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

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Date:   Mon, 1 Oct 2018 18:56:22 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-arm-kernel@...ts.infradead.org, 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

On Sun, Sep 30, 2018 at 04:49:04AM +0200, Jason A. Donenfeld wrote:
> Per the discussion about half-way down in [1], the kernel doesn't
> actually support the ARMv3 ISA, but selects it for some ARMv4 ISA
> hardware that benefits from ARMv3 code generation.

The issue is to do with the half-word stores in the ARMv4 ISA, which
need to be avoided on StrongARM RiscPC - the bus from the processor
card (which was designed for ARM610 and ARM710) does not support
anything except 8-bit and 32-bit accesses, so the 16-bit load/store
instructions don't work correctly.

Obviously, the reason for having the compiler use ARMv3 is to avoid
those instructions which we have no other way to prevent - however,
the use of ARMv3 with the assembler ensures that ldrh/strh are not
accidentally used.

We could argue that the ARMv3 assembly files are now stable, so the
chances of ldrh/strh being introduced is low, which would make this
change tolerable, but the commit message needs to spell out that
we lose this protection.

> Such a consideration,
> then, only applies to the compiler but not to the assembler. This commit
> passes -march=armv4 to the assembler in those cases, so that code
> written for ARMv4 will continue to compile and run fine, without needing
> module-specific asflags-y overrides.

Note that "code written for ARMv4" will not be usable on this platform
if it makes use of ldrh/strh, so depending on which instructions the
assembler is complaining about, it could very well be a real "you're
doing something wrong" case.

The side effect of this patch is that such cases will now be hidden
rather than evaluated on a case-by-case basis.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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