DP AUX Protocol Support

The DisplayPort AUX (Auxiliary) channel is a half-duplex bidirectional serial interface used for sideband communication in DisplayPort and eDP (embedded DisplayPort) connections. Operating at 1 MHz Manchester-encoded signaling on a single differential pair (AUX_CH_P and AUX_CH_N), the AUX channel handles critical functions including EDID/DisplayID reading, DPCD (DisplayPort Configuration Data) register access, link training negotiation, content protection (HDCP) authentication, and MST (Multi-Stream Transport) sideband messaging. The source device (GPU or display controller) initiates transactions by writing to or reading from DPCD address space on the sink device (monitor or display panel). Link training — the process of establishing the main video link at the optimal lane count and bit rate — relies entirely on AUX channel communication to configure and verify link parameters. The AUX channel is present in all DisplayPort, mini-DisplayPort, USB-C DisplayPort Alternate Mode, and eDP connections, making it ubiquitous in monitors, laptops, tablets, docking stations, and automotive displays. Protocol analysis for the AUX channel is critical because display connectivity problems often originate in the AUX handshake — failed link training, EDID read errors, and HDCP authentication failures all manifest as blank screens or degraded video quality. Engineers must decode AUX transactions to diagnose the root cause of display issues.