http://bugzilla.kernel.org/show_bug.cgi?id=6519





------- Additional Comments From [EMAIL PROTECTED]  2006-05-15 11:48 -------


On Mon, 15 May 2006, Andrew Morton wrote:
> > @@ -873,7 +866,8 @@ static int acpi_processor_get_power_info
> >     for (i = 1; i < ACPI_PROCESSOR_MAX_POWER; i++) {
> >             if (pr->power.states[i].valid) {
> >                     pr->power.count = i;
> > -                   pr->flags.power = 1;
> > +                   if (pr->power.states[i].type >= ACPI_STATE_C2)
> > +                           pr->flags.power = 1;
> >             }
> >     }
> >  
> > --------8<--------
> > 
> > The last change in the diff, in acpi_processor_get_power_info, is what 
> > makes the
> > difference: commenting out the "if (pr->power.states[i].type >= 
> > ACPI_STATE_C2)"
> > restores the power saving in 2.6.16.14 .

That last change is also the thing that fixed the lock-up for Maneesh.

We had this particular discussion at some point earlier, and didn't get to 
any resolution. The ACPI people wanted to undo the thing, which didn't 
make a lot of sense because (a) the comment says otherwise and (b) C1 is 
available even without ACPI and (c) nobody ever explained why it locked up 
without the "type >= ACPI_STATE_C2" test.

Now, what happens is that your ACPI tables show that C2+ is unusable, 
which leaves only C1 usable (that, btw, you might be able to fix with 
different tables - possibly through a BIOS update).

However, at that point it's totally pointless to even _try_ to use ACPI 
CPU power management for this case, since ACPI can't do any better than 
the normal C1 stuff in the bog-standard non-ACPI x86 idle routine.

Do you actally see anything running hotter?

Or is it just that the "CPU%d (power states:..)" message disappeared?

Please realize that we will _always_ use C1 (aka "halt") in the idle state 
quite regardless of ACPI - unless you've done "idle=poll" on the kernel 
command line. So the fact that we don't use ACPI for it shouldn't make us 
actually run any hotter (quite the reverse - we'll go into C1 state with 
_less_ work).

So I don't see the downside. Am I missing something?

                 Linus



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