linux-kernel - [PATCH] drm/amdgpu: Re-enable DCN for 64-bit powerpc

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Date:   Mon, 25 Jul 2022 22:39:18 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     <linuxppc-dev@...ts.ozlabs.org>
Cc:     alexdeucher@...il.com, amd-gfx@...ts.freedesktop.org,
        linux@...ck-us.net, <torvalds@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>, dan@...ny.cz,
        tpearson@...torengineering.com
Subject: [PATCH] drm/amdgpu: Re-enable DCN for 64-bit powerpc

Commit d11219ad53dc ("amdgpu: disable powerpc support for the newer
display engine") disabled the DCN driver for all of powerpc due to
unresolved build failures with some compilers.

Further digging shows that the build failures only occur with compilers
that default to 64-bit long double.

Both the ppc64 and ppc64le ABIs define long double to be 128-bits, but
there are compilers in the wild that default to 64-bits. The compilers
provided by the major distros (Fedora, Ubuntu) default to 128-bits and
are not affected by the build failure.

There is a compiler flag to force 128-bit long double, which may be the
correct long term fix, but as an interim fix only allow building the DCN
driver if long double is 128-bits by default.

The bisection in commit d11219ad53dc must have gone off the rails at
some point, the build failure occurs all the way back to the original
commit that enabled DCN support on powerpc, at least with some
toolchains.

Depends-on: d11219ad53dc ("amdgpu: disable powerpc support for the newer display engine")
Fixes: 16a9dea110a6 ("amdgpu: Enable initial DCN support on POWER")
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2100
---
 arch/powerpc/Kconfig                | 4 ++++
 drivers/gpu/drm/amd/display/Kconfig | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

Alex, are you OK if I take this via the powerpc tree for v5.19?

cheers

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 7aa12e88c580..287cc2d4a4b3 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -281,6 +281,10 @@ config PPC
 	# Please keep this list sorted alphabetically.
 	#

+config PPC_LONG_DOUBLE_128
+	depends on PPC64
+	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 0ba0598eba20..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 && !(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
-- 
2.35.3