* \param cmd command.
* \param arg pointer to a resulting drm_auth structure.
* \return zero on success, or a negative number on failure.
*
* If there is a magic number in drm_file::magic then use it, otherwise
* searches an unique non-zero magic number and add it associating it with \p
* \param cmd command.
* \param arg pointer to a resulting drm_auth structure.
* \return zero on success, or a negative number on failure.
*
* If there is a magic number in drm_file::magic then use it, otherwise
* searches an unique non-zero magic number and add it associating it with \p