[Bug 36014] Re: kernel can't scale cpu frequency

[Nacim Rahal]

I had this problem with a Dell Inspiron 9200 (pentium m 1.7 GHz) with
Edgy (kernel 2.6.17-10-generic). But now everything works fine.

I had scaling_max_freq = scaling_min_freq = 600000 and I was unable to
change i :

nacim at nacim-laptop:~$ sudo -i
root at nacim-laptop:~# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
600000
root at nacim-laptop:~# echo 1700000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
root at nacim-laptop:~# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
600000

As a consequence the scaling didn't work like it used to, about one
month ago. I noticed that under Windows everything worked fine but when
I played games the GPU was hotter than usual (65°C instead of 40°C). And
in fact, the GPU fan didn't work at all. The CPU was also slightly
hotter (45°C vs 40°C).

I cleaned my laptop and got rid of the dust that was preventing the GPU
fan from spinning. Now, temperatures while gaming are normal. With
Ubuntu, scaling_max_freq = 1700000, and the frequency scaling works
correctly.

I'm not skilled enough to explain why an abnormally hot temperature of
the GPU, or the fact that its fan doesn't spin, makes the CPU run at the
lowest available frequency using Ubuntu (scaling worked with Windows). I
can only suppose that some hardware device, or the BIOS, gives a wrong
scaling_max_freq to the kernel to prevent parts of the hardware from
being overheated. But it doesn't explain why, when something goes wrong
with the GPU, the CPU is affected.

I hope this will help understanding this bug and the fact that it seems
to appear randomly.

-- 
kernel can't scale cpu frequency
https://launchpad.net/bugs/36014