Re: [PATCH v3 02/17] ARM64 / ACPI: Get RSDP and ACPI boot-time tables

[Jon Masters <jcm@redhat.com>]

On 09/09/2014 02:05 PM, Sudeep Holla wrote:
> 
> 
> On 09/09/14 18:50, Lorenzo Pieralisi wrote:
>> On Tue, Sep 09, 2014 at 06:15:41PM +0100, Mark Rutland wrote:
>>> On Tue, Sep 09, 2014 at 05:41:51PM +0100, Jon Masters wrote:
>>>> On 09/09/2014 12:26 PM, Catalin Marinas wrote:
>>>>> On Mon, Sep 01, 2014 at 03:57:40PM +0100, Hanjun Guo wrote:
>>>>>> diff --git a/arch/arm64/include/asm/acenv.h b/arch/arm64/include/asm/acenv.h
>>>>>> new file mode 100644
>>>>>> index 0000000..3899ee6
>>>>>> --- /dev/null
>>>>>> +++ b/arch/arm64/include/asm/acenv.h
>>>>>> @@ -0,0 +1,18 @@
>>>>>> +/*
>>>>>> + * ARM64 specific ACPICA environments and implementation
>>>>>> + *
>>>>>> + * Copyright (C) 2014, Linaro Ltd.
>>>>>> + *   Author: Hanjun Guo <hanjun.guo@linaro.org>
>>>>>> + *   Author: Graeme Gregory <graeme.gregory@linaro.org>
>>>>>> + *
>>>>>> + * This program is free software; you can redistribute it and/or modify
>>>>>> + * it under the terms of the GNU General Public License version 2 as
>>>>>> + * published by the Free Software Foundation.
>>>>>> + */
>>>>>> +
>>>>>> +#ifndef _ASM_ACENV_H
>>>>>> +#define _ASM_ACENV_H
>>>>>> +
>>>>>> +#define ACPI_FLUSH_CPU_CACHE() WARN_ONCE(1, "Not currently supported on ARM64")
>>>>>
>>>>> Does this mean that it will be supported at some point? Looking at the
>>>>> places where this function is called, I don't really see how this would
>>>>> ever work on ARM. Which means that we add such macro just to be able to
>>>>> compile code that would never be used on arm64. I would rather see the
>>>>> relevant ACPI files only compiled on x86/IA-64 rather than arm64.
>>>>
>>>> That specific cache behavior is a part of e.g. ACPI C3 state support
>>>> (e.g. ACPI5.1 8.1.4 Processor Power State C3).
>>>
>>> Per table 5-35, if neither WBINVD or WBINVD_FLUSH are set in the FADT,
>>> we don't get S1, S2, or S3 states either.
>>>
>>>> As you note, it's not going to work on 64-bit ARM as it does on x86,
>>>> but it's optional to implement C3 and early 64-bit ARM systems should
>>>> not report Wbindv flags in the FADT anyway.
>>>
>>> Unless the arm cache architecture changes, I wouldn't expect any 64-bit
>>> ARM system to set either of the WBINVD flags.
>>>
>>>> They can also set FADT.P_LVL3_LAT > 1000, which has the effect of
>>>> disabling C3 support, while also allowing for use of _CST objects to
>>>> define more flexible C-States later on.
>>>
>>> It sounds like we should be sanity checking these in the arm64 ACPI code
>>> for the moment. I don't want us to discover that current platforms
>>> report the wrong thing only when new platforms come out that might
>>> actually report things correctly.
>>
>> I think that the kernel must ignore most of the stuff mentioned above
>> in HW_REDUCED_ACPI mode. And to be frank I still think that the problem
>> is not even there. The problem is trying to compile code that basically
>> has no defined behaviour - ie it is unspecified - on ARM64, that's what
>> Catalin pointed out.
>>
>> I understand it is compiled in by default on x86, but that's not a reason
>> why we should add empty hooks all over the place to compile code that
>> does not stand a chance to be doing anything sensible apart from
>> returning an error code, in the best case scenario.
>>
> 
> I had pointed out this earlier, even if we make it compile there's
> every possibility that it can blow up if some vendor adds S- states
> to their ACPI tables. One clear reason why it could blow up is:
> 
> "
>       /* This violates the spec but is required for bug compatibility. */
>       acpi_write_bit_register(ACPI_BITREG_SCI_ENABLE, 1);
> "
> 
> I don't think this can ever work on ARM platforms. So better to fix it
> properly.

Hanjun,

How do you want to proceed? I'm not sure it should be !HW_REDUCED_MODE
for the cache behavior, because an embedded x86 box would still probably
define those, but removing the hooks on ARM may make sense.

Jon.