Re: [PATCH v2] acpi: Fix regression where _PPC is not read at boot even when ignore_ppc=0

["Darrick J. Wong" <djwong@us.ibm.com>]

On Thu, Apr 16, 2009 at 12:01:11PM +0200, Thomas Renninger wrote:
> Hi,
> 
> be careful, this could break the T60 again.

So long as T60 owners boot with ignore_ppc=1, they should still be fine.

> Can you and Ingo place acpidump of your machines somewhere, please.
> 
> On Thursday 16 April 2009 02:27:12 Darrick J. Wong wrote:
> > Earlier, Ingo Molnar posted a patch to make it so that the kernel would avoid
> > reading _PPC on his broken T60.  Unfortunately, it seems that with Thomas
> > Renninger's patch last July to eliminate _PPC evaluations when the processor
> > driver loads, the kernel never actually reads _PPC at all!
> This is wrong. _PPC is only evaluated when a cpufreq driver got registered.

I put a printk just before the call to acpi_evaluate_integer in
acpi_processor_get_platform_limit.  The printk did not appear unless (a)
I triggered the Notify event to get the kernel to reevaluate _PPC or (b)
I took a CPU offline and online.

An alternate way to describe the situation, I think, is that ignore_ppc
doesn't go from -1 to 0 until acpi_processor_ppc_notifier gets called,
and that only seems to happen during an ACPI Notify event.

> > This is problematic
> > if you happen to boot your non-T60 computer in a state where the BIOS _wants_
> > _PPC to be something other than zero.
> Your machine should suffer from that since Ingo's T60 patch?

Yes, but the particular machine I have didn't exist until a couple of
weeks ago, and before that our BIOSes were written so that _PPC always
returned zero, which masked the problem.

The _PPC method in this (preproduction) machine's BIOS also sets a flag
that enables the sending of _PPC Notify events.  I'm not sure if that's
the proper way to do such things, though it seems logical that if an OS
never reads _PPC then sending Notify events for it is pointless.

> Reading the _PPC part of the ACPI spec again:
> ---
> In order to support dynamic changes of _PPC object, Notify events on
> the processor object. Notify events of type 0x80 will cause OSPM to
> reevaluate any _PPC objects residing under the particular processor object
> notified.
> ---
> The *reevaluate* implies that the _PPC value has been evaluated/initialized
> by the OS already and Ingo's patch would be wrong then.
> I'd like to have a look at the T60's ACPI parts and find out what exactly
> (or if at all) makes _PPC to return sane values, I expect it's _PDC.

I recall that on the T60 BIOS, the _PPC was programmed to read the value
out of some register in the embedded controller, but I'll have to go
find a T60 to see what the latest BIOSes do.  There's nothing in the T60
BIOS update changelogs to indicate that they found and corrected a
problem with _PPC... but that doesn't eliminate the possibility that
they "forgot" to document one.

Though I do recall seeing some weird bug with that T60 where putting the
machine to sleep would confuse it into "1ghz only" mode, though I never
noticed this symptom after a fresh boot.

> Hmm, I could also imagine that Ingo's T60 patch is not needed anymore since
> Yakui's patch (0ac3c571315a53c14d2733564f14ebdb911fe903).
> This one could make sure that _PDC is evaluated first making the internal
> ACPI _PPC state initialize and makes sure _PPC gets only called afterwards.
> 
> If this patch does not break Ingo's T60, I think this should go in.
> Due to Yakui's reordering/cleanup of ACPI function calls, I think also
> the notifier chain I introduced is not needed anymore and I can clean this
> up if I find some time.

> You are more or less reverting Ingo's patch (e4233dec749a3519069d9390561b5636a75c7579):

Yes, but preserving the ignore_ppc=1 override.

--D