DFU boot USB failures

Steps to reproduce

1. Use the DFU bootloader to load hackrf_usb.dfu
2. Repeat until there is an enumeration failure of the firmware running from RAM (It usually fails once within about 50 attempts.)

Expected behaviour

The firmware should execute from RAM and should enumerate over USB as a HackRF in Hi-Speed USB mode.

Actual behaviour

Enumeration fails, some messages appear in dmesg indicating failures to enumerate full-speed and/or low-speed devices. No Hi-Speed device is detected. Use of an oscilloscope on the USB data lines reveals a never-ending stream of 12 Mbps data produced by the microcontroller.

Workaround

Reset the device and retry. Since this bug only happens about 3% of the time, retrying usually works.

Version information

Operating system: observed with various Linux distros

hackrf_info output: tested only on 2017.02.1 and more recent firmware