12.5. Embedded Controller Firmware¶
The embedded controller firmware must obey the following rules in order to be ACPI-compatible:
SMI Processing. Although it is not explicitly stated in the command specification section, a shared embedded controller interface has a separate command set for communicating with each environment it plans to support. In other words, the embedded controller knows which environment is generating the command request, as well as which environment is to be notified upon event detection, and can then generate the correct interrupts and notification values. This implies that a system management handler uses commands that parallel the functionality of all the commands for ACPI including query, read, write, and any other implemented specific commands.
SCI/SMI Task Queuing. If the system design is sharing the interface between both a system management interrupt handler and OSPM, the embedded controller should always be prepared to queue a notification if it receives a command. The embedded controller only sets the appropriate event flag in the status (EC_SC) register if the controller has detected an event that should be communicated to the OS or system management handler. The embedded controller must be able to field commands from either environment without loss of the notification event. At some later time, the OS or system management handler issues a query command to the embedded controller to request the cause of the notification event.
Notification Management. The use of the embedded controller means using the query (QR_EC) command to notify OSPM of system events requiring action. If the embedded controller is shared with the operating system, the SMI handler uses the SMI_EVT flag and an SMI query command (not defined in this document) to receive the event notifications. The embedded controller doesn’t place event notifications into the output buffer of a shared interface unless it receives a query command from OSPM or the system management interrupt handler.