Re: [PATCH 1/2] drm/i915: Ensure _print_param() builds with Clang

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Oct 16, 2018 at 03:29:37PM +0300, Jani Nikula wrote:
> When building the kernel with Clang with defconfig and CONFIG_64BIT
> disabled, vmlinux fails to link because of the BUILD_BUG in
> _print_param.
> 
> ld: drivers/gpu/drm/i915/i915_params.o: in function `i915_params_dump':
> i915_params.c:(.text+0x56): undefined reference to
> `__compiletime_assert_191'
> 
> This function is semantically invalid unless the code is first inlined
> then constant folded, which doesn't work for Clang because semantic
> analysis happens before optimization/inlining.
> 
> [The above written by Nathan Chancellor <natechancellor@gmail.com>]
> 
> Use WARN_ONCE() instead of BUILD_BUG() to avoid the problem. The
> WARN_ONCE() should get optimized away unless there's a type that's not
> handled by _print_param().
> 
> References: https://github.com/ClangBuiltLinux/linux/issues/191
> References: http://mid.mail-archive.com/20181009171401.14980-1-natechancellor@gmail.com
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Nathan Chancellor <natechancellor@gmail.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

Thanks for the quick turnaround and help getting this fixed, Jani!

> ---
>  drivers/gpu/drm/i915/i915_params.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
> index bd6bd8879cab..8d71886b5f03 100644
> --- a/drivers/gpu/drm/i915/i915_params.c
> +++ b/drivers/gpu/drm/i915/i915_params.c
> @@ -184,7 +184,8 @@ static __always_inline void _print_param(struct drm_printer *p,
>  	else if (!__builtin_strcmp(type, "char *"))
>  		drm_printf(p, "i915.%s=%s\n", name, *(const char **)x);
>  	else
> -		BUILD_BUG();
> +		WARN_ONCE(1, "no printer defined for param type %s (i915.%s)\n",
> +			  type, name);
>  }
>  
>  /**
> -- 
> 2.11.0
> 
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