The USB Serial Driver Layer, Part II

In the previous description of how the USB serial device becomes bound to a specific USB serial driver, the kernel driver core is called a number of times. This happens because the USB serial core is represented as a bus within the kernel driver model, allowing multiple ports to be present on a single USB device.

For example, the following device is an eight-port USB-to-serial device on the first USB bus in the system. Its location in sysfs is /sys/devices/pci0/00:09.0/usb1/1-1/1-1.1. Within that directory are the following directories and files: 1-1.1:0/, bcdDevice, bConfigurationValue, bDeviceClass, bDeviceProtocol, bDeviceSubClass, bmAttributes, bMaxPower, bNumConfigurations, bNumInterfaces, idProduct, idVendor, manufacturer, name, power, product, serial, speed, ttyUSB0/, ttyUSB1/, ttyUSB2/, ttyUSB3/, ttyUSB4/, ttyUSB5/, ttyUSB6/ and ttyUSB7/.

The files in this directory provide the USB-specific information for this device, as do the files in the 1-1.1:0/ directory, which is the first interface on this device. The ttyUSB* directories are created by the USB serial core and contain the following files: dev, name and power.

The dev file contains the major and minor number for this specific device, which then can be used to determine the proper device node for talking to it. In the /sys/bus/usb directory, this USB device is seen as being bound to the io_edgeport USB driver (Figure 3).

Figure 3. The /sys/bus/usb Tree

There is also a usb-serial bus, which shows the individual USB serial ports that are registered with the kernel (Figure 4). As these individual ports are tty devices, they also show up in the tty class directory (Figure 5).

Figure 4. The /sys/bus/usb-serial Tree

Figure 5. The /sys/class/tty Tree

Through all of these different links back to the single USB device, the type of USB device, how many tty ports it has and what type of USB serial driver controls it, easily can be determined. This is also much more information than what was shown in the /proc/tty/driver/usb-serial file, as described in Part I of this article.

The sysfs interface is described here only briefly, but it contains a wealth of information about all physical and virtual devices that are contained in a system at a given point in time. For a better description of sysfs and the kernel driver model, see Pat Mochel's 2003 linux.conf.au paper at www.kernel.org/pub/linux/kernel/people/mochel/doc/lca.