P-state setting suddenly disappeared, what gives?

[Jung-uk Kim]

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On 2013-11-14 17:41:44 -0500, Adrian Chadd wrote:
> Hi all,
> 
> I have this Lenovo T400 that I've been doing FreeBSD development on
> for a while.
> 
> It has a P8700 in it:
> 
> CPU: Intel(R) Core(TM)2 Duo CPU     P8700  @ 2.53GHz (2527.07-MHz
> 686-class CPU)
> 
> Now, up until yesterday, ACPI exported the required twiddles to
> enter various different P-states.
> 
> However, as of sometime yesterday, it stopped being able to do so.
> 
> sysctl dev.cpu.0.freq returns nothing. Setting it to something
> retuns "device not configured." The frequency list (ie, the P-state
> list) is still fine.
> 
> I had to load cpufreq.ko to get the enhanced speedstep stuff to
> show up, but (a) it doesn't support this CPU (and it seems to have
> stopped growing EST bits after Pentium M CPUs..) and (b) setting
> the frequency using it versus P-state settings doesn't save as much
> power.
> 
> I'd like to try and debug why the heck this is.
> 
> The laptop still works fine, things are just not as "nice" as they
> once were.
> 
> Any ideas? Any suggestions on where to start debugging this?

SSDT tables for Intel processors are usually dynamic and often times
additional tables are loaded per _OSC or _PDC. [1]  Basically, we
advertise our capabilities from sys/dev/acpica/acpi_cpu.c, depending
on loaded device drivers.  Unfortunately, some times it is too late
and some SSDTs are not properly loaded.  Also, warm booting from other
OSes to FreeBSD may cause similar problems.

To debug the problem, you need to dump DSDT and SSDTs and try to
understand _OSC (or _PDC), _PCT and _PSS for your system.

Jung-uk Kim

1.  See http://docs.freebsd.org/cgi/mid.cgi?4B698DD8.4010404 for
current implementation.
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