Descriptor Data Structures

The Linux USB subsystem describes the hierarchical structure of descriptors by extending or embedding the standard USB descriptors with or in a subsystem specific structure. This structure helps storing pointers to the selected configuration and interfaces.

The elements of these structures are only explained in detail as far as they are necessary for subsequent API calls. Detailed information about the descriptors can be found in usb.h and [4] section 9.5.

struct usb_device{
  ...
  struct usb_config_descriptor *actconfig;/* the active configuration */
  ...
  struct usb_device_descriptor descriptor;/* Descriptor */
  struct usb_config_descriptor *config;	/* All of the configs */
}

The usb_device structure is the root of all USB specific descriptors. Sometimes it is necessary to parse the descriptors within a driver to configure the device or to setup transfer requests properly.

• Accessing all available configuration descriptors can be done like this:

  
  for (i = 0; i < dev->descriptor.bNumConfigurations; i++) {
    struct usb_config_descriptor *cfg = &dev->config[i];
    ...
  }
  

• Accessing all available interface descriptors of a particular configuration is done like this:

  
  for (j = 0; j < cfg->bNumInterfaces; j++) {
    struct usb_interface *ifp = &cfg->interface[j];
    ...
  }
  

  To start the parsing of the active configuration simply use the dev->actconfig pointer.

• Accessing all alternate settings of a particular interface can be done like this:

  
  for (k = 0; k < ifp->num_altsetting; k++) {
    struct usb_interface_descriptor *as = &ifp->altsetting[k];
    ...
  }
  

  The active alternate setting can be accessed via
  *as = &ifp->altsetting[ifp->act_altsetting]

• Accessing all endpoint descriptors of a particular alternate setting can done like this:

  
  for(l = 0; l < as->bNumEndpoints; l++) {
    struct usb_endpoint_descriptor *ep=&as->endpoint[k];
    ...
  }