C-state C3 missing?

[Frans Pop]

Hi,

Thanks for this great utility. Hopefully it will help the drive for power 
efficiency.

I've tried powertop 1.0 both with my current 2.6.18 kernel (Debian Etch) 
and with 2.6.22-rc1 with CONFIG_TIMER_STATS enabled to see the details.

There is something weird with powertop and how many C states it shows.

The first time I ran it on a 2.6.18 kernel, I remembered seeing 4 C states 
(C0 to C3). But later, when I tried it under 2.6.22-rc1, it only showed 
C0, C1 and C2. I tried a number of times to see if I would get C3 back, 
but no luck.

Now today powertop suddenly does show C3 again (using 1.1 on 2.6.18):
Cn          Avg residency (5s)  Long term residency avg
C0 (cpu running)        ( 6.4%)
C1                0.0ms ( 0.0%)                   0.0ms
C2                3.0ms (93.6%)                   2.9ms
C3                0.0ms ( 0.0%)                   0.0ms

The duration for C2 matches what powertop gives: most all time is spent in 
C2, none in C1. Processor is Mobile Intel(R) Pentium(R) 4 (2.80GHz).
Most testing was done while connected to mains so mains/battery is not an 
issue here.

It looks like maybe powertop only shows states that have a usage/duration 
greater than zero. This is what I had when powertop did not show C3:

$ cat /proc/acpi/processor/CPU0/power
active state:            C2
max_cstate:              C8
bus master activity:     f007ffff
states:
    C1:                  type[C1] promotion[C2] demotion[--] latency[000] 
usage[00000010] duration[00000000000000000000]
   *C2:                  type[C2] promotion[C3] demotion[C1] latency[001] 
usage[00254084] duration[00000000001696262530]
    C3:                  type[C3] promotion[--] demotion[C2] latency[085] 
usage[00000000] duration[00000000000000000000]

Now, when it does show C3, I have:
$ cat /proc/acpi/processor/CPU0/power
active state:            C2
max_cstate:              C8
bus master activity:     ffffffff
states:
    C1:                  type[C1] promotion[C2] demotion[--] latency[000] 
usage[00000010] duration[00000000000000000000]
   *C2:                  type[C2] promotion[C3] demotion[C1] latency[001] 
usage[08499497] duration[00000000085733891845]
    C3:                  type[C3] promotion[--] demotion[C2] latency[085] 
usage[00000228] duration[00000000000001637839]

If that is indeed the cause, it seems like a bug to me. IMO powertop 
should always display all available C states, whether they get activated 
or not. How else are you supposed to know that there is major room for 
improvement? :-)

Cheers,
FJP
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