Mentor Graphics supplies a preliminary GPL’d high speed stack , building on the gadget API and used with chips integrating their silicon design. The endpoint maxpacket field is updated according to the chosen descriptor. That lets you develop some parts of a gadget driver on a normal PC, without any special hardware, and perhaps with the assistance of tools such as GDB running with User Mode Linux. Other controller and gadget drivers are in development, but are unreleased or not published here. Binding configurations assigns global resources including string IDs, and per-configuration resources such as interface IDs and endpoints. For example, the functions on a composite device might give hosts concurrent access to local audio, internet, and file servers.

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It’s easy to backport current 2. Do real work and perform data transfers, possibly involving changes to interface settings or switching to new configurations, until the device is disconnect ed from the host.

Except for the driver data, all fields in this structure are read-only to the gadget driver. For example, building configuration descriptors from vectors of descriptors for the configurations interfaces and endpoints is now automated, and many drivers now use autoconfiguration to choose hardware endpoints and initialize their descriptors.

Gadget driver responsibilities include:.

Again, this is directly analagous to what host side drivers do, though producer and consumer roles are probably reversed. The bind function will be libux to bind it to a gadget before this registration call returns.


Some hardware also constrains other descriptors. The first is a layer that doesn’t exist: There are also several discrete USB controllers.

This copies descriptors into the response buffer, building a descriptor for that configuration. It’s designed for flexibility: Most of those are full speed controllers. Such interdependency may be managed in any way, so long as all of the descriptors complete by the time the composite driver returns from its gsdgetfs routine. These are directly analagous to host side “interface drivers”, except for multi-function “composite” gadget drivers. You can think of that device as being the virtual hardware to which the higher level drivers are written.

Hardware tends to have arbitrary restrictions, relating to transfer types, addressing, packet sizes, buffering, and availability.

Other than providing gadtetfs additional OTG descriptor in each configuration, only the HNP-related differences are particularly visible to driver code. Before returning from bindvarious fields in the template descriptor may be overridden.

USB/Linux USB Layers/GadgetFs

There may also be class-specific or vendor-specific requests to handle. There will often be several such layers, perhaps including user mode components.

Convenience functions make it easier to test which of the currently configured controller chips is being used. On disconnect, the drivers go back to step 3 above. Tests are available too, which can speed hardware bringup substantially. For example, the functions on a composite device might give hosts concurrent access to local audio, internet, and file servers.

In addition to Gadget Zero used primarily for testing and development with drivers for usb controller hardwareother gadget drivers exist. Controller Drivers Drivers for several different usb device chips are available. When the gadget driver module is being unloaded, the driver unbind callback is issued. You should assign those addresses using a registered IEEE company id ; this will also make the device appear to Linux hosts as an “ethN” interface, not as “usbN”.


The core API is sufficient for writing drivers for composite USB devices with more than one function in a given configurationand also multi-configuration devices also more than one function, but not necessarily sharing a given configuration.

USB Gadget API for Linux — The Linux Kernel documentation

Gadget Drivers use that gadget API, and can often be written to be hardware-neutral. Support for other controllers is expected to be developed and contributed over time, as this driver framework evolves. Otherwise, a Net PCI card lets you work on a standard PC, developing or debugging gadget drivers on computers you may already own.

This driver can interoperate with hosts running Linux, MS-Windows, and other operating systems; if the OS can talk to a cable modem, it probably supports this driver. Returns length of descriptor always even or negative errno. An example of multiple activations might be a CDC ACM function that supports two or more distinct instances within the same configuration, providing gadgehfs independent logical data links to a USB host.