Perform the following steps to program MST AUX Messaging:
- Wait for DOWN_REQUEST_BUFFER_READY status in interrupt, and read from DOWN_REQUEST_BUFFER. Continue to collect side-band messages as per DisplayPort Standard v1.2a Section 2.1.11.9. After a complete sideband message is received, the software processes the message and writes the reply to DOWN_REPLY_BUFFER.
- After the response is written, set DOWN Reply Buffer Message to 1 in the Remote Command New register.
- Wait for DOWN_REPLY_BUFFER_READ status in interrupts and continue writing
responses.
During the MST AUX messaging phase, the required PBN (available BW) is calculated and sent to the source. The source then sends allocation requests based on available bandwidth. Internally, Sink HW updates the VC Payload Table by monitoring the AUX transactions. Alternatively, if you are an advanced user, you can use a software control to the VC Payload Table (Setting the bit 1 in 0x0D0 enables it), with which the software maintains a VC Payload manager (by monitoring the interrupt bit 28 of 0x014 register and 0x06C register) and writes the resulting stream allocations to 0x800-0x8FF. Once the software finishes writing to the VC payload table, software has to set bit 4 in 0x0D0.