linux-kernel - Re: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3

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Date:   Tue, 2 Oct 2018 09:51:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        arm-soc <arm@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3

On Tue, Oct 2, 2018 at 5:53 AM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hi Arnd,
>
> Apologies for the delay in getting back to you. I had some MTA issues
> and stupidly assumed ARM developers were taking the day off instead...
>
> On Tue, Oct 2, 2018 at 5:33 AM Arnd Bergmann <arnd@...db.de> wrote:
> > -arch-$(CONFIG_CPU_32v3)                =-D__LINUX_ARM_ARCH__=3 -march=armv3
> > +arch-$(CONFIG_CPU_32v3)                =-D__LINUX_ARM_ARCH__=3 -march=armv3m
>
> Unfortunately this doesn't really cut it in my case, as it's not only
> those multiplications:
> chacha20-arm.S:402: Error: selected processor does not support `bxeq
> lr' in ARM mode
>
> I think we're going to wind up playing whack-a-mole in silly ways. The
> fact of the matter is that the ARM assembly I'm adding to the tree is
> for ARMv4 and up, and not for ARMv3.

I don't see what issues remain. The 'reteq lr' that Ard mentioned
is definitely the correct way to return from assembly (you also need
that for plain armv4, as 'bx' was added in armv4t), and Russell
confirmed that using -march=armv3m is something we want
anyway for mach-rpc.

> I think there are three options to work around this issue:
>
> 1) Not build my assembly when CONFIG_CPU_32v3 via a Kconfig "depends".
> 2) Set asflags-$(CONFIG_CPU_32v3) inside my module locally to select
> -march=armv4.
> 3) This patch.
>
> My initial plan was (1). ArdB recommended I do (2) instead. I thought
> that was a bit too nuanced and submitted (3).

I suspect all three of the above fail to work for armv4.

         Arnd