linux-kernel - Re: [PATCH] arm64: io: specify asm operand width for _

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Message-ID: <20181129161401.GY30658@n2100.armlinux.org.uk>
Date:   Thu, 29 Nov 2018 16:14:01 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Will Deacon <will.deacon@....com>, Jens Axboe <axboe@...nel.dk>,
        Julien Thierry <Julien.Thierry@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <nick.desaulniers@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: io: specify asm operand width for __iormb()

On Thu, Nov 29, 2018 at 09:10:39AM -0700, Nathan Chancellor wrote:
> On Thu, Nov 29, 2018 at 10:49:03AM +0000, Will Deacon wrote:
> > On Thu, Nov 29, 2018 at 09:03:54AM +0000, Julien Thierry wrote:
> > >
> > >
> > > On 29/11/18 04:19, Nick Desaulniers wrote:
> > > > Fixes the warning produced from Clang:
> > > > ./include/asm-generic/io.h:711:9: warning: value size does not match
> > > > register size specified by the constraint and modifier
> > > > [-Wasm-operand-widths]
> > > >         return readl(addr);
> > > >                ^
> > > > ./arch/arm64/include/asm/io.h:149:58: note: expanded from macro 'readl'
> > > >                                                           ^
> > > > ./include/asm-generic/io.h:711:9: note: use constraint modifier "w"
> > > > ./arch/arm64/include/asm/io.h:149:50: note: expanded from macro 'readl'
> > > >                                                   ^
> > > > ./arch/arm64/include/asm/io.h:118:25: note: expanded from macro '__iormb'
> > > >         asm volatile("eor       %w0, %1, %1\n" \
> > > >                                      ^
> > >
> > > Why does the "eor %0, %1, %1" become "eor %w0, %1, %1" ?
> > > The variable passed to the inline assembly for %0 is unsigned long, so
> > > always 64-bits wide on arm64. Why is clang trying to use a 32-bit
> > > register for it?
> 
> Sorry, this was my fault, I accidentally added a w during testing to see
> what constraints were valid (given that my assembly knowledge is nearly
> non-existence so forgive the non-sensical experimentation) and I used
> that message rather than the original one. This is the unadulterated one.
> 
> In file included from arch/arm64/kernel/asm-offsets.c:24:
> In file included from ./include/linux/dma-mapping.h:11:
> In file included from ./include/linux/scatterlist.h:9:
> In file included from ./arch/arm64/include/asm/io.h:209:
> ./include/asm-generic/io.h:695:9: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
>         return readb(addr);
>                ^
> ./arch/arm64/include/asm/io.h:147:58: note: expanded from macro 'readb'
> #define readb(c)                ({ u8  __v = readb_relaxed(c); __iormb(__v); __v; })
>                                                                        ^
> ./include/asm-generic/io.h:695:9: note: use constraint modifier "w"
> ./arch/arm64/include/asm/io.h:147:50: note: expanded from macro 'readb'
> #define readb(c)                ({ u8  __v = readb_relaxed(c); __iormb(__v); __v; })
>                                                                ^
> ./arch/arm64/include/asm/io.h:118:24: note: expanded from macro '__iormb'
>         asm volatile("eor       %0, %1, %1\n"                           \
>                                     ^
> 
> >
> > Yeah, the message above looks bogus to me. I can see %1 being 32-bit for
> > read[bwl], so maybe clang is just getting the diagnostic wrong. If so,
> > I wonder if the following fixes the problem:
> >
> 
> This doesn't appear to work, I get this error:
> 
> In file included from arch/arm64/kernel/asm-offsets.c:24:
> In file included from ./include/linux/dma-mapping.h:11:
> In file included from ./include/linux/scatterlist.h:9:
> In file included from ./arch/arm64/include/asm/io.h:209:
> ./include/asm-generic/io.h:695:9: error: expected expression
>         return readb(addr);
>                ^
> ./arch/arm64/include/asm/io.h:147:50: note: expanded from macro 'readb'
> #define readb(c)                ({ u8  __v = readb_relaxed(c); __iormb(__v); __v; })
>                                                                ^
> ./arch/arm64/include/asm/io.h:120:28: note: expanded from macro '__iormb'
>                      : "=r" (tmp) : "r" (unsigned long)(v) : "memory"); \
>                                          ^
> 
> >
> > diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> > index d42d00d8d5b6..13befec8b64e 100644
> > --- a/arch/arm64/include/asm/io.h
> > +++ b/arch/arm64/include/asm/io.h
> > @@ -117,7 +117,7 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
> >          */                                                             \
> >         asm volatile("eor       %0, %1, %1\n"                           \
> >                      "cbnz      %0, ."                                  \
> > -                    : "=r" (tmp) : "r" (v) : "memory");                \
> > +                    : "=r" (tmp) : "r" (unsigned long)(v) : "memory"); \

The parens around the passed value are part of the asm() syntax, which
is:

	"contraint" (expression)

so should be:

+                    : "=r" (tmp) : "r" ((unsigned long)(v)) : "memory"); \

-- 
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