reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-66-g0196afe1
        # https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd.git/commit/?id=62ee7cf235fa52381d3170035469abfd1057c6f7
        git remote add jarkko-tpmdd https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd.git
        git fetch --no-tags jarkko-tpmdd v4l2-loopback
        git checkout 62ee7cf235fa52381d3170035469abfd1057c6f7
        # 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 drivers/media/v4l2-core/