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

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Date:	Tue, 09 Sep 2014 13:33:14 -0400
From:	Jon Masters <jcm@...hat.com>
To:	Mark Rutland <mark.rutland@....com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Olof Johansson <olof@...om.net>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"graeme.gregory@...aro.org" <graeme.gregory@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Will Deacon <Will.Deacon@....com>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Mark Brown <broonie@...nel.org>, Rob Herring <robh@...nel.org>,
	Robert Richter <rric@...nel.org>,
	Lv Zheng <lv.zheng@...el.com>,
	Robert Moore <robert.moore@...el.com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 02/17] ARM64 / ACPI: Get RSDP and ACPI boot-time tables

On 09/09/2014 01:15 PM, 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@...aro.org>
>>>> + *   Author: Graeme Gregory <graeme.gregory@...aro.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.

That's not quite what it says. You could still define an \_S1 state on
such a system. That table simply makes an assumption that if the stride
parameters are not defined and neither is wbindv then there's no way to
reliably flush caches, which of course isn't a valid conclusion, it's
just language that needs to be updated to reflect reality. So it's not
"don't get", it's "machine cannot support", which is an assumption.

>> 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.

Indeed.

>> 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.

It's reasonable to do this sooner rather than later, sure.

Jon.

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