Cc: Pierre Ossman <drzeus-list@...>, Pavel Machek <pavel@...>, LKML <linux-kernel@...>, Adam Belay <abelay@...>, Andi Kleen <andi@...>, Lee Revell <rlrevell@...>
Modern Intel mobile processors have a feature called "C2 popup"
that allows the processor to retire DMA from C3 without
breaking into C0. Instead the processor pops up to C2
where the cache snoop can allow the DMA to retire --
then it returns to C3, all transparent to software.
That is why powertop does not see it.
ak> Normally such things are not visible in the DSDT, but hidden in SMM traps.
no, this mechanism is totally visible to the OS via a _CST re-evaluation on AC/DC transition.
This is very commmon, as the reference BIOS code does it.
It isn't a secret. There are two common techniques -- either
re-define C3 to enter hardware C4, or simply add C4 as 4th C-state.
the latest cpuidle code actually publishes the details of the C-states being used.
$ pwd
/sys/devices/system/cpu/cpu0/cpuidle/state3
$ grep . *
desc:ACPI FFH INTEL MWAIT 0x20
latency:17
name:C3
power:250
time:1862590932
usage:9028970
You'll see "desc" change if ACPI pulls a _CST change on you.
if you disable it at run-time, Linux puts it in C1.
If you never boot it in the first place (eg. maxcpusp=1), the BIOS leaves it in the deepest available C-sate.
The former will prevent the package from entering a deep package C-state,
as c-states are coordinated in hardware. The later will allow the package
to enter whatever C-state the running core chooses.
yup, that's why we put it in. Is it insufficient?
-Len
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