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Documentation/rpmsg: add RPMsg Port UART transport documentation
Add documentation for RPMsg Port UART, a transport layer that enables RPMsg communication between SoCs via UART when shared memory is not available. The documentation covers: - Hardware requirements (UART with flow control) - Software architecture - Escape coding protocol for command/data separation - Connection establishment protocol - Data frame format - Low power support with ping-pong wake mechanism Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>
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Bowen Wang
GUIDINGLI
@@ -11,3 +11,4 @@ for inter-processor communication in Asymmetric Multiprocessing (AMP) systems.
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concepts
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rpmsg_port
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rpmsg_port_uart
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@@ -0,0 +1,190 @@
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RPMsg Port UART
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===============
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Overview
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--------
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RPMsg Port UART is a transport layer implementation that enables RPMsg
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communication between two SoCs connected via UART. This is useful when
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shared memory is not available between chips, such as two separate chips
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on the same PCB.
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Hardware Requirements
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---------------------
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The UART interface requires four signals with hardware flow control support:
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::
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┌─────────────┐ ┌─────────────┐
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│ SoC A │ │ SoC B │
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│ │ TX ────────> │ │
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│ UART │ RX <──────── │ UART │
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│ │ RTS ────────> │ │
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│ │ CTS <──────── │ │
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└─────────────┘ └─────────────┘
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Requirements:
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- Hardware flow control (RTS/CTS) must be supported to prevent data loss
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- RX pin should support wake-up for low power mode
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- Data corruption during wake-up is not allowed, but data loss is acceptable
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(handled by software)
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Software Architecture
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---------------------
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::
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┌────────────────────────────────────────┐
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│ RPMsg Framework │
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├────────────────────────────────────────┤
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│ RPMsg Port │
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│ (Buffer Management, Name Service) │
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├────────────────────────────────────────┤
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│ RPMsg Port UART │
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│ (Escape Coding, Connection Protocol, │
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│ Data Transfer, Low Power Support) │
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├────────────────────────────────────────┤
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│ UART Driver │
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└────────────────────────────────────────┘
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Protocol Details
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----------------
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Escape Coding
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~~~~~~~~~~~~~
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Special characters (0x70-0x7f) are used to distinguish commands from data:
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.. code-block:: c
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#define RPMSG_PORT_UART_START 0x7f /* Frame start */
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#define RPMSG_PORT_UART_END 0x70 /* Frame end */
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#define RPMSG_PORT_UART_ESCAPE 0x7c /* Escape character */
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#define RPMSG_PORT_UART_CONNREQ 0x7e /* Connection request */
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#define RPMSG_PORT_UART_CONNACK 0x7d /* Connection acknowledge */
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#define RPMSG_PORT_UART_ESCAPE_MASK 0x20 /* Escape XOR mask */
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Encoding example:
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::
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Original: [0x01, 0x7f, 0x02, 0x70, 0x03]
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Encoded: [START] [0x01] [ESC] [0x5f] [0x02] [ESC] [0x50] [0x03] [END]
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[0x7f] [0x01] [0x7c][0x5f] [0x02] [0x7c][0x50] [0x03] [0x70]
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When a byte falls in the 0x70-0x7f range, it is escaped by sending the
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ESCAPE character followed by the original byte XORed with 0x20.
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Connection Protocol
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~~~~~~~~~~~~~~~~~~~
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The connection protocol supports:
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- Either side can start first during system boot
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- Reconnection after either side restarts
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::
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Normal startup:
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┌────────┐ ┌────────┐
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│ CPU A │ │ CPU B │
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└────────┘ └────────┘
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│ │
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│ ──── CONNREQ (0x7e) ────────> │
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│ │ Initialize resources
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│ <──── CONNACK (0x7d) ──────── │
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│ Initialize resources │
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│ │
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│ Connection established │
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Restart scenario (CPU B restarts):
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┌────────┐ ┌────────┐
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│ CPU A │ │ CPU B │
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│(running)│ │(restart)│
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└────────┘ └────────┘
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│ │
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│ <──── CONNREQ (0x7e) ──────── │
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│ Reinitialize resources │
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│ ──── CONNACK (0x7d) ────────> │
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│ │ Initialize resources
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│ Connection established │
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Data Frame Format
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~~~~~~~~~~~~~~~~~
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::
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┌───────┬─────────────────────────────────┬───────┐
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│ START │ Header + Payload (escaped) │ END │
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│ 0x7f │ │ 0x70 │
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└───────┴─────────────────────────────────┴───────┘
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Header structure (rpmsg_port_header_s):
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┌────────┬────────┬────────┬────────┬──────────────┐
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│ crc │ cmd │ avail │ len │ payload │
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│ 2 byte │ 2 byte │ 2 byte │ 2 byte │ N bytes │
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└────────┴────────┴────────┴────────┴──────────────┘
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- crc: CRC16 checksum (optional, enabled via CONFIG_RPMSG_PORT_UART_CRC)
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- cmd: Command field (unused for data frames, set to 0)
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- avail: Available space (unused for data frames, set to 0)
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- len: Total length including header
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- payload: Actual RPMsg data
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Low Power Support
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-----------------
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RPMsg Port UART implements low power support without additional GPIOs using
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special wake/sleep commands:
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.. code-block:: c
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#define RPMSG_PORT_UART_STAYWAKE1 0x79 /* Wake request 1 */
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#define RPMSG_PORT_UART_STAYWAKEACK1 0x78 /* Wake acknowledge 1 */
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#define RPMSG_PORT_UART_STAYWAKE2 0x75 /* Wake request 2 */
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#define RPMSG_PORT_UART_STAYWAKEACK2 0x74 /* Wake acknowledge 2 */
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#define RPMSG_PORT_UART_RELAXWAKE 0x77 /* Allow sleep */
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Wake/Sleep Flow
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~~~~~~~~~~~~~~~
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::
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┌────────┐ ┌────────┐
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│ CPU A │ │ CPU B │
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│(sender)│ │(asleep)│
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└────────┘ └────────┘
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│ │
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│ Acquire TX PM lock │
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│ ──── STAYWAKE1 ──────────────────> │
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│ │ Acquire RX PM lock
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│ <──── STAYWAKEACK1 ─────────────── │
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│ │
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│ ════ Data Transfer ══════════════> │
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│ │
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│ ──── RELAXWAKE ──────────────────> │
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│ Release TX PM lock │ Release RX PM lock
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│ │
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Ping-Pong Wake Mechanism
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~~~~~~~~~~~~~~~~~~~~~~~~
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Two sets of wake commands (STAYWAKE1/ACK1 and STAYWAKE2/ACK2) are used
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alternately to prevent false wake acknowledgments:
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::
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First transfer: STAYWAKE1 → STAYWAKEACK1 → Data → RELAXWAKE
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Second transfer: STAYWAKE2 → STAYWAKEACK2 → Data → RELAXWAKE
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Third transfer: STAYWAKE1 → STAYWAKEACK1 → Data → RELAXWAKE
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...
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This prevents the sender from mistakenly accepting a delayed ACK from a
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previous transfer as confirmation for the current transfer.
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Source Files
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------------
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- ``nuttx/drivers/rpmsg/rpmsg_port_uart.c``
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