Troubleshooting • Re: Raspberry Pi Pico USB disconnect issue on Linux

Dear RPi Forum Team,

The Device Number is part of the C struct usb_device in the Linux kernel USB API.

struct usb_device, the kernel representation of a USB device.

The USB device number devnum is the first variable in the usb_device struct.

int devnum; device number; address on a USB bus

Please find a link here.
https://www.kernel.org/doc/html/v4.14/d ... usb_device

The limit of 127 USB devices may be attached to any one host at any one time. Which explains the modulo 127. The working assumption at the moment is 127 per USB bus on the host. But it might be 127 in total. Will have to look into that a bit more.

It might be due to Power Management of USB's
https://www.kernel.org/doc/html/v4.14/d ... ement.html

And device power management more generally in the Linux kernel
https://www.kernel.org/doc/html/v4.14/d ... vices.html

Have not yet found the definitive architecture rationale of the USB Device Number devnum increment when plugged in but then not a devnum decrement when the device is unplugged.

Yours,
York

Statistics: Posted by York Earwaker — Sun Dec 28, 2025 2:28 pm

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