CpuFreq Laptop Scaling broken?

From: Harry van Haaren <harryhaaren@gmail.com>
To: linux-rt-users@vger.kernel.org
Cc: rjw@sisk.pl, viresh.kumar@linaro.org, tglx@linutronix.de,
	rostedt@goodmis.org
Subject: CpuFreq Laptop Scaling broken?
Date: Mon, 25 Aug 2014 13:28:27 +0100	[thread overview]
Message-ID: <CAKudYbOjXe3JRGDsE3A_=Vd7NEU+_eDO3po3+47CS80=TN363g@mail.gmail.com> (raw)

Hi -rt users, and CC's,

I'm attempting to squeeze the lowest audio-latency out of a laptop as possible,
and in doing so would like to set the CPU to performance.

CPU in laptop: Intel(R) Core(TM)2 Duo CPU T7300 @ 2.00GHz

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
ondemand performance

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
acpi-cpufreq

# Attempt to set performance governer
$ echo "performance" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
bash: echo: write error: Invalid argument

# Attempt to set min freq of 2GHz (max) of this core2duo
$ echo 2000000 > /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq
bash: /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq: Permission denied

As far as I can tell, it is currently impossible on the -rt kernel to
change CPU governer?
This is necessary for reliable low-latency audio for laptop musicians.

What is necessary to fix cpufreq on -rt? Cheers, -Harry

PS: I've CC'd maintainers of cpufreq, I hope that's OK, otherwise
please inform me of
normal practices on linux-rt-users ML