linux-kernel - Re: [Linaro-acpi] [PATCH v5 18/18] Documentation: ACPI for ARM64

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Date:	Wed, 7 Jan 2015 20:05:30 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Jon Masters <jcm@...hat.com>, linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Rob Herring <robh@...nel.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Robert Moore <robert.moore@...el.com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lv Zheng <lv.zheng@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Olof Johansson <olof@...om.net>
Subject: Re: [Linaro-acpi] [PATCH v5 18/18] Documentation: ACPI for ARM64

On Wed, Jan 07, 2015 at 08:48:48PM +0100, Arnd Bergmann wrote:
> On Wednesday 07 January 2015 12:44:56 Jon Masters wrote:
> > On 01/07/2015 12:27 PM, Mark Brown wrote:

> > > That level of hardware compatibility does partly come from the need to
> > > run existing software.  I'd expect that similar effects will start to
> > > come into play with ARMv8 ACPI systems if they become successful; people
> > > will do things like ensure compatibility with common IPs that have
> > > existing Linux drivers that distros tend to include as standard.

> > Agreed.

> There are two problems I see in trying to do the same thing on ARM:

> * we don't have a single vendor that makes de-facto standards that
>   everyone else has to copy in the way that the few remaining x86
>   vendors copy everything that Intel does. In fact, we prefer to
>   have a large number of independent vendors.

Right, I'd guess that (modulo any standards being defined and becoming
successful) it'll more be a case of vendors keeping compatibility with
their own stuff.  We *are* seeing greater reliance on off the shelf IPs
for more boring things like DMA and basic bus controllers but there's
plenty of other areas that still affect servers.

> * There is a general mindset about deprecating unwanted features
>   early. ARMv8 aarch32 bit mode removes support for older instructions
>   or makes them optional. Even the virtualization mode doesn't allow
>   to trap on architecture version specific differences, so you can't
>   completely emulate an older architecture level.
>   This is nice for implementers but not so much for users that rely
>   on old (mis-)features. It's also not just the CPU core, other
>   components also get easily replaced, like a GICv3 that is not
>   a strict superset of GICv2.

This is indeed worrying, though hopefully the fact that we're already
seeing negative impacts in the app ecosystem for Android will have
focused some minds - once you're talking about full system images it
gets even more fun.

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