tinypilot: HID descriptor problems when attached to some hardware KVM swtiches

Description

Spun off from https://github.com/mtlynch/tinypilot/issues/78#issuecomment-783755302

Some hardware KVM switches seems to “take issue” with the combined mouse and keyboard HID descriptors provided through TinyPilot’s use of libcomposite gadget behaviors. One particular case is interesting, because at least one

What’s the behavior that you expect?

Keyboard & Mouse are able to be activated through init-usb-gadget script, and successfully pass through the KVM switch hardware to the end device

What’s happening instead?

Only one device descriptor can be activated at a time, or neither will pass through.

What are the steps to reproduce this behavior?

1. Attach TinyPilot to KVM swtich
2. Observe “missing” keyboard / mouse on end device
3. See errors in TinyPilot logs regarding writes of keypresses

Research

I’ve collected some notes in this Gist.

• Kernel logs showing the difference in reports. Notice, the strings are missing when passing through the CKLau 🤔
• When both HID devices are activated, there is no kernel message. This seems to indicate an issue within the switch hardware/firmware, though due to a specific compatibility issue only yet observed here. Could be a HID definition compliance issue, or could be something more obscure like report_length misalignment. 🤷
• Included a full HID dump from a hardware combo device which “seemingly” has no issues when directly attached to the KVM switch.