Why does lsusb show devices connected to a USB 3 port as connected to USB2 root hub?

I'm running Ubuntu on a Dell Chromebook 11, which has two USB 3 ports, and an SD card reader. I was trying to establish whether the SD card reader would be capable of USB 3 speeds (and thus whether it's worth buying a 130MB/s SD card), but I can't tell whether it's physically connected to USB 3 internally.

One way I thought I might do this is to run lsusb after plugging in an SD card - however, if I plug in a USB2 device to one of the USB 3 ports, it comes up as being connected to the USB 2 hub anyway. If I put a USB 3 device into the very same port, it shows as a being connected to the USB 3 hub.

1. Why does lsusb show USB2 devices connected to a USB3 port as being attached to the USB 2 root?
2. Is there a way to get the physical rather than logical mapping of USB ports to hubs?
3. Coming back to my original reason for looking into this, is there a way to tell what speed the internal SD card reader is capable of before buying one?

Why does lsusb show USB2 devices connected to a USB3 port as being attached to the USB 2 root?

Answer :

In USB 3.0, dual-bus architecture is used to allow both USB 2.0 (Full Speed, Low Speed, or High Speed) and USB 3.0 (SuperSpeed) operations to take place simultaneously, thus providing backward compatibility.

Source: https://en.wikipedia.org/wiki/USB_3.0

For instance, on a machine that contains two 2.0 USB ports and one USB 3.0 port, the output of the lsusb command shows:

Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 2.0 root hub 
Bus 003 Device 002: ID 8087:8004 Intel Corp. 
Bus 003 Device 001: ID 1d6b:0008 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 058f:6386 Alcor Micro Corp. Multi Flash Reader
Bus 002 Device 001: ID 1d6b:0007 Linux Foundation 3.0 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

The extra USB 2.0 root hub is part of the USB 3.0 root hub and is available there to provide backward compatibility with USB 2.0 standards. As a result of this, when you connect USB2 devices into USB3 port, they are shown connected to 2.0 root hub.

Is there a way to get the physical rather than logical mapping of USB ports to hubs?

Answer: Use lsusb -t

-t     Tells lsusb to dump the physical USB device hierarchy as a tree.

Source: lsusb man page

Coming back to my original reason for looking into this, is there a way to tell what speed the internal SD card reader is capable of before buying one?

This can be done by using the following steps. Here we are considering a laptop with a built-in card reader from Alcor Micro Corp.

1. Use lsusb command

   $ lsusb 
   Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 2.0 root hub 
   Bus 003 Device 002: ID 8087:8004 Intel Corp. 
   Bus 003 Device 001: ID 1d6b:0008 Linux Foundation 2.0 root hub
   Bus 001 Device 002: ID 058f:6386 Alcor Micro Corp. Multi Flash Reader
   Bus 002 Device 001: ID 1d6b:0007 Linux Foundation 3.0 root hub
   Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
   

   Find out the Bus number of the card reader. In this example it is 001

2. Use lsusb -t

   $ lsusb -t 
   /:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/14p, 480M 
   |__ Port 5: Dev 2, If 0, Class=Mass Storage, Driver=usb-storage, 480M
   

   At the end of each line, the negotiated communication speed for each device is shown in Mbits/s.
   The 480Mbit/s is called "Hi-Speed" and is used with USB2.0.

For USB 3.0, the negotiated communication speed is 5000Mbit/s called "SuperSpeed" (5Gbit/s)

Source: https://en.wikipedia.org/wiki/USB

Result: the card reader from Alcor Micro Corp supports USB 2.0 standards and not USB 3.0 standards as 480M was mentioned in the output.

From the man page:

DESCRIPTION

       lsusb  is  a  utility for displaying information about USB buses in the
       system and the devices connected to them.

I suggest you use lshw (you may have to sudo apt-get install lshw first). From the lshw man page:

DESCRIPTION

       lshw  is  a  small tool to extract detailed information on the hardware
       configuration of the machine. It can report exact memory configuration,
       firmware version, mainboard configuration, CPU version and speed, cache
       configuration, bus speed, etc. on DMI-capable x86 or IA-64 systems  and
       on some PowerPC machines (PowerMac G4 is known to work).

       It  currently  supports  DMI  (x86 and IA-64 only), OpenFirmware device
       tree (PowerPC only), PCI/AGP, CPUID (x86), IDE/ATA/ATAPI, PCMCIA  (only
       tested on x86), SCSI and USB.