Re: [linux-pm] Runtime PM discussion notes

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Wednesday, July 13, 2011, Paul Walmsley wrote:
> (cc'ing Len)
> 
> Hi Mark,
> 
> On Mon, 11 Jul 2011, Mark Brown wrote:
> 
> > The interesting bits are things like being able to kill lots of the SoC
> > core supplies when the RAM is in retention mode - the CPU needs to go
> > through its shutdown procedures.
> 
> This is indeed possible on OMAP3+ chips with TWL4030+ PMICs.  Probably 
> other PMICs also.  TI calls it "off-mode."  The N900 shipped with this 
> feature enabled.  Not sure how many other similar products did.
> 
> This can be enabled in mainline, but not all of the mainline drivers have 
> context save/restore code merged yet, so in mainline it only works with a 
> subset of drivers.
> 
> > Actually, it just occurred to me that if we're waiting for a system
> > timer and can hand that off to a suitable timer in the PMIC then we can
> > do a suspend to RAM for the deep idle state from the hardware point of
> > view.
> 
> Yep.  At LinuxCon Cambridge two years ago, we had a discussion about 
> whether it would be possible to enter ACPI S-states from CPUIdle (or some 
> idle governor) on Intel chips.  If I remember correctly, the conclusion 
> was that ACPI always disables the screen/backlight, so it would only be 
> useful for situations where that was acceptable.

The reason why you can't enter ACPI S-states from CPUidle is because you
need to go out of the idle loop to execute some ACPI-specific stuff.  Which
is not even specific to Intel chips, but to ACPI in general.

So entering ACPI S-states from idle is a no-no and I don't think it'll
change in foreseeable future.

> To the best of my (limited) knowledge, that's the only case I know of 
> where there's a hardware limitation that prevents dynamic idle from 
> reaching the same low power state as system suspend.  If someone has hard 
> details of a similar example, it would be great to know about it.

Google G1 had this problem IIRC, but I don't have any details.

Thanks,
Rafael