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From:   Michael Ellerman <michael@ellerman.id.au>
To:     Dan =?utf-8?Q?Hor=C3=A1k?= <dan@danny.cz>,
        amd-gfx@lists.freedesktop.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Guenter Roeck <linux@roeck-us.net>,
        linux-kernel@vger.kernel.org
Cc:     Alex Deucher <alexdeucher@gmail.com>,
        <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH] amdgpu: re-enable DCN for ppc64le
In-Reply-To: <20220722082122.571974-1-dan@danny.cz>
References: <20220722082122.571974-1-dan@danny.cz>
Date:   Fri, 22 Jul 2022 22:32:06 +1000
Message-ID: <87o7xhcoqh.fsf@mpe.ellerman.id.au>
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Hi Dan,

[ Cc += linuxppc-dev  ]

Dan Horák <dan@danny.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)
+
 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