Dynamic USB descriptors: enable/disable, change HID descriptors, etc.

[dhalbert]

A couple of forum posts recently have asked if HID or MSC could be disabled for security or device interference reasons. We need some kind of dynamic USB descriptors to do this.

https://forums.adafruit.com/viewtopic.php?f=8&t=136248#p683158
https://forums.adafruit.com/viewtopic.php?f=60&t=137940#p682494

Other uses for dynamic USB descriptors include adding new HID devices, disabling selected HID devices, and varying an HID device (e.g., changing the joystick coordinate ranges).

[ATMakersBill]

This would be helpful in some of our projects as well. One way I could see this working is to check for the presence (or absence) of a file or config option in the BOOT filesystem. So, when CP's UF2 is loaded, it checks for that file (maybe a config file?) and checked which USB descriptors to present.

This would make it possible to enter the bootloader mode, tell it to not share the mass storage device or the HID (or the Serial port?) and restart. If you ever wanted to change that, just double click to get into the boot loader.

Just a thought

[tannewt]

The idea I had was to control the descriptors from boot.py which runs before usb enumeration.

[ATMakersBill]

Where does that live? On the BOOT drive or on the CIRCUITPY drive? How does a user edit it?

[tannewt]

CIRCUITPY just like code.py. You can edit it the same way but it won't be rerun automatically. Instead, one needs to eject the drive and then reset the board (using the button or unplugging).

[ATMakersBill]

I understand how that would be helpful for the HID descriptors, etc. But shouldn't the Mass Storage descriptor be controllable by something not on that drive?

[tannewt]

I think its ok. You can change files through the REPL or use the REPL to reboot to safe mode which skips all user code.

A common technique for the similar problem of flipping read-only for the file system is to read an external line connected to a switch to determine whether to do it or not on boot.