Re: [PATCH] ACPI: create "processor.bm_check_disable" boot param

[Len Brown <lenb@kernel.org>]

> > Note also that an alternative for newer systems
> > is to use the intel_idle driver, which always
> > ignores BM_STS, relying Linux device drivers
> > to register constraints explicitly via PM_QOS.
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=15886
> 
> Thanks.  I don't fully understand why the check for this option
> is in a different place than the register check in the earlier patch? 

Technically, it could have been.

There are a comple of constraints in the layout of this code.

The _CST flag is x86 (actually Intel) specific -- so the detection
went into arch/x86/kernel/acpi/cstate.c

However, the operation of the that flag is per C-state,
not necessarily per system -- so we remember the flag
in in a cx->bm_sts_skip flag and check it in the 
'acpi generic' drivers/acpi/processor_idle.c

But we can't test a per cx flag inside acpi_idle_bm_check()
because it doesn't have access to the cx, so i put that
test at the site of its only caller.

In this 2nd patch...
we added a 'generic' ACPI bootparam that applies
to all C-states.  So it overrides any per-cstate flag
and it is static to the processor_idle.c file,
so it seemed cleanest (to me)
to push it down inside acpi_idle_bm_check()
rather than in its only caller.

> This needs to be also documented in Documentation/kernel-parameters.txt

I thought about that and decided against it.
While we do document some driver specific modparams
in kernel-parameters.txt, I do not expect this one to
be used that often -- mostly for diagnosis of BIOS bugs.
I know of two machines that need it,
and both of those machines have a BIOS update
or a BIOS update in progress that make it unnecessary.

thanks for caring.

Len Brown, Intel Open Source Technology Center.