docs/licensing: Clarify wording about "GPL" and "Proprietary"

There are currently some doubts about out-of-tree kernel modules licensed
under GPLv3 and if they are supposed to be able to use symbols exported
using EXPORT_SYMBOL_GPL.

Clarify that "Proprietary" means anything non-GPL2 even though the
license might be an open source license. Also disambiguate "GPL
compatible" to "GPLv2 compatible".

Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241115103842.585207-2-ukleinek@kernel.org

Author: Uwe Kleine-König

Documentation/kernel-hacking/hacking.rst

@@ -587,7 +587,7 @@ Defined in ``include/linux/export.h``
 
 Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols
 exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by
-modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL
+modules with a :c:func:`MODULE_LICENSE()` that specifies a GPLv2
 compatible license. It implies that the function is considered an
 internal implementation issue, and not really an interface. Some
 maintainers and developers may however require EXPORT_SYMBOL_GPL()

Documentation/process/license-rules.rst

@@ -471,14 +471,16 @@ _`MODULE_LICENSE`
 				  source files.
 
     "Proprietary"		  The module is under a proprietary license.
-				  This string is solely for proprietary third
-				  party modules and cannot be used for modules
-				  which have their source code in the kernel
-				  tree. Modules tagged that way are tainting
-				  the kernel with the 'P' flag when loaded and
-				  the kernel module loader refuses to link such
-				  modules against symbols which are exported
-				  with EXPORT_SYMBOL_GPL().
+				  "Proprietary" is to be understood only as
+				  "The license is not compatible to GPLv2".
+                                  This string is solely for non-GPL2 compatible
+                                  third party modules and cannot be used for
+                                  modules which have their source code in the
+                                  kernel tree. Modules tagged that way are
+                                  tainting the kernel with the 'P' flag when
+                                  loaded and the kernel module loader refuses
+                                  to link such modules against symbols which
+                                  are exported with EXPORT_SYMBOL_GPL().
     ============================= =============================================