USB-C DisplayPort Alt Mode on Rockchip RK3588/RK3576, Starting with Flipper One

This explains how Collabora enabled USB-C DisplayPort Alternate Mode on Rockchip RK3588/RK3576 silicon to let a single USB-C port drive a display (demonstrated on the Flipper One). The core is USB Power Delivery (USB‑PD), which negotiates orientation, power and announces alternate modes via vendor-defined messages (VDMs). Rockchip SoCs pair a Synopsys DWC3 USB3 controller and a DesignWare DisplayPort controller that share a combined PHY (USBDP), allowing the board to swap orientation and multiplex four differential pairs between USB3 and DisplayPort without external analog switches. An external USB‑PD controller (e.g., fusb302) is required for CC signaling while the kernel’s TCPM state machine performs PD negotiation and AltMode handshakes. Device tree bindings describe the connector, altmodes, and USBDP ports so the kernel can route lanes correctly.

The software flow runs three TCPM phases: initial PD negotiation (roles, voltage/current), optional role swaps, then SVID/VDM queries to enter AltMode and probe typec_displayport. Successful entry exposes sysfs entries (pin_assignment, configuration, hpd) and triggers USBDP’s mode-switch and orientation-switch per DT flags. Because USB‑C lacks a physical HPD pin, DP hotplug state is carried in PD VDMs and the Rockchip DP controller’s HPD multiplexer selects a register-fed HPD source. The result is a working DP AltMode implementation on RK3588/RK3576 that routes lanes and HPD over USB‑PD so the Flipper One can drive an external screen from one USB‑C port.