The move follows a recent announcement by many kernel developers about licence enforcement.
The Linux kernel, which is used widely by the four companies named, is released under the GNU General Public Licence version 2.0. A later version of this licence includes an approach that offers users an opportunity to comply with the licence.
The kernel developers made their announcement due to efforts being made by some to enforce the licence more vigorously. Companies that violate the GPL often do so because of inadvertent errors and the developers wanted to be more reasonable in enforcing the licence under which their code is released.
"Before filing or continuing to prosecute any legal proceeding or claim (other than a Defensive Action) arising from termination of a Covered Licence, [Company] commits to extend to the person or entity ('you') accused of violating the Covered License the following provisions regarding cure and reinstatement, taken from GPL version 3. As used here, the term ‘this License’ refers to the specific Covered License being enforced.
"However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your licence, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
"Moreover, your licence from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this Licence (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
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"[Company] may modify this Commitment by publishing a new edition on this page or a successor location."