reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-52-g1cf3d98c-dirty
        # https://git.kernel.org/pub/scm/linux/kernel/git/vishal/linux.git/commit/?id=2e7ab04d7b9be08918c4166412b02550d545819c
        git remote add vishal https://git.kernel.org/pub/scm/linux/kernel/git/vishal/linux.git
        git fetch --no-tags vishal vv/dax_abi
        git checkout 2e7ab04d7b9be08918c4166412b02550d545819c
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=x86_64 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=x86_64 SHELL=/bin/bash drivers/dax/