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

[Joel Stanley <joel@jms.id.au>]

On Fri, 13 May 2022 at 21:52, Arnout Vandecappelle <arnout@mind.be> wrote:
>
>
>
> 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

Yes, mpe and I discussed this. x86 does a similar thing:

ifeq ($(ARCH),x86)
  ifeq ($(shell uname -m),x86_64)
        KBUILD_DEFCONFIG := x86_64_defconfig
  else
        KBUILD_DEFCONFIG := i386_defconfig
  endif
else
        KBUILD_DEFCONFIG := $(ARCH)_defconfig
endif

If you were building on an eg. Arm or PowerPC host with ARCH=x86, you
would get unexpected results.

>
>   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...

I noticed that. It is similarly broken if you expected a kernel that
worked on armv5.

Should we disable BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG for
platforms that don't have a single defconfig?

The other option I considered was removing ARCH_DEFAULT_CONFIG and
providing a default for platforms where we could suggest one. It would
look something like this:

--- a/linux/Config.in
+++ b/linux/Config.in
@@ -176,10 +176,15 @@ endchoice
 config BR2_LINUX_KERNEL_DEFCONFIG
        string "Defconfig name"
        depends on BR2_LINUX_KERNEL_USE_DEFCONFIG
+       default "ppc64le_defconfig" if BR2_powerpc_powerpc64le
+       default "ppc64_defconfig" if BR2_powerpc_powerpc64
+       default "ppc_defconfig" if BR2_powerpc_powerpc
+       default "mutli_v5_defconfig" if BR2_ARM_CPU_ARMV5
+       default "mutli_v7_defconfig" if BR2_ARM_CPU_ARMV7A
+       default "defconfig"
        help
-         Name of the kernel defconfig file to use, without the
-         trailing _defconfig.  The defconfig is located in
-         arch/<arch>/configs in the kernel tree.
+         Name of the kernel defconfig file to use. The defconfig is located in
+         arch/<arch>/configs in the kernel tree

It would need to change the option to include the _defconfig (in order
to support plain old "defconfig"), or we could add some logic in the
.mk to add _defconfig only if the name is not "defconfig".
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