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

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Date:	Sun, 8 Jul 2012 00:29:20 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 00/36] AArch64 Linux kernel port


> 1. The AArch64 architecture is significantly different from AArch32 (the
> official name of the 32-bit ARM architecture), it is not an extension.
> It has a new exception model, new instruction set (even the register
> names are different), new C ABI, PCS. It has a hardware compat mode but
> that's about it, there is no interworking between the two (not allowed
> by hardware, nor that the ABIs would make this possible). 
> 
> 2. Easier code maintenance. While I started the kernel port from the
> arch/arm/ code base (it was at a time when Linux didn't even have
> generic unistd.h and the AArch64 specs kept changing) the code diverged
> significantly both because of architecture differences and software
> requirements like new ABI, generic headers (unistd.h, stat.h etc). The
> port undergone significant rewrite following the rules for new
> architecture ports and some of the few similarities will be moved to a
> more generic place (like the mmu_gather patches from peterz). AArch64
> SoCs are going into a more standardised direction and the SoC code will
> be kept to a minimum and moved under drivers/. So merging AArch32 and
> AArch64 together would be rather detrimental to code maintenance for
> both architectures (#ifdef's or duplicate 32/64 files).
> 
> 3. Flexibility. It's early days for AArch64, we don't have a hardware
> platform supported yet and there are ongoing debates around ACPI,
> firmware standardisation for CPU booting, hotplug, power management. We
> need the flexibility to improve the code base without worrying about
> backwards compatibility (apart from the compat layer which isn't at all
> affected by the hardware developments).

These are the same reasons the x86_64 people gave and regretted later.

Alan
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