For those still running a Nokia N900, the Linux 3.16 kernel lands a modem driver for the phone. With this morning's HSI pull request for Linux 3.16 kernel comes the N900 modem driver called nokia-modem. The N900 modem is connected via a Synchronous Serial Interface (SSI) as the legacy version of HSI (High-Speed Synchronous Serial Interface). This basic driver handles enabling and resetting the modem along with data exchange but doesn't yet support voice data.
This HSI client driver for the Nokia N900 modem was written this year by Sebastian Reichel. This new Nokia modem driver along with a few other changes can be found with the 3.16 HSI pull request.