Writing Device Drivers

The Descriptor Tree

Parsing descriptors involves aligning structure members at natural boundaries and converting the structure members to the endianness of the host CPU. Parsed standard USB configuration descriptors, interface descriptors, and endpoint descriptors are available to the client driver in the form of a hierarchical tree for each configuration. Any raw class-specific or vendor-specific descriptor information also is available to the client driver in the same hierarchical tree.

Call the usb_get_dev_data(9F) function to retrieve the hierarchical descriptor tree. The “SEE ALSO” section of the usb_get_dev_data(9F) man page lists the man pages for each standard USB descriptor. Use the usb_parse_data(9F) function to parse raw descriptor information.

A descriptor tree for a device with two configurations might look like the tree shown in the following figure.

Figure 20–3 A Hierarchical USB Descriptor Tree

Diagram shows a tree of pairs of descriptors for each
interface of a device with two configurations.

The dev_cfg array shown in the above figure contains nodes that correspond to configurations. Each node contains the following information:

The node that represents the second interface of the second indexed configuration is at dev_cfg[1].cfg_if[1] in the diagram. That node contains an array of nodes that represent the alternate settings for that interface. The hierarchy of USB descriptors propagates through the tree. ASCII strings from string descriptor data are attached where the USB specification says these strings exist.

The array of configurations is non-sparse and is indexed by the configuration index. The first valid configuration (configuration 1) is dev_cfg[0]. Interfaces and alternate settings have indices that align with their numbers. Endpoints of each alternate setting are indexed consecutively. The first endpoint of each alternate setting is at index 0.

This numbering scheme makes the tree easy to traverse. For example, the raw descriptor data of endpoint index 0, alternate 0, interface 1, configuration index 1 is at the node defined by the following path:


An alternative to using the descriptor tree directly is using the usb_lookup_ep_data(9F) function. The usb_lookup_ep_data(9F) function takes as arguments the interface, alternate, which endpoint, endpoint type, and direction. You can use the usb_lookup_ep_data(9F) function to traverse the descriptor tree to get a particular endpoint. See the usb_get_dev_data(9F) man page for more information.