Comment 0 for bug 516325

Alex Chiang (achiang) wrote :

Please backport the following patch into Karmic:

From: Len Brown <email address hidden>

upstream in 2.6.33-rc: 5d76b6f6c17572e662f5c99c2023adae92100855

Refreshed here for 2.6.32.y, applies w/ offset back to 2.6.29.y.

Linux has always ignored ACPI BIOS C2 with exit latency > 100 usec,
and the ACPI spec is clear that is correct FADT-supplied C2.
However, the ACPI spec explicitly states that _CST-supplied C-states
have no latency limits.

So move the 100usec C2 test out of the code shared
by FADT and _CST code-paths, and into the FADT-specific path.

This bug has not been visible until Nehalem, which advertises
a CPU-C2 worst case exit latency on servers of 205usec.
That (incorrect) figure is being used by BIOS writers
on mobile Nehalem systems for the AC configuration.
Thus, Linux ignores C2 leaving just C1, which is
saves less power, and also impacts performance
by preventing the use of turbo mode.