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

Lorenzo Pieralisi
2014-09-09T17:50:10Z

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.

Lorenzo