Wednesday, 8 April 2015

sysfs structures for Linux USB

Q: What are the sysfs structures for Linux USB?
A: For example the directory will have something like:
# ls  /sys/bus/usb/devices/
1-0:1.0      1-1.3        1-1.3.1:1.0  1-1:1.0
1-1          1-1.3.1      1-1.3:1.0    usb1

The names that begin with "usb" refer to USB controllers. More accurately, they refer to the "root hub" associated with each controller. The number is the USB bus number. In the example there is only one controller, so its bus is number 1. Hence the name "usb1".

"1-0:1.0" is a special case. It refers to the root hub's interface. This acts just like the interface in an actual hub an almost every respect; see below.
All the other entries refer to genuine USB devices and their interfaces. The devices are named by a scheme like this:

 bus-port.port.port ...
In other words, the name starts with the bus number followed by a '-'. Then comes the sequence of port numbers for each of the intermediate hubs along the path to the device.
For example, "1-1" is a device plugged into bus 1, port 1. It happens to be a hub, and "1-1.3" is the device plugged into port 3 of that hub. That device is another hub, and "1-1.3.1" is the device plugged into its port 1.

The interfaces are indicated by suffixes having this form:

That is, a ':' followed by the configuration number followed by '.' followed by the interface number. In the above example, each of the devices is using configuration 1 and this configuration has only a
single interface, number 0. So the interfaces show up as;

 1-1:1.0  1-1.3:1.0  1-1.3.1:1.0

A hub will never have more than a single interface; that's part of the USB spec. But other devices can and do have multiple interfaces (and sometimes multiple configurations). Each interface gets its own entry in sysfs and can have its own driver.

overall summery of the sysfs structure path :

|_usb root hub - bus number -1
    |_ port number - 1 of root hub
       |_port number - 3 of intermediate hub
          |_current configuration number - 1
            |_ current interface number - 0


Salient features of sysfs includes:

  1.  A hierarchical view of the complete device tree where the various components of the driver model are organized as directories, attributes as files, and interconnections as links.
  2. A type-safe kernel interface.
  3. A facility by which any layer can export its attributes.
  4. A true representation of the various relationships that exist between devices, buses, and drivers in the system.

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