linux-kernel - Re: [PATCH v3 00/17] Introduce ACPI for ARM64 based on ACPI 5.1

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Date:	Tue, 16 Sep 2014 11:12:56 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	"grant.likely@...aro.org" <grant.likely@...aro.org>
Cc:	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Mark Rutland <Mark.Rutland@....com>,
	Olof Johansson <olof@...om.net>,
	"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>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>
Subject: Re: [PATCH v3 00/17] Introduce ACPI for ARM64 based on ACPI 5.1

On Mon, Sep 15, 2014 at 11:48:00PM +0100, Grant Likely wrote:
> On Mon, 15 Sep 2014 10:15:16 +0100, Catalin Marinas <catalin.marinas@....com> wrote:
> > On Mon, Sep 15, 2014 at 05:31:16AM +0100, Grant Likely wrote:
> > > On Thu, 11 Sep 2014 16:37:39 +0100, Catalin Marinas <catalin.marinas@....com> wrote:
> > > > On Thu, Sep 11, 2014 at 02:29:34PM +0100, Grant Likely wrote:
> > > > > Regarding the requests to refactor ACPICA to work better for ARM. I
> > > > > completely agree that it should be done, but I do not think it should be
> > > > > a prerequisite to getting this core support merged. That kind of
> > > > > refactoring is far easier to justify when it has immediate improvement
> > > > > on the mainline codebase, and it gives us a working baseline to test
> > > > > against. Doing it the other way around just makes things harder.
> > > > 
> > > > I have to disagree here. As I said, I'm perfectly fine with refactoring
> > > > happening later but I'm not happy with compiling in code with undefined
> > > > behaviour on ARM that may actually be executed at run-time.
> > > > 
> > > > I'm being told one of the main advantages of ACPI is forward
> > > > compatibility: running older kernels on newer hardware (potentially with
> > > > newer ACPI version tables). ACPI 5.1 includes partial support for ARM
> > > > but the S and C states are not defined yet. We therefore assume that
> > > > hardware vendors deploying servers using ACPI would not provide such
> > > > yet to be defined information in ACPI 5.1 tables.
> > > 
> > > We're good on this front. ACPI-future platforms aren't allowed to use
> > > new features when booting an older kernel.
> > 
> > Do you mean ACPI-future firmware should not provide new information to
> > older kernels?
> 
> I mean that ACPI has the mechanism so that the platform (via AML)
> doesn't try to do things that the OS doesn't understand.  AML methods are
> to use the \_REV object to determine whether or not it can use a
> feature. If, for example, the _REV is 5, then the AML must switch itself
> to code paths that don't use ACPI 6 features.

So you are talking about AML code paths and from what I understand they
should be fine.

However, what I'm talking about is _kernel_ code paths and functions
like acpi_suspend_enter(). Do you mean this function can only be called
as a result of some AML execution (it doesn't look so to me)? If not,
what guarantees do we have that ACPI-future tables exposed to an older
kernel would not trigger this kernel code path (if compiled in)?

-- 
Catalin
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