Replicant

Yes, it's related to the modem. It took me quite some time to get rid of the reboots. There are multiple causes. The modem is able to request a reboot. This happens for example when something went wrong during the firmware upload or when there is an error during EFS sync. As the modem boot code is not yet part of the Replicant images, the reason here is a different one. The kernel creates a list of power management devices that should include the modem interfaces. But those aren't created because the modem is never booted and stays in Download Mode. After a timeout, the kernel triggers a reboot.

The fix for this issue would be to make the modem boot program described in QMI-RIL part of QMI-RIL as a separate thread and get QMI-RIL ready for inclusion in Replicant. A shortcut would be to make the modem-boot program run as a daemon, much like its proprietary counterpart, but I'd like to mirror the design of Samsung-RIL which gives us more control and cleaner and less buggy code in the long run.
The kernel patches from my qmi branch are needed as well.