Bluetooth Profile: AVRCP

This component implements the Audio Video Remote Control Profile (AVRCP) as specified by the Bluetooth SIG in the official specification.

The bt-avrcp component is a system service that is created when needed - typically as a side effect of launching the bt-a2dp-sink or bt-avrcp-target components. Once launched, bt-avrcp will persist until explicitly terminated.

The component registers both the Controller (CT) and Target (TG) roles with the BR/EDR Profile Service.

Build configuration

Follow the steps for the A2DP sink profile and add this module to your build with --with //src/connectivity/bluetooth/profiles/bt-avrcp to include AVRCP with your build. For product configurations in which AVRCP support is desired, include the bt-avrcp-core-shard.

It's recommended to include the bt-avrcp-controller package to provide a simple REPL to send AVRCP commands to peers either by including the bt tools package in your build or directly with --with //src/connectivity/bluetooth/tools/bt-avrcp-controller.

eg: fx set core.x64 --with //src/media/bundles:services --with //src/connectivity/bluetooth/tools/bt-avrcp-controller --with src/connectivity/bluetooth/profiles/bt-avrcp --with //src/connectivity/bluetooth/profiles/bt-a2dp

Using AVRCP from the shell

1. AVRCP doesn't start automatically and service manager should launch the bt_avrcp service. The best way to do that today is by running the bt-avrcp-controller tool from the Fuchsia shell with a fake peer id and then immediately exiting the shell.

   Eg:

   #> bt-avrcp-controller 12345
   avrcp# exit
   >
   

2. Start the bt-a2dp-sink service with ffx component run /core/ffx-laboratory:bt-a2dp fuchsia-pkg://fuchsia.com/bt-a2dp-sink#meta/bt-a2dp-sink.cm

3. (If you haven't paired previously) pair your fuchsia device with your test device -

   • Make the fuchsia device discoverable but running run bt-cli from the fuchsia shell.
   • Use the discoverable command to make the fuchsia device discoverable
   • Once your device is paired exit the bt-cli with the exit command.

4. Obtain the peer id for the paired device

   • Run the bt-cli from the fuchsia shell.
   • Use the list-peers commands in bt-cli and copy down the peer ID.
   • Exit bt-cli with the exit command.

5. Run bt-avrcp-controller <peer id> (using the peer id from bt-cli) to obtain a controller for the peer.

6. Issue commands to the peer using the AVRCP tool. Use the help command to get a list of supported commands available.

Inspect

The bt-avrcp component includes support for component inspection. To view component metrics or the current state of any connected peers , use ffx inspect show core/bt-avrcp.

Hierarchy

root:
  peers:
    peer_#:
      peer_id
      browse = (connected / connecting / disconnected)
      control = (connected / connecting / disconnected)
      target_info
      controller_info
      last_connected
  metrics:
    distinct_peers
    control_connections
    browse_connections
    connection_errors
    control_channel_collisions
    target_peers_supporting_browsing
    distinct_target_peers_supporting_browsing
    target_peers_supporting_cover_art
    distinct_target_peers_supporting_cover_art
    controller_peers_supporting_browsing
    distinct_controller_peers_supporting_browsing
    controller_peers_supporting_cover_art
    distinct_controller_peers_supporting_cover_art

One peer child exists in the hierarchy for each peer that has been discovered regardless of its current connection status.

Code Layout

peer_manager mod

PeerManager

• Central hub of AVRCP
• Routes requests from the FIDL front end services and from the BREDR service to the correct peer.

TargetDelegate

• Functions as stateful proxy to the currently registered target handler and absolute volume handler.
• Replies with error responses when the underlying handlers are not set.
• Resolves outstanding notifications when target and absolute volume handlers close.
• Typically used by the ControlCommandHandler for target profile commands.

peer mod

RemotePeerHandle

• Handle representing a remote peer.
• Provides a public API for interacting and connecting to a remote peer and observing changes.
• Typically used by Controller

RemotePeer

• Internal state object representing peer
• Contains the the discovered service attributes of the peer as reported by the BR/EDR service.
• Owns any associated connections currently open to the peer.
• Contains a vector of controller listener channels waiting for any incoming notifications by the peer. Events received by the peer are dispatched to all listening control listeners.

Controller

• Public interface vended by peer manager to service and test frontends to communicate with a specific peer.
• Encapsulates communication with peer manager to send commands requests to a peer and to handle the responses to those commands.
• Provides an event stream to receive incoming notifications from the peer.

peer/tasks mod

• Provides a state_watcher task that is spawned to observe and react to state changes on each RemotePeer created
• Provides connection statement management.
• Makes outgoing connections to the remote peer when needed.
• Spawns tasks to handle incoming events from the control and browse streams.
• Owns a command handler (ControlCommandHandler) to respond to incoming AVC commands from the peer. Depending on the role we are functioning in after doing SDP discovery, this command handler will dispatch events to the target handler or to an absolute volume via the TargetDelegate

peer/handlers mod

ControlCommandHandler

• Handles AV\C commands received from the peer on the control channel.
• Primarily used when when we are acting in AVRCP target role with A2DP source or for absolute volume handling with A2DP sink.
• Maintains state for the peer as target for continuations and registered notifications by the peer.
• Dispatches target commands to media session or a registered test target handler for most target commands.
• Dispatches absolute volume commands to the registered absolute volume handler (typically A2DP sink).

service mod

• Encapsulates and abstracts (to facilitate testing and mocking) outward facing AVRCP FIDL services from the peer manager.
• Handles FIDL request to obtain an AVRCP Controller for a given peer id.
• Incoming requests for a controller to a peer are dispatched to the PeerManager who in turn vend off a PeerController to interact with the requested peer.
• Handles setting over absolute volume handlers and test target handlers that will be registered into the PeerManager.

profile mod

• Encapsulates and abstracts (to facilitate testing) the BR/EDR FIDL service from the peer manager.
• Sets up SDP service definitions that are registered with the BD/EDR service and service searches.
• Vends off incoming L2CAP connections and discovered peer attributes events to the peer manager and gives the peer manager a method for making outgoing L2CAP connections to discovered peers.

packets mod

• Contains the AV\C control channel and browse channel vendor dependent specific packet encoders/decoders.
• Provides methods for encoding the vendor dependent header/preambles on most commands.
• Contains encoding logic for AV\C continuations/fragmentation in packet decoding and encoding of vendor dependent commands.