title: How do I specify header files for make headers_install in an out-of-tree kernel module?
body: <p>I couldn't find any information regarding this in kbuild <a href="https://www.kernel.org/doc/Documentation/kbuild/modules.txt" rel="nofollow">modules</a> documentation and only an oblique reference (suggesting that it's possible) in the <a href="https://www.kernel.org/doc/Documentation/kbuild/headers_install.txt" rel="nofollow">headers_install</a> documentation. </p>
  
  <blockquote>
    <p>The "make headers_install" command can be run in the top level directory of the
    kernel source code (or using a standard out-of-tree build).</p>
  </blockquote>
  
  <p>I have an out-of-tree kernel module with an associated header file (I have #defines for an IOCTL), and I want to provide that header file as part of the module installation. It seems like the in-tree way for kernel modules to provide header files (for userspace use) is to place them in include/linux/. What do I have to do for this to work for out-of-tree?</p>