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Date:   Wed, 06 Mar 2019 08:58:43 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Robin Murphy <robin.murphy@....com>
Cc:     linux@...linux.org.uk, nicolas.pitre@...aro.org,
        mark.rutland@....com, arnd@...db.de, ard.biesheuvel@...aro.org,
        will.deacon@....com, julien.thierry@....com,
        ndesaulniers@...gle.com, linux-kernel@...r.kernel.org,
        thierry.reding@...il.com, natechancellor@...il.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] ARM: drop unnecessary WASM

On 06.03.2019 00:39, Robin Murphy wrote:
> Hi Stefan,
> 
> On 2019-03-05 10:18 pm, Stefan Agner wrote:
>> The W macro for generating wide instructions when targeting Thumb-2
>> is not required for the preload data instructions (pld, pldw) since
>> they are only available as wide instructions. The GNU assembler seems
>> to work with or without the .w appended when compiling an Thumb-2
>> kernel. However, Clang's integrated assembler does not consider the
>> .w variants as valid instructions:
>>
>>    ./arch/arm/include/asm/processor.h:133:5: error: invalid instruction
>>                            "pldw.w\t%a0 \n"
>>                             ^
>>    <inline asm>:2:1: note: instantiated into assembly here
>>    pldw.w  [r0]
>>    ^
>>    1 error generated.
> 
> Have you filed a bug against Clang for that? Something like "pldwal.w"
> may be super-redundant, but it's still perfectly valid syntax. The
> "Standard assembler syntax fields" section of the Arm ARM even calls
> out that "...the .W qualifier has no effect" on ARM/A32 instructions
> since they are inherently 32-bit; that should equally apply for 32-bit
> only Thumb/T32 instructions. There are certainly a few instructions
> which don't allow a condition code (even "AL"), but off-hand I don't
> remember any not having the optional {<q>} field in their syntax.

Good point, bug filed:
https://bugs.llvm.org/show_bug.cgi?id=40972

Will send a v2 and note that this is a work around for Clang and
reference the bug.

--
Stefan

> 
> That said, obviously the patch itself is no problem since the syntax
> *is* redundant here, but it really is just a workaround for an
> assembler bug.
> 
> Robin.
> 
>> Drop the macro to make sure non-wide variants of pld and pldw are
>> emitted in all cases.
>>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> ---
>>   arch/arm/include/asm/processor.h | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
>> index 120f4c9bbfde..042d77cf686d 100644
>> --- a/arch/arm/include/asm/processor.h
>> +++ b/arch/arm/include/asm/processor.h
>> @@ -131,8 +131,8 @@ static inline void prefetchw(const void *ptr)
>>   	__asm__ __volatile__(
>>   		".arch_extension	mp\n"
>>   		__ALT_SMP_ASM(
>> -			WASM(pldw)		"\t%a0",
>> -			WASM(pld)		"\t%a0"
>> +			"pldw\t%a0",
>> +			"pld\t%a0"
>>   		)
>>   		:: "p" (ptr));
>>   }
>>

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