By default, all original contributions are assumed to be under GPL-2.0+. However, we must be sure that content under an incompatible license has not been copied from another project. Also, newcomers must know this check has been done.

All files should have copyright information. It is general convention to include a full notice, but we only need these lines:

// Copyright (c) 1970 Jane Doe <jane@example.org> et al.
// License: GPL-2.0+

Copyright
This line states that the file is copyrighted and that we do not tolerate innocent infringement. A valid copyright notice needs:

1. Symbol © or Copyright (c).
2. The year of first publication.
3. The name or pseudonym of an author. Email address is optional. et al. (which means "and others") is optional if there is only one author, but it is recommended because this is a collaborative project.

License
This line states which license applies. After License: there should be a valid SPDX license expression.