DisplayPort MST Topology Manager and Monitor Mapping

This article explains the core functions of the Topology Manager within a DisplayPort Multi-Stream Transport (MST) architecture. It covers the discovery mechanisms, sideband messaging protocols, and virtual channel allocations that allow a single DisplayPort source to identify, map, and manage multiple downstream monitors across a daisy-chain or hub setup.

What is a Topology Manager?

The Topology Manager is a core component within the DisplayPort source device—typically integrated into the GPU's display driver or graphics firmware. In standard DisplayPort Single-Stream Transport (SST), a source simply connects to a single display endpoint. In contrast, DisplayPort MST allows a single physical port to route independent video and audio streams to multiple displays. The Topology Manager is the governing system responsible for discovering the physical network structure, tracking connection changes, parsing device capabilities, and allocating link bandwidth across that shared interface.

The DisplayPort Configuration Data (DPCD) Handshake

When a device connects to a DisplayPort port, the Topology Manager initiates a handshake using the bidirectional Auxiliary (AUX) Channel. It first reads the DisplayPort Configuration Data (DPCD) registers of the directly connected downstream device. By checking specific registers (notably MSTM_CAP), the Topology Manager determines whether the connected hardware is an MST-capable "Branch Device" (such as a multi-port hub or a monitor with an MST out port) or an SST "Sink Device" (a standard display).

Once MST support is confirmed, the Topology Manager enables MST mode on the link and prepares to map the broader network.

Device Discovery via Sideband Messaging

Because downstream monitors might be connected several hops away through multiple hubs or daisy-chained screens, the Topology Manager cannot communicate with them purely through direct pin-to-pin signals. Instead, it relies on the MST Sideband Messaging Protocol carried over the AUX channel.

The mapping process unfolds through specific steps:

  1. Link Address Queries: The Topology Manager transmits a LINK_ADDRESS message to the primary branch device. The device responds with a list of its input and output ports, the status of each port (connected, disconnected, or non-functional), and whether each port leads to another branch device or a terminal sink device (a monitor).
  2. Recursive Traversal: If a connected port leads to another branch device, the Topology Manager issues another LINK_ADDRESS command targeting that specific downstream branch. This recursive query continues through every branch until every active endpoint in the chain is accounted for.
  3. Assigning Relative Addresses (RAD): To route messages accurately, the Topology Manager uses Relative Addressing (RAD). A device's RAD acts as a navigational path through the tree. For instance, a monitor plugged into Port 2 of a hub that is connected to Port 1 of the primary hub receives an address indicating that exact sequence of port hops.

Querying Monitor Capabilities (EDID)

Once the network tree is drawn, the Topology Manager queries each mapped endpoint using REMOTE_I2C_READ messages. This allows the host to read the Extended Display Identification Data (EDID) from every individual monitor across the sideband link. Through the EDID, the Topology Manager discovers each monitor’s manufacturer, supported resolutions, refresh rates, and color formats.

Bandwidth Allocation and Virtual Channels

The physical DisplayPort link between the GPU and the first branch device has a finite bandwidth limit dictated by the link rate (e.g., HBR2, HBR3, UHBR) and lane count. The total bandwidth of this physical link is divided into 64 time slots, quantified using Payload Bandwidth Numbers (PBN).

Using the EDID information and user settings, the Topology Manager performs the following:

  1. Bandwidth Calculation: It calculates the required PBN for each monitor's requested resolution and refresh rate.
  2. Payload Allocation: It verifies that the sum of all requested PBNs does not exceed the link capacity.
  3. Virtual Channel Payload ID Table (VCPID): The Topology Manager transmits an ALLOCATE_PAYLOAD sideband message to program each branch device along the path. This assigns specific time-slot ranges to Virtual Channel IDs.
  4. Stream Transmission: The GPU transmits the multiplexed data across the main link. As the data passes through each branch device, the hardware reads the time-slot tags established by the Topology Manager and directs the designated video stream to the correct physical output port.

Dynamic Topology Management

The Topology Manager continuously monitors the network state. If a user disconnects or adds a display, the nearest branch device generates a Hot Plug Detect (HPD) interrupt via an HPD_IRQ signal over the AUX channel. The Topology Manager receives this signal, queries the network to identify what changed, updates its internal topological map, deallocates or reallocates PBN time slots, and reconfigures the active video streams without interrupting unaffected displays.