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

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Date:	Wed, 4 Mar 2015 21:11:03 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	"Li, Aubrey" <aubrey.li@...ux.intel.com>,
	"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


* 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.

> so we can clear it on hw reduced, but also in other cases. hw 
> reduced is one way, but I'd be surprised if there weren't other ways 
> (like quirks) where we'd want to do the same things

Exactly. The key step is the proper, clean separation out of hardware 
interfaces.

Thanks,

	Ingo
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