This is affecting the
> kernel time keeping as the timer frequecy has been changed. What is the
> mechanism for managing this through the CPU freq/some other Linux Framework
> ?

The CPUfreq framework offers CPU transition notifiers for exactly this
purpose. See e.g. this snippet from arch/x86/kernel/tsc.c:

	...
        cpufreq_register_notifier(&time_cpufreq_notifier_block,
                                CPUFREQ_TRANSITION_NOTIFIER);
	...

static struct notifier_block time_cpufreq_notifier_block = {
        .notifier_call  = time_cpufreq_notifier
};

	...

static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
                                void *data)
{
	...

        if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
                        (val == CPUFREQ_POSTCHANGE && freq->old > freq->new) ||
                        (val == CPUFREQ_RESUMECHANGE)) {
                *lpj = cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new);

                tsc_khz = cpufreq_scale(tsc_khz_ref, ref_freq, freq->new);
		...
        }
	...
}

Hope this helps.

Best,
	Dominik