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This now captures historical rationale along with cross-linking code templates with license guidelines. Closes #3962.
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107 lines
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.. SPDX-License-Identifier: CC-BY-SA-4.0
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.. Copyright (C) 2018.
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.. COMMENT: RTEMS Foundation, The RTEMS Documentation Project
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.. _LicensingRequirements:
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Licensing Requirements
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**********************
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All artifacts shall adhere to RTEMS Project licensing
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requirements. Currently, the preferred licenses are:
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* "Two Clause BSD" (BSD-2-Clause) for source code, and
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* CC-BY-SA-4.0 license for documentation
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Historically, RTEMS has been licensed under the GPL v2 with linking
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exception (https://www.rtems.org/license). It is preferred that new
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submissions be under one of the two preferred licenses. If you have
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previously submitted code to RTEMS under a historical license, please
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grant the project permission to relicense. See
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https://devel.rtems.org/ticket/3053 for details.
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For example templates for what to include in source code and
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documentation, see :ref:`FileHeaderCopyright`.
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Rationale
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---------
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.. COMMENT: Thanks to Gedare Bloom for his 2013 blog which
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.. COMMENT: discussed the rationale for RTEMS License section.
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.. COMMENT: http://gedare-csphd.blogspot.com/2013/05/software-licenses-with-rtems.html
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RTEMS is intended for use in real-time embedded systems in which the
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application is statically linked with the operating system and all
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support libraries. Given this use case, the RTEMS development team
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evaluated a variety of licenses with with the goal of promoting use
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while protecting both users and the developers.
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Using the GNU General Public License Version 2 (GPLv2) unmodified
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was considered but discarded because the GPL can only be linked statically
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with other GPL code. Put simply, linking your application code statically
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with GPL code would cause your code to become GPL code. This would force
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both licensing and redistribution requirements onto RTEMS users. This
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was completely unacceptable.
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The GNU Lesser General Public License Version 2 (LGPLv2) was also
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considered and deemed to not be a suitable license for RTEMS. This is
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because it either requires use of a shared library that can be re-linked,
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or release of the linked (application) code. This would require an
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RTEMS-based embedded system to provide a "relinking kit." Again, this
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license would force an unacceptable requirement on RTEMS users and deemed
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unacceptable.
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Newer versions of the GPL (i.e. version 3) are completely unsuitable
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for embedded systems due to the additions which add further restrictions
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on end user applications.
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The historical RTEMS `License <https://www.rtems.org/license>`_ is a
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modified version of the GPL version 2 that includes an exception to permit
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including headers and linking against RTEMS object files statically. This
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was based on the license used by GCC language runtime libraries at that
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time. This license allows the static linking of RTEMS with applications
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without forcing obligations and restrictions on users.
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A problem for RTEMS is there are no copyleft licenses that are compatible
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with the deployment model of RTEMS. Thus, RTEMS Project has to reject any
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code that uses the GPL or LGPL, even though RTEMS has historically appeared
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to use the GPL itself -- but with the exception for static linking, and also
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because an upstream GPL version 2 project could at any time switch to
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GPL version 3 and become totally unusable. In practice, RTEMS can only
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accept original code contributed under the RTEMS License and code that
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has a permissive license.
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As stated above, the RTEMS Project has defined its preferred licenses.
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These allow generation of documentation and software from specification
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as well as allow end users to statically link with RTEMS and not incur
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obligations.
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In some cases, RTEMS includes software from third-party projects. In those
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cases, the license is carefully evaluated to meet the project licensing
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goals. The RTEMS Project can only include software under licenses which follow
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these guidelines:
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* 2- and 3-clause BSD, MIT, and other OSI-approved non-copyleft licenses
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that permit statically linking with the code of different licenses
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are acceptable.
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* The historical RTEMS `License <https://www.rtems.org/license>`_ is
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acceptable for software already in the tree. This software is being
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relicensed to BSD-2-Clause, rewritten, or removed.
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* GPL licensed code is NOT acceptable, neither is LGPL.
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* Software which is dual-licensed in a manner which prevents free use
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in commercial applications is not acceptable.
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* Advertising obligations are not acceptable.
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* Some license restrictions may be permissible. These will be considered
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on a case-by-case basis.
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In practice, these guidelines are not hard to follow. Critically,
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they protect the freedom of the RTEMS source code and that of end users
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to select the license and distribution terms they prefer for their
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RTEMS-based application.
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