reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git/commit/?id=6d1abeffbce3bf1c78052c72b02530c421e2e4e8
        git remote add mcgrof https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git
        git fetch --no-tags mcgrof blk-iobuf-pool-v2
        git checkout 6d1abeffbce3bf1c78052c72b02530c421e2e4e8
        # apt-get install sparse
        # sparse: v0.6.5-rc1

        # 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=i386 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=i386 SHELL=/bin/bash fs/iomap/