I have an Ubuntu 20.04 system (Intel 2400K, ASRock Z68 Extreme3 Gen3) with two 90W powered 16-port USB 3 hubs connected to two USB 2 ports on the motherboard. There are 16 external USB 3 drives connected to the first USB hub, and will have soon have 16 external USB 3 drives connected to the second USB 3 hub.
When the hubs are both connected to two USB 3 ports, dmesg shows the log message
xhci_hcd 0000:07:00.0: Max number of devices this xHCI host supports is 32.
Switching both hubs to two USB 2 ports avoids this error message.
Is the 32 device limit still there? lsusb shows these devices connected using ehci instead of xchi?
$ lsusb -t
/: Bus 04.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 5000M
/: Bus 03.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 480M
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ehci-pci/2p, 480M
|__ Port 1: Dev 2, If 0, Class=Hub, Driver=hub/8p, 480M
|__ Port 3: Dev 98, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 1: Dev 99, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 3: Dev 109, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 1: Dev 101, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 114, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 2: Dev 104, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 2: Dev 100, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 1: Dev 103, If 0, Class=Hub, Driver=hub/3p, 480M
|__ Port 1: Dev 108, If 0, Class=Mass Storage, Driver=uas, 480M
|__ Port 4: Dev 115, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 2: Dev 106, If 0, Class=Hub, Driver=hub/3p, 480M
|__ Port 1: Dev 112, If 0, Class=Mass Storage, Driver=uas, 480M
|__ Port 3: Dev 110, If 0, Class=Hub, Driver=hub/3p, 480M
|__ Port 1: Dev 116, If 0, Class=Mass Storage, Driver=uas, 480M
|__ Port 3: Dev 102, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 1: Dev 107, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 2: Dev 113, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 3: Dev 118, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 120, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 105, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 3: Dev 119, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 1: Dev 111, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 121, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 2: Dev 117, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 65, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 3: Dev 69, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 1: Dev 66, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 3: Dev 97, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 96, If 0, Class=Mass Storage, Driver=usb-storage, 480M
|__ Port 4: Dev 71, If 0, Class=Hub, Driver=hub/4p, 480M
|__ Port 2: Dev 67, If 0, Class=Hub, Driver=hub/4p, 480M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ehci-pci/2p, 480M
|__ Port 1: Dev 2, If 0, Class=Hub, Driver=hub/6p, 480M
|__ Port 2: Dev 3, If 0, Class=Human Interface Device, Driver=usbhid, 12M
