The most powerful contributor agreement

The general kernel policy is not to take code whose author doesn't want it contributed to the kernel, despite the GPL giving the project the right to do so. (In general, this is wise, since it's possible that some code was written for a Windows (or chip reference) driver and leaked to a third party, and the third party combined it with the GPL kernel without any license to some of the code, such that the author of the first part never had to accept the GPL and the kernel wouldn't have a proper license to that part of the code.)

Even when the code is structured such that it is clear that it must be licensed by the copyright holder to any recipients under the GPL, it's just seen as impolite to take code into the project against the author's wishes.