Date:	Thu, 11 Sep 2014 16:37:39 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Grant Likely <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 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.

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? If we can't guarantee this,
we rather fix it now. Some suggestions:

a) Make sure code which doesn't have a clear behaviour on ARM is not
   compiled in and doesn't even try to interpret such tables on ARM (you
   could just go for the latter but I'm not sure how feasible it is)
b) Ensure that the current patches don't support anything other than
   ACPI 5.1 (6 or later would not be supported; can we get information
   about which errata update do vendor tables cover?)
c) Consider the current ACPI code broken for ARM (rather than feature
   incomplete) and strongly recommend vendors not to use it

Point (a) looks the sanest to me. Point (b) kind of defeats one of the
ACPI goals while point (c) would question why we even bother discussing
merging now.

-- 
Catalin

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