Re: [Buildroot] [PATCH] linux: Fix powerpc64le defconfig selection

[Michael Ellerman <mpe@ellerman.id.au>]

Arnout Vandecappelle <arnout@mind.be> writes:
> On 10/05/2022 04:20, Joel Stanley wrote:
>> The default defconfig target for the 64 bit powerpc kernel is
>> ppc64_defconfig, the big endian configuration.
>> 
>> When building for powerpc64le users want the little endian kernel as
>> they can't boot LE userspace on a BE kernel.
>> 
>> Fix up the defconfig used in this case. This will avoid the following
>> autobuilder failure:
>> 
>>   VDSO32A arch/powerpc/kernel/vdso32/sigtramp.o
>>   cc1: error: ‘-m32’ not supported in this configuratioin
>>   make[4]: *** [arch/powerpc/kernel/vdso32/Makefile:49: arch/powerpc/kernel/vdso32/sigtramp.o] Error 1
>> 
>>   http://autobuild.buildroot.net/results/dd76d53bab56470c0b83e296872d7bb90f9e8296/
>> 
>> Note that the failure indicates the toolchain is configured to disable
>> the 32 bit target, causing the kernel to fail when building the 32 bit
>> VDSO. This is only a problem on the BE kernel as the LE kernel disables
>> CONFIG_COMPAT, aka 32 bit userspace support, by default.
>> 
>> Signed-off-by: Joel Stanley <joel@jms.id.au>
>
>   Applied to master, thanks. However, the defconfig mechanism for *all* powerpc 
> seems pretty broken. Here's what we have in 5.16, before that there was 
> something similar:
>
> # If we're on a ppc/ppc64/ppc64le machine use that defconfig, otherwise just use
> # ppc64_defconfig because we have nothing better to go on.
> uname := $(shell uname -m)
> KBUILD_DEFCONFIG := $(if $(filter ppc%,$(uname)),$(uname),ppc64)_defconfig
>
>   So I guess we should use a specific defconfig for *all* powerpc.
>
>   The arch-default defconfig is generally not really reliable, for example for 
> arm it always takes v7_multi, but that won't work for v7m targets...

There's a fundamental problem that just the "arch" is not sufficient
detail when you're building a kernel.

Two CPUs that implement the same user-visible "arch" may differ enough
at the kernel level to require a different defconfig.

Having said that I think we could handle this better in the powerpc
kernel. Other arches allow specifying a different value for ARCH, which
then is fed into the defconfig.

That way you could at least pass ARCH=ppc/ppc64/ppc64le, and get an
appropriate defconfig.

I'll work on some kernel changes for that.

cheers