linux-kernel - Re: [PATCH] [v3] arm64/io: add constant-argument check

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Date: Wed, 5 Jun 2024 12:10:08 +0100
From: Will Deacon <will@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Arnd Bergmann <arnd@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Nathan Chancellor <nathan@...nel.org>,
	Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Baoquan He <bhe@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH] [v3] arm64/io: add constant-argument check

On Wed, Jun 05, 2024 at 10:15:38AM +0100, Mark Rutland wrote:
> On Tue, Jun 04, 2024 at 10:59:57PM +0200, Arnd Bergmann wrote:
> > Move the check for constant arguments into the inline function to ensure
> > it is still constant if the compiler decides against inlining it, and
> > mark them as __always_inline to override the logic that sometimes leads
> > to the compiler not producing the simplified output.
> > 
> > Note that either the __always_inline annotation or the check for a
> > constant value are sufficient here, but combining the two looks cleaner
> > as it also avoids the macro. With clang-8 and older, the macro was still
> > needed, but all versions of gcc and clang can reliably perform constant
> > folding here.
> > 
> > Fixes: ead79118dae6 ("arm64/io: Provide a WC friendly __iowriteXX_copy()")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> I have a trivial nit below, but either way this looks good to me, so
> regardless of that:
> 
> Reviewed-by: Mark Rutland <mark.rutland@....com>
> 
> > +static __always_inline void
> > +__iowrite32_copy(void __iomem *to, const void *from, size_t count)
> >  {
> > -	if (count == 8 || count == 4 || count == 2 || count == 1) {
> > +	if (__builtin_constant_p(count) &&
> > +	    (count == 8 || count == 4 || count == 2 || count == 1)) {
> >  		__const_memcpy_toio_aligned32(to, from, count);
> >  		dgh();
> >  	} else {
> >  		__iowrite32_copy_full(to, from, count);
> >  	}
> >  }
> > +#define __iowrite32_copy(to, from, count) __iowrite32_copy(to, from, count)
> 
> Normally we'd make this:
> 
> #define __iowrite32_copy __iowrite32_copy
> 
> ... so that it's clear it's just providing the preprocessor symbol, and
> doesn't have to be updated if the prototype changes.
> 
> [...]
> 
> > +#define __iowrite64_copy(to, from, count) __iowrite64_copy(to, from, count)
> 
> Likewise here.

I can fold these two changes in.

Will