libcamera: Summarize licensing terms in COPYING.rst

Storing detailed licensing information in SPDX headers and in the DEP5
file gives precise information to handle license compliance, but lacks a
high-level overview. Summmarize the licensing terms in a COPYING.rst
file to facilitate understanding of the libcamera project licenses, and
to clarify that closed-source third-party IPA modules are permitted.

Signed-off-by: Laurent Pinchart <[email protected]>
Reviewed-by: Kieran Bingham <[email protected]>
Acked-by: Niklas Söderlund <[email protected]>
Reviewed-by: Tomasz Figa <[email protected]>

COPYING.rst

@@ -0,0 +1,71 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+==========
+ Licenses
+==========
+
+TL;DR summary: The libcamera core is covered by the LGPL-2.1-or-later license.
+IPA modules included in libcamera are covered by a free software license.
+Third-parties may develop IPA modules outside of libcamera and distribute them
+under a closed-source license, provided they do not include source code from
+the libcamera project.
+
+The libcamera project contains multiple libraries, applications and utilities.
+Licenses are expressed through SPDX tags in text-based files that support
+comments, and through the .reuse/dep5 file otherwise. A copy of all licenses is
+stored in the LICENSES directory.
+
+The following text summarizes the licenses covering the different components of
+the project to offer a quick overview for developers. The SPDX and DEP5
+information are however authoritative and shall prevail in case of
+inconsistencies with the text below.
+
+The libcamera core source code, located under the include/libcamera/ and
+src/libcamera/ directories, is fully covered by the LGPL-2.1-or-later license,
+which thus covers distribution of the libcamera.so binary. Other files located
+in those directories, most notably the meson build files, and various related
+build scripts, may be covered by different licenses. None of their source code
+is incorporated in the in the libcamera.so binary, they thus don't affect the
+distribution terms of the binary.
+
+The IPA modules, located in src/ipa/, are covered by free software licenses
+chosen by the module authors. The LGPL-2.1-or-later license is recommended.
+Those modules are compiled as separate binaries and dynamically loaded by the
+libcamera core at runtime.
+
+The IPA module API is defined in headers located in include/libcamera/ipa/ and
+covered by the LGPL-2.1-or-later license. Using the data types (including
+classes, structures and enumerations) and macros defined in the IPA module and
+libcamera core API headers in IPA modules doesn't extend the LGPL license to
+the IPA modules. Third-party closed-source IPA modules are thus permitted,
+provided they comply with the licensing requirements of any software they
+include or link to.
+
+The libcamera Android camera HAL component is located in src/android/. The
+libcamera-specific source code is covered by the LGPL-2.1-or-later license. The
+component additionally contains header files and source code, located
+respectively in include/android/ and src/android/metadata/, copied verbatim
+from Android and covered by the Apache-2.0 license.
+
+The libcamera GStreamer and V4L2 adaptation source code, located respectively
+in src/gstreamer/ and src/v4l2/, is fully covered by the LGPL-2.1-or-later
+license. Those components are compiled to separate binaries and do not
+influence the license of the libcamera core.
+
+The cam and qcam sample applications, as well as the unit tests, located
+respectively in src/cam/, src/qcam/ and test/, are covered by the
+GPL-2.0-or-later license. qcam additionally includes an icon set covered by the
+MIT license. Those applications are compiled to separate binaries and do not
+influence the license of the libcamera core.
+
+Additional utilities are located in the utils/ directory and are covered by
+various licenses. They are not part of the libcamera core and do not influence
+its license.
+
+Finally, copies of various Linux kernel headers are included in include/linux/
+to avoid depending on particular versions of those headers being installed in
+the system. The Linux kernel headers are covered by their respective license,
+including the Linux kernel license syscall exception. Using a copy of those
+headers doesn't affect libcamera licensing terms in any way compared to using
+the same headers installed in the system from kernel headers packages provided
+by Linux distributions.