Re: [Mactel-linux-users] C4 state problem

[Sven A. <an...@an...>]

Sheer El-Showk schrieb:
> I haven't coded in a while and its quite late my time so maybe I'm
> being stupid but looking at the processor_idle.c file and even the
> entire drivers/acpi directory can't find any mention of state C4.  If
> you grep for C3 you get a lot of hits but C4 shows nothing.  Are you
> sure linux even support C4?

Yes, the old C0 to C3 states are handles special in ACPI (they even
have special definitions in the ACPI table). All additional c-states
are handled via the _CST method, which is queried by a seperate
function.
What I'm not sure about is: Are all of the processors deeper sleep
modes handled via the single ACPI state C4 or do we have to define
even an state C5..Cn ?

> From some observation (and the usage counter) I agree that C3 is used
> a lot and since, from the PDF file you sent a while ago, C4 saves
> about 8 watts more battery than C3 (~10 watts vs ~2 watts) I think
> this might really be important so it would be nice to see if we can
> even test this somehow.
> 
> Incidentally there is some discussion on the kernel mailing list about
> hidden c-states in Linux where someone claims that remove the USB
> module will expose additional c-states but on the MBP since this is
> needed for the keyboard this is obviously hard to even test, let alone
> apply.

The "hidden" states are only hidden, if ACPI did not expose it to Linux
(like here). The problem with USB is only, that the processors cannot
switch in the C3/C4 if there is some busmaster activity. Some USB devices
(or it's drivers) will cause this.
I do not think the keyboard will be the problem only devices that are
causing a lot of interrupts are. One candidate may be the touchpad,
because it's generating every 8 ms a data package from it's sensors.

Hopefully this problem will be solved by the USB auto-suspend-mechanism,
which are currently under development.

Regards
 Sven

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