When a usb mass-storage device is attached to the host, enumeration of the device happens and the process where an address is given to the device, getting the descriptors occurs, then the core checks for the class driver required and assigns the mass-storage driver. What happens after that? Where does the SCSI comes in picture?
Check the following
[23870.089465] usb 3-2: new high-speed USB device number 5 using xhci_hcd [23870.158769] usb 3-2: New USB device found, idVendor=058f, idProduct=6387 [23870.158778] usb 3-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3 [23870.158782] usb 3-2: Product: Mass Storage [23870.158785] usb 3-2: Manufacturer: Generic [23870.158788] usb 3-2: SerialNumber: EEA1EE5B [23870.202363] usb-storage 3-2:1.0: USB Mass Storage device detected [23870.202469] scsi6 : usb-storage 3-2:1.0