Well yes, I agree with you, but I am not sure this is an issue we should
fix in Ubuntu ourselves, overriding the current Linux Kernel policies.
The speedstep_centrino module is included in i386 (but doens't work
there anyhow - at least on my machine) and is ommitted on amd64. I guess
this is the same issue as can be observed with KDE3 vs. KDE4 in Kubuntu
- the new system/driver is not ready for general usage yet, but is
"conceptionally" better since it has a more polished API. Of course,
this "conceptional" advantage doesn't really help the users affected by
it in reality.

Concerning the DSDT hacking - well of course the vendor would not
support such a modified system, but the ACPI tables contain the adresses
of the registers where those supported clock frequencies and voltages
are stored for those CPUs. As far as I got it, this speedstep-centrino
did some sort of autodetection of certain CPU types (like the Dothan-M)
and based on that switch to certain hardcoded addresses. Hardcoded in
the way that they were hardcoded in the driver, not in the DSDT/SSDT
tables.

Looking at it from this perspective, this was the same as running this
modified system with the altered ACPI tables. The changes in the ACPI
tables are not persistent, so if you reboot to another kernel or even
operating system without this override the previous state is restored.

Sticking with this acpi-cpufreq driver, the problem could be addressed
by including some sort of "repaired" ACPI tables which replace other
broken tables at bootup. This would have the same effect as the
speedstep-centrino driver, but would conform to the new non-deprecated
unified driver. But I guess in most systems there are such broken
tables, since most systems are designed to run in Windows where some
hacks are included which override those buggy tables. I assume that
those hacks are included in some driver updates from the system vendors.
Anyhow, they don't even seem to work correctly: On my P8400 CPU only two
clock settings are used: 1600 MHz and the full 2260 MHz - I believe the
CPU itself has the ability to be clocked down a lot more.

-- 
cpufreq modules missing in hardy and intrepid amd64
https://bugs.launchpad.net/bugs/246434
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