On my system, I used to have powernow-k8 compiled into the kernel, but not acpi-cpufreq. I run Gentoo Linux. With sys-kernel/gentoo-sources-3.6.8, I got the following relevent dmesg output: powernow-k8: Found 1 AMD Phenom(tm) II X6 1090T Processor (6 cpu cores) (version 2.20.00) powernow-k8: Core Performance Boosting: on. powernow-k8: 0 : pstate 0 (3200 MHz) powernow-k8: 1 : pstate 1 (2400 MHz) powernow-k8: 2 : pstate 2 (1600 MHz) powernow-k8: 3 : pstate 3 (800 MHz) Now, with sys-kernel/gentoo-sources-3.7.0, dmesg showed the following: powernow-k8: this CPU is not supported anymore, using acpi-cpufreq instead. This lead me into thinking that acpi-cpufreq was used when it actually wasn’t, because it was missing in my kernel (since I don’t know all config options by heart: I had to check for that). I found that the fans were a bit noisier than normal, so I looked in /proc/cpuinfo, which showed all 6 cores were at 3.2 GHz. I then recompiled the kernel with acpi-cpufreq compiled in and removed powernow-k8. Now /proc/cpuinfo shows 800 MHz on all or most of the cores when the system is mostly idle. Apparently, acpi-cpufreq doesn’t show any output at all, except: acpi-cpufreq: overriding BIOS provided _PSD data My suggestion would be to make powernow-k8 check for the availability of acpi-cpufreq and print out a warning should it not be available. And I also suggest a short “acpi-cpufreq is used for frequency scaling” so one can see that it was loaded alright.
This should be fixed now. The patch titled powernow-k8: Add a kconfig dependency on acpi-cpufreq was added to the 3.7-stable tree. The filename of the patch is: powernow-k8-add-a-kconfig-dependency-on-acpi-cpufreq.patch and it can be found in the queue-3.7 subdirectory. Since 3.7.4 is already out, I assume it will be part of 3.7.5 and later.
A patch referencing this bug report has been merged in Linux v3.8-rc5: commit 631e8ac18fd59722e7ceb15fceeef3f368c250f9 Author: Borislav Petkov <bp@suse.de> Date: Fri Jan 18 00:19:37 2013 +0100 powernow-k8: Add a kconfig dependency on acpi-cpufreq
After running 'make oldconfig' from 3.7.3 to 3.7.7, it silently removed the "CONFIG_X86_POWERNOW_K8=m" line from .config. I would expect it to add the new dependencies.