Mastering USB Permissions

The "Always Allow" Checkbox

One of the most frustrating aspects of using Android with custom hardware is the relentless permission popup:

Allow MAVLink GPS to access [Device Name]?

Even if you check "Always open MAVLink GPS when ... is connected", the popup often returns the next day. Why?

The Anatomy of a Match (DeviceFilter)

When you check that box, Android creates a "Filter" based on the connected device's unique signature and saves it to a system XML file. To bypass the popup next time, the new connection must match every field in that saved filter.

The comparison logic is strict (AOSP DeviceFilter.java):

1. Vendor ID (VID): Must match.
2. Product ID (PID): Must match.
3. Serial Number: Must match exactly (if stored).

The Serial Number Trap

This is where cheap Flight Controllers fail.

Scenario 1: The "Randomizer" (Bad)

Some bootloaders generate a random USB Serial Number on every boot to avoid conflicts.

• Day 1: You connect. Serial=ABC-123. You check "Always Allow". Android saves [VID:1234, PID:5678, Serial:ABC-123].
• Day 2: You reboot the drone. Serial=DEF-456.
• The Check: Android compares DEF-456 vs stored ABC-123. Mismatch.
• Result: The permission is denied, and the popup appears again.

Scenario 2: The "Clone" (Good?)

Cheap clones often have no serial number or a generic one (00000000).

• If the device reports no serial, Android saves [VID:1234, PID:5678, Serial:null].
• Any subsequent device with that VID/PID will match. This actually makes the "Always Allow" feature work reliably, albeit less securely.

Hardware Recommendations

To ensure a true "Plug and Fly" experience, use flight controllers from reputable manufacturers (Hex Cube, Holybro, mRo) that burn a unique, static UUID into the STM32 USB descriptor.

Source Code Reference

• Matching Logic: frameworks/base/core/java/android/hardware/usb/DeviceFilter.java - See the matches(UsbDevice device) method.