reproduce (this is a W=1 build):
        # https://github.com/kdave/btrfs-devel/commit/fda37caa264ff597b0fc03039dc95e392432bbc5
        git remote add kdave-btrfs-devel https://github.com/kdave/btrfs-devel.git
        git fetch --no-tags kdave-btrfs-devel ext/dv/fscrypt
        git checkout fda37caa264ff597b0fc03039dc95e392432bbc5

        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # apt-get install sparse
        # sparse: v0.6.5-rc1

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-8.5.0 ~/lkp-tests/kbuild/make.cross W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=alpha olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-8.5.0 ~/lkp-tests/kbuild/make.cross W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=alpha SHELL=/bin/bash fs/btrfs/