reproduce (this is a W=1 build):
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # https://github.com/jgunthorpe/linux/commit/2c39e478f1bbc1680ad47e8b418bbd76701a4496
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout 2c39e478f1bbc1680ad47e8b418bbd76701a4496
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=hexagon olddefconfig
        ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=hexagon SHELL=/bin/bash drivers/iommu/