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SubjectRe: Help: Using cpufreq from kernel level
On Sat, Dec 17, 2005 at 01:10:33PM -0500, Claudio Scordino wrote:
> Hi all,
>
> I'm writing a kernel module that needs to get info about the available
> frequencies on the current processor and to periodically change the current
> frequency.

So it seems you're writing a governor. See
Documentation/cpu-freq/governors.txt and
drivers/cpufreq/cpufreq_conservative.c
drivers/cpufreq/cpufreq_ondemand.c
drivers/cpufreq/cpufreq_performance.c
drivers/cpufreq/cpufreq_powersave.c
drivers/cpufreq/cpufreq_userspace.c

for a first reference.

> At user level it can be done through
>
> /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
> /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
> /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
>
> but I have no idea how to implement it at kernel level.
>
> I tried to declare
>
> extern struct cpufreq_driver *cpufreq_driver;
> extern struct cpufreq_policy *cpufreq_cpu_data[NR_CPUS];
> extern spinlock_t cpufreq_driver_lock;
> extern ssize_t show_available_freqs (struct cpufreq_policy *policy, char
> *buf);
>
> and to do
>
> char buffer [100000] = "\n";
> spin_lock_irqsave(&cpufreq_driver_lock, flags);
> show_available_freqs(cpufreq_cpu_data[0], buffer);
> spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
>
> but it crashes the system.

of course it does, this is just the sysfs -show function and you're
passing it a null pointer as first parameter. If you look at
show_available_freqs code you'll see that cpufreq_cpu_data is
dereferenced and this happening with interrupts disabled..

hth
--
mattia
:wq!
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