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When inserting a USB pen drive, I get the following message

[20236.720694] usb-storage 1-1.2:1.0: USB Mass Storage device detected
[20236.721212] scsi host6: usb-storage 1-1.2:1.0

I think usb-storage is the name of the kernel module. 1-1.2 means USB bus 1, with location 1.2 in the hirarchy (the root hub has on port 1 a hub, and on this hub the device is on port 2).

But what does the ":1.0" suffix mean?

Entire dmesg output:

$ sudo dmesg -c
[20236.607122] usb 1-1.2: new high-speed USB device number 28 using ehci-pci
[20236.720003] usb 1-1.2: New USB device found, idVendor=0951, idProduct=1665
[20236.720007] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[20236.720009] usb 1-1.2: Product: DataTraveler 2.0
[20236.720012] usb 1-1.2: Manufacturer: Kingston
[20236.720014] usb 1-1.2: SerialNumber: 08606E6D402EFD91D7025267
[20236.720694] usb-storage 1-1.2:1.0: USB Mass Storage device detected
[20236.721212] scsi host6: usb-storage 1-1.2:1.0
[20237.766610] scsi 6:0:0:0: Direct-Access     Kingston DataTraveler 2.0 1.00 PQ: 0 ANSI: 4
[20237.767789] sd 6:0:0:0: Attached scsi generic sg2 type 0
[20237.768454] sd 6:0:0:0: [sdc] 15131636 512-byte logical blocks: (7.75 GB/7.21 GiB)
[20237.769158] sd 6:0:0:0: [sdc] Write Protect is off
[20237.769163] sd 6:0:0:0: [sdc] Mode Sense: 45 00 00 00
[20237.769903] sd 6:0:0:0: [sdc] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[20237.774661]  sdc: sdc1 sdc2
[20237.777526] sd 6:0:0:0: [sdc] Attached SCSI removable disk

I'm on Ubuntu 17.04.

1

The 1 in ...:1.0 is the configuration (most USB devices only have one configuration), the 0 is the interface.

Each USB device can have multiple configurations with multiple interfaces. Only one configuration can be enabled at a time, and different configurations can have different power characteristics. The interfaces of one configuration are accessible at the same time, and group endpoints that perform some function.

See e.g. here for more details.

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