linux-kernel - Re: [PATCH] x86: Bypass legacy PIC and PIT on ACPI hardware reduced platform

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Date:	Thu, 5 Mar 2015 12:36:41 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>,
	"alan@...ux.intel.com" <alan@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, Len.Brown@...el.com,
	x86@...nel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Bypass legacy PIC and PIT on ACPI hardware reduced
 platform


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

> On 2015/3/5 4:11, Ingo Molnar wrote:
> > 
> > * Arjan van de Ven <arjan@...ux.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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