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

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Date:	Wed, 11 Jul 2012 14:56:49 +0930
From:	Rusty Russell <rusty@...abs.org>
To:	Arnd Bergmann <arnd@...db.de>, 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\@vger.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 Tue, 10 Jul 2012 16:52:18 +0000, Arnd Bergmann <arnd@...db.de> wrote:
> 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

I know it's a crazy idea, but why don't we try some actual analysis?

In 2.5.5, when arch/x86_64 was introduced it was 35412 lines of code.
hashmatch (http://www.samba.org/~tridge/hashmatch) says that about 24642
are identical with arch/i386 in the same kernel.  That's 70%.  Some of
that's boilerplate: 9610 lines are in common with arch/sparc64 (27%),
so let's say that 43% of x86-64 was specifically sharable with i386.

arch/aarch64/ is 22016 line of code.  Hashmatch says 12509 (57%) is in
common with arch/arm.  But only 3232 lines (15%) are in common with
sparc.  So let's say that 42% of aarch64 is specifically sharable with
arm.

Looks equivalent to me.  They will merge eventually.

That said:
1) It's nice to have a clear division of maintainer responsibilities in
   the near term.

2) PowerPC only "merged" by removing a raft of older platforms, and I
   don't think ARM is ready for that.

And yes, aarch64 is a stupid name.

Cheers,
Rusty.
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