linux-kernel - Re: [PATCH] amdgpu: re-enable DCN for ppc64le

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Message-Id: <20220722145453.eb37bd3a99c4b738ed2e26b9@danny.cz>
Date:   Fri, 22 Jul 2022 14:54:53 +0200
From:   Dan Horák <dan@...ny.cz>
To:     Michael Ellerman <michael@...erman.id.au>
Cc:     amd-gfx@...ts.freedesktop.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-kernel@...r.kernel.org, Alex Deucher <alexdeucher@...il.com>,
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] amdgpu: re-enable DCN for ppc64le

On Fri, 22 Jul 2022 22:32:06 +1000
Michael Ellerman <michael@...erman.id.au> wrote:

> Hi Dan,
> 
> [ Cc += linuxppc-dev  ]
> 
> Dan Horák <dan@...ny.cz> writes:
> > Commit d11219ad53dc disabled the DCN driver for all platforms that
> > define PPC64 due long build issues during "make allmodconfig" using
> > cross-compilation. Cross-compilation defaults to the ppc64_defconfig
> > and thus big-endian toolchain configuration. The ppc64le platform uses a
> > different ABI and doesn't suffer from the build issues.
> 
> Unfortunately it's a bit messier than that.
> 
> The build error occurs when the compiler is built to use a 64-bit long
> double type.
> 
> The ppc64le ABI document says that long double should be 128-bits, but
> there are ppc64le compilers out there that are configured to use 64-bit
> long double, notably the kernel.org crosstool compilers.
> 
> So just testing for CPU_LITTLE_ENDIAN means we'll still get build errors
> on those compilers.
> 
> But I think we can detect the long double size and key off that. Can you
> test the patch below works for you?
> 
> cheers
> 
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 7aa12e88c580..e9f8cd50af99 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -281,6 +281,9 @@ config PPC
>  	# Please keep this list sorted alphabetically.
>  	#
>  
> +config PCC_LONG_DOUBLE_128
> +	def_bool $(success,test "$(shell,echo __LONG_DOUBLE_128__ | $(CC) -E -P -)" = 1)

^^^ there is a typo s/PCC/PPC/ :-)

with that fixed, it then defines AMD_DC_DCN on Fedora 36 with
gcc-12.1.1-1.fc36.ppc64le and we should be OK.


		Dan

> +
>  config PPC_BARRIER_NOSPEC
>  	bool
>  	default y
> diff --git a/drivers/gpu/drm/amd/display/Kconfig b/drivers/gpu/drm/amd/display/Kconfig
> index b4029c0d5d8c..ec6771e87e73 100644
> --- a/drivers/gpu/drm/amd/display/Kconfig
> +++ b/drivers/gpu/drm/amd/display/Kconfig
> @@ -6,7 +6,7 @@ config DRM_AMD_DC
>  	bool "AMD DC - Enable new display engine"
>  	default y
>  	select SND_HDA_COMPONENT if SND_HDA_CORE
> -	select DRM_AMD_DC_DCN if (X86 || PPC64) && !(KCOV_INSTRUMENT_ALL && KCOV_ENABLE_COMPARISONS)
> +	select DRM_AMD_DC_DCN if (X86 || PPC_LONG_DOUBLE_128) && !(KCOV_INSTRUMENT_ALL && KCOV_ENABLE_COMPARISONS)
>  	help
>  	  Choose this option if you want to use the new display engine
>  	  support for AMDGPU. This adds required support for Vega and