ppc64 crossbuild fails

[naguam]

Hello,

I am not yet knowledgeable on kernel internals, however I was curious about building a kernel with clang.

I followed instructions here https://docs.kernel.org/kbuild/llvm.html

And made my tests on a clean debian stable vm with llvm/clang 14 and 18 (the later from apt-llvm)

The kernel 6.7.5 built perfectly with clang for my native x86 pc and crossbuild with ARCH=riscv

However the ppc64 build failed on both :

make LLVM=1 ARCH=powerpc defconfig
make LLVM=1 ARCH=powerpc -j4

(debian system clang 14)

and

make LLVM=-18 ARCH=powerpc defconfig
make LLVM=-18 ARCH=powerpc -j4

(apt llvm)

I hope I gave enough info to reproduce.

(I used apt build-dep linux to install the dependencies and ensured the kernel could be built with gcc. so not a dependency problem)

I am happy to contribute at my level,

(And sorry I don't have the build log/error anymore, but it is very fast to reproduce)

[naguam]

Ok in a debian container where I am now I was able to reproduce an output for llvm-14 and llvm-18 which have the same error (I tried llvm-18 thinking that for other arch than x86, llvm minimum versions could be higher)

objcopy: Unable to recognise the format of the input file `vmlinux'
make[2]: *** [arch/powerpc/boot/Makefile:399: arch/powerpc/boot/zImage.pseries] Error 1
make[1]: *** [arch/powerpc/Makefile:238: zImage] Error 2

[nathanchance]

Thanks for the report! With ARCH=powerpc, there is still a dependency on GNU binutils in arch/powerpc/boot/wrapper, so you will need to install binutils-powerpc64le-linux-gnu and provide CROSS_COMPILE=powerpc64le-linux-gnu- to your make command (#1601)

[naguam]

Ok thanks I did not see that issue.

Maybe the Linux kernel documentation should be changed so it does not state LLVM=1 in the chart :).

[nathanchance]

Well LLVM=1 works for the main kernel build up through vmlinux and modules, so I don't think recommending CC=clang is fair, but I agree the documentation could be clearer around this situation... I'll think about how to do that.