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

["Li, Aubrey" <aubrey.li@linux.intel.com>]

On 2015/3/5 19:36, Ingo Molnar wrote:
> 
> * 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().
> 
I'll do more investigation above items but I want to leave at least
these two as the quirk today unless I am convinced I can do that because
from my understanding, UEFI runtime services should not be supported in
reduced hw mode.

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

Thanks,
-Aubrey

> Thanks,
> 
> 	Ingo
> 
>