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

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Message-ID: <CAHmME9pFnuRx2F=xeyK8d3bwv3bJY+WvE1KZn__+2oFWS0RCnA@mail.gmail.com>
Date:   Tue, 2 Oct 2018 14:30:29 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>, arm@...nel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3

Hi Arnd,

On Tue, Oct 2, 2018 at 9:58 AM Arnd Bergmann <arnd@...db.de> wrote:
> > 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'll do that. I can confirm that after changing it to `reteq lr`,
everything works well with armv3m.

> > 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.

Armv4 does actually work in this configuration, in fact. But anyway,
I'll go with your primary suggestion and we therefore can move ahead
with changing the global cflag to march=armv3m. Would you like me to
submit the patch for this, or would yo like to handle it?

Jason