Re: [ACPI] _PDC method in DSDT

[David Moore <dcm@acm.org>]

On Mon, 2003-06-23 at 14:37, Dominik Brodowski wrote:
> 
> Firstly, please let me explain the goal of the cpufreq project: it is a
> cross-architecture, _generic framework_ which provides

Thanks for your explanation.  My fault was that I was too busy looking
at 2.4.21-ac1 and I didn't realize cpufreq had matured a lot in the
development series and resolved the duplication between it and the ACPI
drivers.

> Because of this, the ACPI P-States driver became a "cpufreq driver" in the
> _development_ series -- see arch/i386/kernel/cpu/cpufreq/acpi.c in latest 
> 2.5. kernels.
> 

Yes, I see that now.  That looks like exactly the right way to do it.

> IMO the ACPI P-States driver should only include the ACPI 2.0 P-States driver,
> and not try to support all other sorts of legacy support and become one
> "mega-driver". Instead, the other information stored in _PCT you talk about 
> should be available to other processor-specific cpufreq drivers which see 
> a need to obtain this information -- ACPI as a library, available to those 
> who want it.
> 

I see your rationale for wanting the P-state driver to be ACPI 2.0-only,
but I think there is some advantage to adding Enhanced Speedstep support
to i386/kernel/cpu/cpufreq/acpi.c rather than just added ACPI query
capabilities to speedstep-centrino.c:

speedstep-centrino.c is very picky about the precise CPU because its
tables are hard-coded with no margin for error.  With ACPI, the test for
Enhanced Speedstep would be more general:
if cpu_has(cpu, X86_FEATURE_EST) is true, and after calling _PDC, if
_PTC returns FFH registers, then treat those registers as MSRs instead
of IO ports.  Thus, it supports any Enhanced Speedstep hardware with a
correct BIOS, not just the ones enumerated by speedstep-centrino.c.

I guess my main point is that in order for speedstep-centrino.c to call
ACPI as a library, it would basically need to use every function already
written in acpi.c.  Why not just dispatch the wrmsr() from acpi.c rather
than the other way around?  It would take probably a 15-line patch to
add Enhanced Speedstep support to acpi.c, but probably would be much
more complicated to query ACPI from speedstep-centrino.c.  For AMD and
Transmeta processors, there would be no point in specific support for
them in acpi.c since ACPI doesn't really provide useful information for
their performance control anyway.

-David