Writing Device Drivers

Device Configuration Facilities

This section describes functions related to device configuration.

Getting Interface Numbers

If you are using a multiple-interface device where the usb_mid(7D) driver is making only one of its interfaces available to the calling driver, you might need to know the number of the interface to which the calling driver is bound. Use the usb_get_if_number(9F) function to do any of the following tasks:

Managing Entire Devices

If a driver manages an entire composite device, that driver can bind to the entire device by using a compatible name that contains vendor ID, product ID, and revision ID. A driver that is bound to an entire composite device must manage all the interfaces of that device as a nexus driver would. In general, you should not bind your driver to an entire composite device. Instead, you should use the generic multiple-interface driver usb_mid(7D).

Use the usb_owns_device(9F) function to determine whether a driver owns an entire device. The device might be a composite device. The usb_owns_device(9F) function returns TRUE if the driver owns the entire device.

Multiple-Configuration Devices

USB devices make only a single configuration available to the host at any particular time. Most devices support only a single configuration. However, a few USB devices support multiple configurations.

Any device that has multiple configurations is placed into the first configuration for which a driver is available. When seeking a match, device configurations are considered in numeric order. If no matching driver is found, the device is set to the first configuration. In this case, the usb_mid driver takes over the device and splits the device into interface nodes. Use the usb_get_cfg(9F) function to return the current configuration of a device.

You can use either of the following two methods to request a different configuration. Using either of these two methods to modify the device configuration ensures that the USBA module remains in sync with the device.

Caution – Caution –

Do not change the device configuration by doing a SET_CONFIGURATION USB request manually. Using a SET_CONFIGURATION request to change the configuration is not supported.

Modifying or Getting the Alternate Setting

A client driver can call the usb_set_alt_if(9F) function to change the selected alternate setting of the currently selected interface. Be sure to close all pipes that were opened explicitly. When switching alternate settings, the usb_set_alt_if(9F) function verifies that only the default pipe is open. Be sure the device is settled before you call usb_set_alt_if(9F).

Changing the alternate setting can affect which endpoints and which class-specific and vendor-specific descriptors are available to the driver. See The Descriptor Tree for more information about endpoints and descriptors.

Call the usb_get_alt_if(9F) function to retrieve the number of the current alternate setting.

Note –

When you request a new alternate setting, a new configuration, or a new interface, all pipes except the default pipe to the device must be closed. This is because changing an alternate setting, a configuration, or an interface changes the mode of operation of the device. Also, changing an alternate setting, a configuration, or an interface changes the device's presentation to the system.