reproduce (this is a W=1 build):
        git remote add chrome-os https://chromium.googlesource.com/chromiumos/third_party/kernel
        git fetch --no-tags chrome-os chromeos-6.1
        git checkout f5681d0019946b46effc39bc6501c1ed02f65162
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash drivers/dma-buf/heaps/