He is rather using the "pay what you want" model temporarily just because he felt he was speaking to a community that could understand the importance of his contribution. "Donate" is most probably there because it is customary to be used with paypal and open source developers.

At the same time, he has made the source available and he has contacted and passed the code to the kernel developers to be included for free in future versions.

Sorry for the off-topic, just wanted to mention that in case some people didn't read the whole story.