struct usb_host_conf(9)
NAME
struct_usb_host_config - representation of a device's configuration
SYNOPSIS
struct usb_host_config {
struct usb_config_descriptor desc;
char * string;
struct usb_interface_assoc_descriptor * intf_assoc[USB_MAXIADS];
struct usb_interface * interface[USB_MAXINTERFACES];
struct usb_interface_cache * intf_cache[USB_MAXINTERFACES];
unsigned char * extra;
int extralen;
};
MEMBERS
- desc
- the device´s configuration descriptor.
- string
- pointer to the cached version of the iConfiguration string, if
present for this configuration. - intf_assoc[USB_MAXIADS]
- list of any interface association descriptors in this config
- interface[USB_MAXINTERFACES]
- array of pointers to usb_interface structures, one for each
interface in the configuration. The number of interfaces is stored in desc.bNumInterfaces. These pointers are valid only while the the configuration is active. - intf_cache[USB_MAXINTERFACES]
- array of pointers to usb_interface_cache structures, one for each
interface in the configuration. These structures exist for the
entire life of the device. - extra
- pointer to buffer containing all extra descriptors associated with
this configuration (those preceding the first interface
descriptor). - extralen
- length of the extra descriptors buffer.
DESCRIPTION
USB devices may have multiple configurations, but only one can be
active at any time. Each encapsulates a different operational
environment; for example, a dual-speed device would have separate
configurations for full-speed and high-speed operation. The number of
configurations available is stored in the device descriptor as
bNumConfigurations.
A configuration can contain multiple interfaces. Each corresponds to a
different function of the USB device, and all are available whenever
the configuration is active. The USB standard says that interfaces are
supposed to be numbered from 0 to desc.bNumInterfaces-1, but a lot of
devices get this wrong. In addition, the interface array is not
guaranteed to be sorted in numerical order. Use usb_ifnum_to_if to look
up an interface entry based on its number.
Device drivers should not attempt to activate configurations. The
choice of which configuration to install is a policy decision based on
such considerations as available power, functionality provided, and the
user´s desires (expressed through userspace tools). However, drivers
can call usb_reset_configuration to reinitialize the current
configuration and all its interfaces.