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

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Date:	Sun, 14 Sep 2014 21:31:16 -0700
From:	Grant Likely <grant.likely@...aro.org>
To:	Catalin Marinas <catalin.marinas@....com>
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 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. ACPI has a mechanism to check
version numbers. Similarly, when ACPI-future defines those states, the
kernel shouldn't expose them to an older platform.

> What if in a year time we get ACPI 5.2 or 6 (or an errata update)
> covering the S and C states for ARM? I would expect hardware vendors
> to take advantage and provide such information in ACPI tables. Can we
> guarantee that a kernel with the current ACPI patches wouldn't blow up
> when it tries to interpret the new tables?

Yes - at least as much as we can guarantee anything in systems
programming.

g.
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