mavlink: add message spacing for AVAILABLE_MODES, for low bandwidth

[alexcekay]

Solved Problem

PX4 sends all of the available modes at once via mavlink, this can overflow the TX buffer of low bandwidth radios, causing the ground station to not receive most of them.

Solution

• Add some spacing between the individual messages if the low bandwidth mode is used
• The spacing is calculated for the case of the maximum amount of modes being transmitted, this way it was able to test that it does work independent of the amount of modes
• The spacing does not add more than 2s delay overall to avoid ACK timeouts

Test coverage

• Was tested with a low bandwidth radio and a ARK v6x

[farhangnaderi]

maybe @bkueng could also help. The rest seem ok to me.

[jsm09a]

Curious why the existing "Mav Rate" param is not sufficient to prevent TX overflows ? Can MAVLINK_MODE_LOW_BANDWIDTH and/or LOW_BANDWIDTH_DELAY_US be inferred from the "Mav Rate" ?

[hamishwillee]

FWIW only - The GCS should re-request specific modes it doesn't get first time. It still makes sense to manage the bandwidth of course.

[alexcekay]

@jsm09a: Yes good point. Logic now respects the rate
@hamishwillee: Yes exactly. The logic here is to try to get as much as possible over the link in the first "chunk", as the transfer of individual modes has quite some overhead (1 request and one ack per mode). After the initial chunk the GS can fetch all missing ones.

[github-actions]

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 88 byte (0 %)]

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--------------  -------------- 
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  [DEL]     -32  [ = ]       0    CSWTCH.1927
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  [DEL]     -32  [ = ]       0    CSWTCH.2712
  [NEW]     +32  [ = ]       0    CSWTCH.2718
  [DEL]     -48  [ = ]       0    CSWTCH.3447
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  [NEW]     +48  [ = ]       0    CSWTCH.3453
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-0.8%     -88  [ = ]       0    [Unmapped]
+0.0%    +744  +0.0%     +88    TOTAL

px4_fmu-v6x [Total VM Diff: 80 byte (0 %)]

    FILE SIZE        VM SIZE    
--------------  -------------- 
+0.0%     +80  +0.0%     +80    .text
   +45%     +76   +45%     +76    MavlinkStreamAvailableModes::request_message()
  [NEW]     +60  [NEW]     +60    CSWTCH.1933
  [NEW]     +44  [NEW]     +44    CSWTCH.2718
  [NEW]     +12  [NEW]     +12    CSWTCH.3453
  [NEW]     +12  [NEW]     +12    CSWTCH.3454
  +3.8%      +8  +3.8%      +8    MavlinkStreamAvailableModes::send_single_mode()
  -6.2%      -4  -6.2%      -4    MavlinkMissionManager::init_offboard_mission()
  [DEL]     -12  [DEL]     -12    CSWTCH.3447
  [DEL]     -12  [DEL]     -12    CSWTCH.3448
  [DEL]     -44  [DEL]     -44    CSWTCH.2712
  [DEL]     -60  [DEL]     -60    CSWTCH.1927
+0.0%     +81  [ = ]       0    .debug_abbrev
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  [DEL]     -12  [ = ]       0    CSWTCH.1927
  [NEW]     +12  [ = ]       0    CSWTCH.1933
  [DEL]     -12  [ = ]       0    CSWTCH.2712
  [NEW]     +12  [ = ]       0    CSWTCH.2718
  [DEL]     -24  [ = ]       0    CSWTCH.3447
  [DEL]     -24  [ = ]       0    CSWTCH.3448
  [NEW]     +24  [ = ]       0    CSWTCH.3453
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  +1.3%      +1  [ = ]       0    MavlinkStreamAvailableModes::send_single_mode()
+0.0%     +16  [ = ]       0    .symtab
  [DEL]     -32  [ = ]       0    CSWTCH.1927
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  [DEL]     -32  [ = ]       0    CSWTCH.2712
  [NEW]     +32  [ = ]       0    CSWTCH.2718
  [DEL]     -48  [ = ]       0    CSWTCH.3447
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-1.1%     -80  [ = ]       0    [Unmapped]
+0.0%    +744  +0.0%     +80    TOTAL

Updated: 2025-08-18T15:20:36