> Actually, I posted SPDX patch to ALSA ML, and one guy taught me
> that, Linus want to have C++ style comment for "all" source
> headers.

$ less Documentation/process/license-rules.rst

===
...

2. Style:

   The SPDX license identifier is added in form of a comment.  The comment
   style depends on the file type::

      C source: // SPDX-License-Identifier: <SPDX License Expression>
      C header: /* SPDX-License-Identifier: <SPDX License Expression> */
      ASM:      /* SPDX-License-Identifier: <SPDX License Expression> */
      scripts:  # SPDX-License-Identifier: <SPDX License Expression>
      .rst:     .. SPDX-License-Identifier: <SPDX License Expression>
      .dts{i}:  // SPDX-License-Identifier: <SPDX License Expression>

   If a specific tool cannot handle the standard comment style, then the
   appropriate comment mechanism which the tool accepts shall be used. This
   is the reason for having the "/\* \*/" style comment in C header
   files. There was build breakage observed with generated .lds files where
   'ld' failed to parse the C++ comment. This has been fixed by now, but
   there are still older assembler tools which cannot handle C++ style
   comments.

...
===

checkpatch also warns if you use C++ comment style in headers.