Messages in this thread | | | Date | Mon, 1 Oct 2018 18:56:22 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3 |
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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.
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