reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git/commit/?id=8694697a54096ae97eb38bf4144f2d96c64c68f2
        git remote add pci https://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git
        git fetch --no-tags pci enumeration
        git checkout 8694697a54096ae97eb38bf4144f2d96c64c68f2
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash drivers/mfd/