linux-kernel - Re: [PATCH] ARM: makefile: use ARMv3M mode for RiscPC

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Date:   Tue, 2 Oct 2018 15:28:35 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        arm-soc <arm@...nel.org>, Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] ARM: makefile: use ARMv3M mode for RiscPC

On 2 October 2018 at 15:20, Jason A. Donenfeld <Jason@...c4.com> wrote:
> The purpose of CONFIG_CPU_32v3 is to avoid ldrh/strh on the RiscPC,
> which is pretty much an ARMv4 device, except its bus will choke on the
> half-words. The way to make the C compiler not output ldrh/strh is with
> -march=armv3, which doesn't support them in the ISA. However, this
> prevents certain cryptography code from working that uses instructions
> like umull. Fortunately there's also -march=armv3m that does support
> those, making it possible to continue assembling optimized cryptography
> routines for our beloved RiscPC.
>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Arnd Bergmann <arnd@...db.de>

Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

> ---
>  arch/arm/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index d1516f85f25d..7fd4bcaf0721 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -74,7 +74,7 @@ endif
>  arch-$(CONFIG_CPU_32v5)                =-D__LINUX_ARM_ARCH__=5 $(call cc-option,-march=armv5te,-march=armv4t)
>  arch-$(CONFIG_CPU_32v4T)       =-D__LINUX_ARM_ARCH__=4 -march=armv4t
>  arch-$(CONFIG_CPU_32v4)                =-D__LINUX_ARM_ARCH__=4 -march=armv4
> -arch-$(CONFIG_CPU_32v3)                =-D__LINUX_ARM_ARCH__=3 -march=armv3
> +arch-$(CONFIG_CPU_32v3)                =-D__LINUX_ARM_ARCH__=3 -march=armv3m
>
>  # Evaluate arch cc-option calls now
>  arch-y := $(arch-y)
> --
> 2.19.0
>