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

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Date:   Mon, 25 Jul 2022 17:45:59 +0200
From:   Dan Horák <dan@...ny.cz>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     <linuxppc-dev@...ts.ozlabs.org>, alexdeucher@...il.com,
        amd-gfx@...ts.freedesktop.org, linux@...ck-us.net,
        <torvalds@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
        tpearson@...torengineering.com
Subject: Re: [PATCH] drm/amdgpu: Re-enable DCN for 64-bit powerpc

On Mon, 25 Jul 2022 22:39:18 +1000
Michael Ellerman <mpe@...erman.id.au> wrote:

> 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

LGTM

Reviewed-by: Dan Horák <dan@...ny.cz>


		Dan

> ---
>  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
>