UART Framing Protocol Library (uart_fpl)

uart_fpl is a lightweight, transport-independent binary framing, multiplexing, and protocol negotiation library for transmitting structured packet streams over raw, byte-oriented serial (UART) links.

It provides shared, platform-independent primitives used by host-side background daemons (ffx-uart-driver) and target-side drivers (fdomain-uart-driver) to multiplex multiple concurrent logical communication channels over a single physical or virtual serial interface.

1. Architectural Overview & Component Topology

uart_fpl is structured into five modular layers:

+-------------------------------------------------------------------------+
|                              Client Layer                               |
|        (Host CLI/Daemon: ffx-uart-driver, Target Runner: fdomain)       |
+------------------------------------+------------------------------------+
                                     |
               +---------------------+--------------------+
               |                                          |
               v                                          v
+-----------------------------+            +-----------------------------+
|    Dynamic Negotiation      |            |    Reliable Flow Control    |
|   (handshake::HostHandshake,|            |   (resend_sp::ResendSender, |
|    handshake::TargetHandshake)          |    resend_sp::ResendReceiver)|
+--------------+--------------+            +--------------+--------------+
               |                                          |
               +---------------------+--------------------+
                                     |
                                     v
               +------------------------------------------+
               |        Framing & Checksum Engine         |
               |      (frame::Frame, encode_frame,        |
               |       parser::FrameParser)               |
               +---------------------+--------------------+
                                     |
                                     v
+-------------------------------------------------------------------------+
|                          Physical / Virtual                             |
|                           UART Byte Stream                              |
+-------------------------------------------------------------------------+

2. Logical Channel Multiplexing

A raw serial interface provides only a single flat byte stream. Without packet-level multiplexing, access to the serial link is an exclusive, single-client bottleneck:

• If a developer runs a continuous log monitor (ffx log) or serves package files over UART, the serial link is completely monopolized.
• Concurrent control queries (ffx target echo, ffx component list) or interactive diagnostics are blocked until the streaming command terminates.

uart_fpl solves this by embedding a 16-bit Channel ID into every frame:

Physical / Virtual UART Link
 ├── Channel 0: Control & Protocol Negotiation (Handshake)
 ├── Channel 1: Primary Data Channel (e.g. Diagnostic / Control RPC)
 └── Channel 2+: Auxiliary streaming & diagnostic channels

• Host Multiplexing: The host-side background daemon (ffx-uart-driver) listens on a local UNIX domain socket. Each incoming client connection is assigned a distinct channel_id (channels 1 through 65535).
• Target Demultiplexing: The target driver (fdomain-uart-driver) demultiplexes incoming frames by channel_id, creating an independent FIDL stream socket for each active channel.
• Granular Teardown: When a client terminates, a FrameType::Close frame tears down that specific channel on the target without disrupting concurrent streams on other channels.
• Dedicated Control Plane: Channel 0 is permanently reserved for transport signaling, dynamic handshake negotiation, and in-band reboot detection.

3. Documentation Index

The documentation for uart_fpl is divided into two focused specifications:

• Wire Protocol Specification (PROTOCOL.md):

  • Complete 13-byte binary frame layout diagram and exact byte offsets.
  • Field definitions and wire representations.
  • 1-byte CRC-8 header checksum (HdrChk) and IEEE 802.3 CRC-32 checksum formulas.
  • Dynamic handshake negotiation wire payloads (NEGOTIATE_REQ, NEGOTIATE_RESP).
  • Protocol-level design decisions (session IDs, corrupted length protection, Go-Back-N wire mechanics).

• Rust API Reference & Integration Guide (API.md):

  • Comprehensive public Rust API reference (Frame, FrameParser, HostHandshake, TargetHandshake, ResendSender, ResendReceiver, AckTracker).
  • Method signatures, contracts, and error handling (UnexpectedSeqError).
  • High-level architectural justifications (such as end-to-end backpressure without local receiver queueing).
  • Runnable Rust code examples for handshake negotiation, sender loop, and receiver check-and-commit workflows.

4. Running Tests

Unit tests are defined in BUILD.gn:

# Run host unit tests
fx test //src/developer/lib/uart_fpl:uart_fpl_test --host