[PATCH] drm/amdgpu: drop the long-double-128 powerpc check/hack

[Deucher, Alexander]

[Public]

> -----Original Message-----
> From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> On Behalf Of
> Daniel Kolesa
> Sent: Wednesday, February 1, 2023 1:46 PM
> To: linuxppc-dev at lists.ozlabs.org
> Cc: dan at danny.cz; stable at vger.kernel.org;
> tpearson at raptorengineering.com; amd-gfx at lists.freedesktop.org;
> alexdeucher at gmail.com; torvalds at linux-foundation.org
> Subject: [PATCH] drm/amdgpu: drop the long-double-128 powerpc
> check/hack
> 
> Commit c653c591789b ("drm/amdgpu: Re-enable DCN for 64-bit powerpc")
> introduced this check as a workaround for the driver not building with
> toolchains that default to 64-bit long double.
> 
> The reason things worked on 128-bit-long-double toolchains and not
> otherwise was however largely accidental. The real issue was that some files
> containing floating point code were compiled without -mhard-float, while
> others were compiled with -mhard-float.
> 
> The PowerPC compilers tag object files that use long doubles with a special
> ABI tag in order to differentiate 64-bit long double, IBM long double and IEEE
> 128-bit long double. When no long double is used in a source file, the file
> does not receive the ABI tag.
> Since only regular doubles are used in the AMDGPU source, there is no ABI
> tag on the soft-float object files with 128-bit-ldbl compilers, and therefore no
> error. With 64-bit long double, the double and long double types are equal,
> and an ABI tag is introduced.
> 
> Of course, this resulted in the real bug, which was mixing of hard and soft
> float object files, getting hidden, which makes this check technically
> incorrect.
> 
> Since then, work has been done to ensure that all float code is separately
> compiled. This was also necessary in order to enable
> AArch64 support in the display stack, as AArch64 does not have any soft-float
> ABI, and all code that does not explicitly conatain floats is compiled with -
> mgeneral-regs-only, which prevents float-using code from being compiled at
> all. That means AArch64 support will from now on always safeguard against
> such cases happening ever again.
> 
> In mainline, this work is now fully done, so this check is fully redundant and
> does not do anything except preventing AMDGPU DC from being built on
> systems such as those using musl libc. The last piece of work to enable this
> was commit c92b7fe0d92a
> ("drm/amd/display: move remaining FPU code to dml folder") and this has
> since been backported to 6.1 stable (in 6.1.7).
> 
> Relevant issue: https://gitlab.freedesktop.org/drm/amd/-/issues/2288
> 
> Signed-off-by: Daniel Kolesa <daniel at octaforge.org>

Acked-by: Alex Deucher <alexander.deucher at amd.com>

> ---
>  arch/powerpc/Kconfig                | 4 ----
>  drivers/gpu/drm/amd/display/Kconfig | 2 +-
>  2 files changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig index
> b8c4ac56b..267805072 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -289,10 +289,6 @@ config PPC
>  	# Please keep this list sorted alphabetically.
>  	#
> 
> -config PPC_LONG_DOUBLE_128
> -	depends on PPC64 && ALTIVEC
> -	def_bool $(success,test "$(shell,echo __LONG_DOUBLE_128__ |
> $(CC) -E -P -)" = 1)
> -
>  config PPC_BARRIER_NOSPEC
>  	bool
>  	default y
> diff --git a/drivers/gpu/drm/amd/display/Kconfig
> b/drivers/gpu/drm/amd/display/Kconfig
> index 2efe93f74..94645b6ef 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 || X86_64 || SPARC64 ||
> ARM64
>  	select SND_HDA_COMPONENT if SND_HDA_CORE
>  	# !CC_IS_CLANG:
> https://github.com/ClangBuiltLinux/linux/issues/1752
> -	select DRM_AMD_DC_DCN if (X86 || PPC_LONG_DOUBLE_128 ||
> (ARM64 && KERNEL_MODE_NEON && !CC_IS_CLANG))
> +	select DRM_AMD_DC_DCN if (X86 || PPC64 || (ARM64 &&
> KERNEL_MODE_NEON
> +&& !CC_IS_CLANG))
>  	help
>  	  Choose this option if you want to use the new display engine
>  	  support for AMDGPU. This adds required support for Vega and
> --
> 2.34.1