The ATF registers for the IPI interrupts during boot. The PLM sends IPI to the ATF when any event occurs along with event data in IPI payload. The ATF then sends an SGI to Linux to notify that some event has occurred. This SGI number is provided by Linux during Event Manager driver initialization using IOCTL EEMI call (IOCTL_SET_SGI).
The ATF provides a callback through which Linux can read the IPI data to identify the event that has occurred and sends an IPI acknowledgment to the PLM when Linux reads IPI data.