The TF-A registers for IPI interrupts during boot. The PLM sends IPI to TF-A when any event occurs along with event data in IPI payload. TF-A then sends an SGI to Linux to notify that an event has occurred. This SGI number is provided by Linux during the event manager driver initialization using IOCTL EEMI call (IOCTL_SET_SGI).
The TF-A 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 the IPI data.