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

[Mark Rutland]

On Tue, Jun 04, 2024 at 10:59:57PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> In some configurations __const_iowrite32_copy() does not get inlined
> and gcc runs into the BUILD_BUG():
> 
> In file included from <command-line>:
> In function '__const_memcpy_toio_aligned32',
>     inlined from '__const_iowrite32_copy' at arch/arm64/include/asm/io.h:203:3,
>     inlined from '__const_iowrite32_copy' at arch/arm64/include/asm/io.h:199:20:
> include/linux/compiler_types.h:487:45: error: call to '__compiletime_assert_538' declared with attribute error: BUILD_BUG failed
>   487 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>       |                                             ^
> include/linux/compiler_types.h:468:25: note: in definition of macro '__compiletime_assert'
>   468 |                         prefix ## suffix();                             \
>       |                         ^~~~~~
> include/linux/compiler_types.h:487:9: note: in expansion of macro '_compiletime_assert'
>   487 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>       |         ^~~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
>    39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>       |                                     ^~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
>    59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
>       |                     ^~~~~~~~~~~~~~~~
> arch/arm64/include/asm/io.h:193:17: note: in expansion of macro 'BUILD_BUG'
>   193 |                 BUILD_BUG();
>       |                 ^~~~~~~~~
> 
> 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@xxxxxxxx>

I have a trivial nit below, but either way this looks good to me, so
regardless of that:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>

> +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.

Mark.