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SubjectRe: [PATCH] x86: Bypass legacy PIC and PIT on ACPI hardware reduced platform

* Li, Aubrey <aubrey.li@linux.intel.com> wrote:

> On 2015/3/5 4:11, Ingo Molnar wrote:
> >
> > * Arjan van de Ven <arjan@linux.intel.com> wrote:
> >
> >> On 3/4/2015 1:50 AM, Borislav Petkov wrote:
> >>> On Wed, Mar 04, 2015 at 12:43:08AM -0800, Arjan van de Ven wrote:
> >>>>>
> >>>>> Using 'acpi_gbl_reduced_hardware' flag outside the ACPI code
> >>>>> is a mistake.
> >>>>
> >>>> ideally, the presence of that flag in the firmware table will clear/set more global settings,
> >>>> for example, having that flag should cause the 8042 input code to not probe for the 8042.
> >>>>
> >>>> for interrupts, there really ought to be a "apic first/only" mode, which is then used on
> >>>> all modern systems (not just hw reduced).
> >>>
> >>> Do we need some sort of platform-specific querying interfaces now too,
> >>> similar to cpu_has()? I.e., platform_has()...
> >>>
> >>> if (platform_has(X86_PLATFORM_REDUCED_HW))
> >>> do stuff..
> >>
> >> more like
> >>
> >> platform_has(X86_PLATFORM_PIT)
> >>
> >> etc, one for each legacy io item
> >
> > Precisely. The main problem is the generic, 'lumps everything
> > together' nature of the acpi_gbl_reduced_hardware flag.
> >
> > (Like the big kernel lock lumped together all sorts of locking rules
> > and semantics.)
> >
> > Properly split out, feature-ish or driver-ish interfaces for PIT and
> > other legacy details are the proper approach to 'turn them off'.
> >
> > - x86_platform is a function pointer driven, driver-ish interface.
> >
> > - platform_has(X86_PLATFORM_IT) is a flag driven, feature-flag-ish
> > interface.
> >
> > Both are fine - for something as separate as the PIT (or the PIC)
> > it might make more sense to go towards a 'driver' interface
> > though, as modern drivers are (and will be) much different from
> > the legacy PIT.
> >
> > Whichever method is used, low level platforms can just switch them
> > on/off in their enumeration/detection routines, while the generic
> > code will have them enabled by default.
>
> Whichever method is used, we will face a problem how to determine
> PIT exists or not.
>
> When we enabled Bay Trail-T platform at the beginning, we were
> trying to make the code as generic as possible, and it works
> properly up to now. So we don't have a SUBARCH like
> X86_SUBARCH_INTEL_MID to use the platform specific functions. And
> for now I'm not quite sure it's a good idea to create one.
>
> If we make it as a flag driven, I don't know there is a flag in
> firmware better than ACPI HW reduced flag(Of course it's not good
> enough to cover all the cases). Or if we want to use platform info
> to turn on/off this flag, we'll have to maintain a platform list,
> which may be longer and more complicated than worth doing that.

Well, it's not nearly so difficult, because you already have a
platform flag: acpi_gbl_reduced_hardware.

What I object against is to infest generic codepaths with unreadable,
unrobust crap like:

+ if (acpi_gbl_reduced_hardware) {
+ pr_info("Using NULL legacy PIC\n");
+ legacy_pic = &null_legacy_pic;
+ } else
+ legacy_pic->init(0);

To solve that, add a small (early) init function (say
'x86_reduced_hw_init()') that sets up the right driver
selections if acpi_gbl_reduced_hardware is set:

- in x86_reduced_hw_init() set 'legacy_pic' to 'null_legacy_pic'

- clean up 'global_clock_event' handling: instead of a global
variable, move its management into x86_platform_ops::get_clockevent()
and set the method to hpet/pit/abp/etc. specific handlers that
return the right clockevent device.

- in your x86_reduced_hw_init() function add the hpet clockevent
device to x86_platform_ops::get_clockevent, overriding the default
PIT.

- in x86_reduced_hw_init() set pm_power_off.

- set 'reboot_type' and remove the acpi_gbl_reduced_hardware hack
from efi_reboot_required().

etc.

Just keep the generic init codepaths free of those random selections
based on global flags, okay?

Thanks,

Ingo


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