linux-kernel - Re: [PATCH 1/9] ARM: ARMv7-M uses BE-8, not BE-32

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Date:	Thu, 18 Feb 2016 17:12:13 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Jon Medhurst <tixy@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Maxime Coquelin stm32 <mcoquelin.stm32@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] ARM: ARMv7-M uses BE-8, not BE-32

On Thursday 18 February 2016 11:06:08 Nicolas Pitre wrote:
> On Thu, 18 Feb 2016, Arnd Bergmann wrote:
> 
> > When configuring the kernel for big-endian, we set either BE-8 or BE-32
> > based on the CPU architecture level. Until linux-4.4, we did not have
> > any ARMv7-M platform allowing big-endian builds, but now i.MX/Vybrid
> > is in that category, adn we get a build error because of this:
> > 
> > arch/arm/kernel/module-plts.c: In function 'get_module_plt':
> > arch/arm/kernel/module-plts.c:60:46: error: implicit declaration of function '__opcode_to_mem_thumb32' [-Werror=implicit-function-declaration]
> > 
> > This comes down to picking the wrong default, ARMv7-M uses BE8
> > like ARMv7-A does. Changing the default gets the kernel to compile
> > and presumably works.
> 
> Was it tested without BE8 when it was submitted upstream? I don't think 
> you can switch this freely on a given hardware platform and expect it to 
> still work.
> 
> 

mach-imx contains a number of different SoCs, and one SoC was recently
tested successfully after a number of endianess bugs got fixed. This was
an i.mx6 using a Cortex-A9 core, but we are now also able to build
vybrid vf610 big-endian based on that selection. This SoC supports
Linux running either on its Cortex-A5 or its Cortex-M3 (or M4?) cores.

I am rather sure nobody has ever run Linux in big-endian mode on the
Cortex-M platform, specifically because it was always wrong and could
not be enabled in Kconfig.

	Arnd