A modern cellular modem and an application processor share the same PMIC. Both are SPMI masters. If the AP requests 1.1V for a rail while the modem requests 1.2V, the SPMI arbitration ensures one wins, or the system resolves policy externally.
These master and slave nodes can reside on a single integrated circuit, across separate discrete chips, or within a hybrid multi-chip module. Operating Speeds and Device Classes spmi bus
int ret = spmi_register_write(pmic->spmi, slave_id, reg_addr, &value, 1); A modern cellular modem and an application processor