Add optional DFU mode parameter to MSP_REBOOT command

[sensei-hacker]

User description

Summary

Add optional parameter to MSP_REBOOT (message 68) to trigger DFU mode directly via MSP protocol. This provides a reliable programmatic method for tools and scripts to enter DFU mode without CLI timing issues or manual hardware button presses.

Changes

Firmware (src/main/fc/fc_msp.c):

• Add static variable mspRebootBootloader to store DFU mode flag
• Modify mspRebootFn() to use bootloader flag when calling fcReboot()
• Add mspFcRebootCommand() helper function to read optional parameter
• Update mspFcProcessCommand() to handle MSP_REBOOT with new logic

Documentation (docs/USB Flashing.md):

• Update DFU mode entry methods with proper CLI sequence
• Remove deprecated 'R' character method
• Add MSP_REBOOT method as recommended programmatic approach

Protocol Details

Backwards Compatibility:

• Empty payload (existing behavior) = normal reboot
• Parameter 0x00 = explicit normal reboot
• Parameter 0x01 = reboot to DFU/bootloader mode

MSP Message:

• Message ID: 68 (MSP_REBOOT)
• Payload: 0 or 1 byte
• Direction: IN (configurator → FC)

Testing

Tested on AOCODARCF722AIO hardware:

• ✅ Empty payload reboots normally (backwards compatible with existing tools)
• ✅ Parameter 0x00 reboots normally
• ✅ Parameter 0x01 enters DFU mode (verified with dfu-util -l showing device 0483:df11)

All three test cases passed successfully.

Motivation

Current DFU entry methods have limitations:

• Hardware button: Requires physical access and manual intervention
• CLI sequence: Timing-sensitive, requires waiting for prompt, prone to race conditions
• Single 'R' character: Can be triggered accidentally

The MSP_REBOOT method:

• Works through standard MSP protocol
• No CLI timing issues
• Suitable for automation and scripting
• Backwards compatible with existing configurator implementations

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PR Type

Enhancement, Bug fix

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Description

• Add optional DFU mode parameter to MSP_REBOOT command for programmatic bootloader entry

• Support backwards-compatible reboot behavior with optional bootloader flag parameter

• Improve DFU mode entry documentation with reliable serial CLI sequence

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Diagram Walkthrough

flowchart LR
  MSP["MSP_REBOOT Command"] -->|"Empty payload"| Normal["Normal Reboot"]
  MSP -->|"Parameter 0x00"| Normal
  MSP -->|"Parameter 0x01"| DFU["DFU/Bootloader Mode"]
  DFU --> Device["Device enters bootloader"]
  Normal --> Reboot["System reboots normally"]

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File Walkthrough

	Relevant files
Enhancement	fc_msp.c Implement MSP_REBOOT DFU mode parameter support                    src/main/fc/fc_msp.c Add static variable mspRebootBootloader to store DFU mode flag Create new mspFcRebootCommand() function to parse optional bootloader parameter from MSP payload Modify mspRebootFn() to use bootloader flag when calling fcReboot() Move MSP_REBOOT handling from mspFcProcessOutCommand() to main command processor with proper armed state check +25/-9
Documentation	USB Flashing.md Improve DFU mode entry documentation with reliable methods docs/USB Flashing.md Replace unreliable single 'R' character method with proper serial CLI sequence documentation Add detailed bash example showing #### CLI entry followed by dfu command Add warning about timing requirements and importance of waiting for CLI prompt Add MSP_REBOOT as recommended programmatic DFU entry method for INAV 9.x+ Clarify hardware button method with explicit instructions +18/-6

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