IPI is a key interface between PMU firmware and non-PMU entities on the SoC. PMU includes four Inter-Processor Interrupts (IPI) assigned to it and one set of buffers. PMU firmware uses IPI-0 and associated buffers for communication by default, which is initiated by other masters on SoC to PMU. PMU firmware uses IPI-1 and associated buffers for callbacks from PMU to other masters and for communication initiated by PMU firmware.
The following figure shows the IPI handling stack with interfaces between different components involved in this process. PMU firmware uses IPI driver to send and receive the messages. An IPI manager layer in Base Firmware is implemented over the driver and it takes care of dispatching the IPI message to the registered module handlers based on IPI ID in the first word of the message. The following table displays the message format for IPI.
| Word | Content | Description |
|---|---|---|
| 0 | Header | <target_module_id, api_id> |
| 1 | Payload | Module dependent payload |
| 2 | ||
| 3 | ||
| 4 | ||
| 5 | ||
| 6 | Reserved | Reserved - for future use |
| 7 | Checksum |
IPI-1 is used for the callbacks from PMU to other masters and for communication initiated by PMU firmware. Currently, PM and EM modules use IPIs and this can be taken as reference for implementing custom modules which require IPI messaging.
PMU firmware provides wrapper APIs around IPI driver functions to send and receive IPI messages. During initialization, PMU firmware initializes the IPI driver and enables IPI interrupt from the masters which are IPI assigned.