linux-kernel - Re: [PATCH 00/36] AArch64 Linux kernel port

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Date:	Tue, 10 Jul 2012 16:52:18 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...nel.org>,
	Olof Johansson <olof@...om.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 00/36] AArch64 Linux kernel port

On Tuesday 10 July 2012, Alan Cox wrote:
> > In the AArch32 kernel port many implementation decisions newer
> > architectures were made in a way that preserves backwards compatibility
> > to over 15 years ago (and for good reasons, ARMv4 hardware is still in
> > use). But keeping the same decisions in AArch64 is wrong.
> 
> Same argument as x86-32 v x86-64. Same issues about compatibility.

Similar but not the same. In case of x86-64 the hardware was actually
meant to run old 32 bit kernel binaries and still can. I don't
expect to see any 64 bit ARM systems running 32 bit kernels, and
I will almost certainly reject any attempt to submit such a platform
support to the arm-soc tree.

Another difference is the amount of legacy code for 32 bit ARM, which
no other architecture comes close to. With AArch64 we really want to
do a lot of things in a more modern way, and doing in in a common tree
would mean having to change a lot of old code at once. We might get
there eventually, but I would not want to depend on it.

> > The initial target is servers (see the companies that have announced
> > plans around ARMv8) but I agree, we may see it in other devices in the
> > future. But as the maintainer I have no plans to support a 32-bit SoC on
> > an AArch64/ARMv8 system (which may or may not support AArch32 at kernel
> > level). If an AArch64 SoC would share some devices with an AArch32 SoC,
> > such code will go to drivers/.
> 
> What plans to other maintainers and board vendors have ? Any design choice
> has to cope with these happening if a third party goes and does it.

It is slightly worrying to have multiple SoC vendors working on their
own platform support. There are a few things we can assume though:

* Everyone who has an AArch64 implementation has access to Catalin's
kernel patches in is basing their stuff on top of it.

* Most likely they are all working on server chips, which means they
want to have their hardware supported in upstream kernels and
enterprise distros.

* Unlike on 32 bit, the different platforms cannot be compile-time
exclusive. A platform port that cannot be enabled without breaking
another platform is not getting merged.

* I do not expect board-specific kernel patches, at least no more
than we have them on x86 with the occasional hack to work around
a broken machine.

* The differences between SoCs will be confined to device drivers
to a much larger degree than they are on 32 bit, partly because
the SoC companies are trying to be fit into the single-kernel
model, and partly because we have added the infrastructure to
allow it.

	Arnd
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