Hi Thomas, First bad commit (maybe != root cause): tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master head: 9ccce092fc64d19504fa54de4fd659e279cc92e7 commit: 396c7d94c3bbe07ae00c9f694a5c731878843038 MIPS: alchemy: Fix build breakage, if TOUCHSCREEN_WM97XX is disabled date: 7 months ago config: mips-randconfig-r023-20210503 (attached as .config) compiler: mipsel-linux-gcc (GCC) 9.3.0 reproduce (this is a W=1 build): wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross chmod +x ~/bin/make.cross # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=396c7d94c3bbe07ae00c9f694a5c731878843038 git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git git fetch --no-tags linus master git checkout 396c7d94c3bbe07ae00c9f694a5c731878843038 # save the attached .config to linux build tree COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross W=1 ARCH=mips If you fix the issue, kindly add following tag as appropriate Reported-by: kernel test robot All warnings (new ones prefixed by >>): >> arch/mips/boot/compressed/uart-alchemy.c:4:6: warning: no previous prototype for 'putc' [-Wmissing-prototypes] 4 | void putc(char c) | ^~~~ vim +/putc +4 arch/mips/boot/compressed/uart-alchemy.c aae7e8da806e15 Manuel Lauss 2010-01-13 3 aae7e8da806e15 Manuel Lauss 2010-01-13 @4 void putc(char c) :::::: The code at line 4 was first introduced by commit :::::: aae7e8da806e1536fdee0632a40aaaeed97a2f1b MIPS: Alchemy: debug output for compressed kernels :::::: TO: Manuel Lauss :::::: CC: Ralf Baechle --- 0-DAY CI Kernel Test Service, Intel Corporation https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org