Re: [PATCH] drm/amd/display: Disable CONFIG_DRM_AMD_DC_FP for RISC-V with clang

[Harry Wentland <harry.wentland@amd.com>]

On 2024-06-14 15:54, Nathan Chancellor wrote:
> Commit 77acc6b55ae4 ("riscv: add support for kernel-mode FPU") and
> commit a28e4b672f04 ("drm/amd/display: use ARCH_HAS_KERNEL_FPU_SUPPORT")
> enabled support for CONFIG_DRM_AMD_DC_FP with RISC-V. Unfortunately,
> this exposed -Wframe-larger-than warnings (which become fatal with
> CONFIG_WERROR=y) when building ARCH=riscv allmodconfig with clang:
> 
>   drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c:58:13: error: stack frame size (2448) exceeds limit (2048) in 'DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation' [-Werror,-Wframe-larger-than]
>      58 | static void DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation(
>         |             ^
>   1 error generated.
> 
> Many functions in this file use a large number of parameters, which must
> be passed on the stack at a certain pointer due to register exhaustion,
> which can cause high stack usage when inlining and issues with stack
> slot analysis get involved. While the compiler can and should do better
> (as GCC uses less than half the amount of stack space for the same
> function), it is not as simple as a fix as adjusting the functions not
> to take a large number of parameters.
> 
> Unfortunately, modifying these files to avoid the problem is a difficult
> to justify approach because any revisions to the files in the kernel
> tree never make it back to the original source (so copies of the code
> for newer hardware revisions just reintroduce the issue) and the files
> are hard to read/modify due to being "gcc-parsable HW gospel, coming
> straight from HW engineers".
> 
> Avoid building the problematic code for RISC-V by modifying the existing
> condition for arm64 that exists for the same reason. Factor out the
> logical not to make the condition a little more readable naturally.
> 
> Fixes: a28e4b672f04 ("drm/amd/display: use ARCH_HAS_KERNEL_FPU_SUPPORT")
> Reported-by: Palmer Dabbelt <palmer@rivosinc.com>
> Closes: https://lore.kernel.org/20240530145741.7506-2-palmer@rivosinc.com/
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  drivers/gpu/drm/amd/display/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/Kconfig b/drivers/gpu/drm/amd/display/Kconfig
> index 5fcd4f778dc3..47b8b49da8a7 100644
> --- a/drivers/gpu/drm/amd/display/Kconfig
> +++ b/drivers/gpu/drm/amd/display/Kconfig
> @@ -8,7 +8,7 @@ config DRM_AMD_DC
>  	depends on BROKEN || !CC_IS_CLANG || ARM64 || RISCV || SPARC64 || X86_64
>  	select SND_HDA_COMPONENT if SND_HDA_CORE
>  	# !CC_IS_CLANG: https://github.com/ClangBuiltLinux/linux/issues/1752
> -	select DRM_AMD_DC_FP if ARCH_HAS_KERNEL_FPU_SUPPORT && (!ARM64 || !CC_IS_CLANG)
> +	select DRM_AMD_DC_FP if ARCH_HAS_KERNEL_FPU_SUPPORT && !(CC_IS_CLANG && (ARM64 || RISCV))

Thanks for also making the logic easier to parse.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>

Harry

>  	help
>  	  Choose this option if you want to use the new display engine
>  	  support for AMDGPU. This adds required support for Vega and
> 
> ---
> base-commit: c6c4dd54012551cce5cde408b35468f2c62b0cce
> change-id: 20240614-amdgpu-disable-drm-amd-dc-fp-riscv-clang-31c84f6b990d
> 
> Best regards,


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