Re: [PATCH] arm64: io: specify asm operand width for __iormb()

From: Nathan Chancellor <natechancellor@gmail.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	Julien Thierry <Julien.Thierry@arm.com>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Nick Desaulniers <nick.desaulniers@gmail.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64: io: specify asm operand width for __iormb()
Date: Thu, 29 Nov 2018 09:17:38 -0700	[thread overview]
Message-ID: <20181129161738.GA9805@flashbox> (raw)
In-Reply-To: <20181129161337.GA21349@arm.com>

On Thu, Nov 29, 2018 at 04:13:37PM +0000, Will Deacon wrote:
> 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.
> 
> Aha, that explains it. Thanks for clearing that up.
> 
> > 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"); \
> >                                          ^
> 
> Can you try throwing another set of brackets around it, please?
> 
> 	((unsigned long)(v))
> 
> Will

Thanks, that fixes the warning as well.

Nathan

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