reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/688297d25e0f7174014a30f873b36632eae65c38
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe pcie_switch_groups
        git checkout 688297d25e0f7174014a30f873b36632eae65c38
        # 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 kernel/dma/