mavlink: unified TX architecture - move all service sends to TX thread by dakejahl · Pull Request #26512 · PX4/PX4-Autopilot

dakejahl · 2026-02-17T23:06:18Z

Summary

All MAVLink service responses (parameters, FTP, missions, log handler, timesync) now send through a TX queue instead of directly from the RX thread
The RX thread only receives and dispatches; all sends go through the TX thread with unified bandwidth management
Services and streams share the MAV_RATE budget — streams automatically back off when service traffic is active

Problem

The RX thread (MavlinkReceiver) both received messages AND sent service responses, bypassing MAV_RATE bandwidth management entirely. This caused uncontrolled link bursting that breaks low-bandwidth links with hard throughput limits (e.g. telemetry radios).

Changes

New: MavlinkTxQueue — thread-safe queue wrapping VariableLengthRingbuffer. Services encode messages via mavlink_msg_*_encode_chan() + enqueue_tx() instead of calling mavlink_msg_*_send_struct() directly. The TX thread drains the queue under a per-cycle byte budget.

Converted services:

MavlinkParametersManager — send_param(), send_error(), hash PARAM_VALUE, UAVCAN forwarding
MavlinkFTP — _reply(), resend path, burst reads
MavlinkMissionManager — all 6 send helpers (ack, current, count, item, request, item_reached)
MavlinkLogHandler — send_log_entry(), state_sending_data()
MavlinkReceiver — PING response, MESSAGE_INTERVAL
MavlinkTimesync — TIMESYNC response

Thread safety: Per-service pthread_mutex_t protects shared state between handle_message() (RX thread) and send() (TX thread).

Bandwidth accounting: update_rate_mult() now subtracts measured service traffic (EMA of bytes/sec) from available stream bandwidth, replacing the old sending_parameters() hack.

Test plan

SITL: connect QGC, verify streams, parameter download, mission upload/download, log list/download all work
Set MAV_1_RATE low (e.g. 1200 B/s), trigger param download — streams should back off, heartbeat continues, no TX overruns
Verify mavlink status shows reasonable rate_mult values during service activity

…AV_RATE

github-actions · 2026-02-17T23:14:03Z

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 1488 byte (0.07 %)]

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Updated: 2026-02-17T23:15:35

hamishwillee · 2026-02-18T02:52:25Z

Docs should be added somewhere in https://docs.px4.io/main/en/mavlink/ explaining the pattern. So obviously in https://docs.px4.io/main/en/mavlink/streaming_messages#define-the-streaming-class but also "How to TX a mavlink message" section.

dakejahl added 2 commits February 17, 2026 14:01

claudes first attempt at reworking mavlink rx/tx threads to respect M…

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…AV_RATE

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mavlink: unified TX architecture - move all service sends to TX thread#26512

mavlink: unified TX architecture - move all service sends to TX thread#26512
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jake/mavlink_rework

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dakejahl commented Feb 17, 2026

Summary

Problem

Changes

Test plan

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