C4 state problem
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Len Brown
reviewerQuoted 13 lines · Sven Anders
Hello!
I spend my day analysing why the Linux could not use the C4 state(s). Here my results...
For a start we take a look at the current available c-states:
#> cat /proc/acpi/processor/CPU?/power
active state: C3 max_cstate: C8 bus master activity: 00000000 maximum allowed latency: 8000 usec
Quoted 7 lines
states:
C1: type[C1] promotion[C2] demotion[--] latency[001]
usage[00002590] duration[00000000000000000000]
C2: type[C2] promotion[C3] demotion[C1] latency[001]
usage[00002086] duration[00000000000006795863]
*C3: type[C3] promotion[--] demotion[C2] latency[055]
usage[00003633] duration[00000000000009200376]
First note that there are "hardware C-states", and "ACPI C-states". The table above shows the ACPI C-states that the BIOS makes visible to the OS.
The BIOS (via the FADT or _CST) can map any hardware C-state it wants to ACPI C3. It is likely that you are already getting hardware C4 and simply skipping over hardware C3.
If you provide the complete raw output from acpidump, then it should be possible to tell.
cheers,
-Len