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

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

[Michael Ellerman]

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

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

[Arnd Bergmann]

On Mon, May 16, 2022 at 2:17 PM Michael Ellerman <mpe@ellerman.id.au> wrote:
> 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.

I would recommend against that. It's always a bit hacky, and I think this was
mainly done on x86 to avoid breaking user workflows after arch/i386
and arch/x86_64
got merged.

Since there was never an arch/ppc64le, and arch/{ppc,ppc64}/ are gone for so
long, I see no point in bringing back those interfaces, just use the right
defconfig for what you want.

        Arnd

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

[Arnout Vandecappelle]

On 16/05/2022 15:17, Michael Ellerman wrote:
> 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.

  Yes, which is pretty much unavoidable.

> 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.

  I don't know if it's worth bothering with that. It certainly would not make 
our life easier, because it would mean we need to set ARCH correctly. If we can 
do that, we can just as well set the defconfig correctly.

> 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.

  I think the most important thing is that it makes no sense to rely on uname 
when ARCH and/or CROSS_COMPILE are set.

  Regards,
  Arnout

> 
> cheers

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

[Michael Ellerman]

Arnout Vandecappelle <arnout@mind.be> writes:
> On 16/05/2022 15:17, Michael Ellerman wrote:
>> 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.
>
>   Yes, which is pretty much unavoidable.
>
>> 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.
>
>   I don't know if it's worth bothering with that. It certainly would not make 
> our life easier, because it would mean we need to set ARCH correctly. If we can 
> do that, we can just as well set the defconfig correctly.

OK.

>> 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.
>
>   I think the most important thing is that it makes no sense to rely on uname 
> when ARCH and/or CROSS_COMPILE are set.

I'm not sure I entirely agree.

Neither ARCH or CROSS_COMPILE give us enough information to know which
defconfig to use, so we still have to guess somehow.

CROSS_COMPILE can be set even when you're building on ppc, it's the
easiest way to specfiy a different toolchain from the default.

cheers