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

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Date:   Mon, 25 Jul 2022 22:25:19 +1000
From:   Michael Ellerman <michael@...erman.id.au>
To:     Dan Horák <dan@...ny.cz>
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

Dan Horák <dan@...ny.cz> writes:
> On Fri, 22 Jul 2022 22:32:06 +1000
> Michael Ellerman <michael@...erman.id.au> wrote:
>> 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?
>> 
>> 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/ :-)

Oops, renamed it after testing :}

> 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.

Thanks. I'll send a proper patch.

cheers