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

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Message-Id: <201207170807.25139.arnd@arndb.de>
Date:	Tue, 17 Jul 2012 08:07:24 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Pavel Machek <pavel@....cz>, 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>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 00/36] AArch64 Linux kernel port

On Tuesday 17 July 2012, Christoph Hellwig wrote:
> On Sun, Jul 15, 2012 at 07:43:07PM +0000, Arnd Bergmann wrote:
> > Yes, I agree that's the best way to handle this. Compared to other
> > architectures, I think x86 is the only that allows booting either a
> > 32 or 64 bit kernel on the same system. We used to support 32 bit
> > kernels on 64 bit PowerMac, but nobody used it and we discontinued
> > it long ago. Tile 64 bit is actually incompatible with 32 bit kernels
> > at the architecture level and would require a third mode. On sparc,
> > parisc and mips, AFAIK we could support 32 bit kernels on 64 bit
> > machines, but never did.
> 
> On mips it works just fine.  On Sparc I don't think Linux ever did it,
> but Solaris did for a long time, as did (IIRC) NetBSD/OpenBSD.

Ah, I didn't know about mips doing that. I also just remembered that
s390 supports running 31 bit kernels on all 64 bit machines, but there is
no longer official support for that from IBM's side AFAIK.

I certainly expect ARM to be similar to powperpc and sparc here,
and anyone trying to submit a 32 bit kernel port for a 64 bit platform
will have a hard time arguing why that should be accepted into mainline.

	Arnd

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