panic on hid device enumeration with a particular hub about tinyusb HOT 6 OPEN

A little bit of background. The problem came to my notice in Olimex's reason for creating a USB hub board where they gave some detail, including:

Unfortunately, in most cases, these cheap USB hubs, like the one we have on the web and many others, simply refuse to work. Out of the 10 USB hubs I bought to test, only 2 were functional.

from tinyusb.

With the hub plugged into a Pi, please post the output of sudo lsusb -v. Also busting it open and getting the controller IC part number would be useful.

from tinyusb.

Here's a log after connecting the hub.
Bus 001 Device 002: ID 214b:7250
after-hub-plug.log

from tinyusb.

It appears the hub is very flaky when talking to low-speed devices if operating in full-speed mode. I attach a Teledyne Lecroy analyser trace showing multiple errors - it appears the host hardware eventually gets upset. My suggestion would be to ensure consistency of the expected packet data toggle in the case of smashed ACKs.
pico host usb hid probe.zip

from tinyusb.

Thanks @P33M, so the Pico host eventually gets upset. Did you try traces with other hosts, as Matthew tried a Pi5 and Linux laptop, and they also suffered the hub's flakiness but coped better?

from tinyusb.

Every single other host will use the hub in high-speed mode, which fundamentally changes the signalling involved - communication with the low-speed device is handed off to the hub's transaction translator. See the USB 2.0 spec section 11.14.1.1.

from tinyusb.