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

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Date:	Thu, 05 Mar 2015 19:13:37 +0800
From:	"Li, Aubrey" <aubrey.li@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
CC:	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

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.

Thanks,
-Aubrey
> 
>> 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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