There will also be an OTG Controller Driver , which is visible to gadget and device driver developers only indirectly. Returns length of descriptor always even or negative errno. Most new development is based on 2. Such issues are in the domain of gadget drivers, including knowing about implementation constraints imposed by some USB controllers or understanding that composite devices might happen to be built by integrating reusable components. At this writing, there are people at work on drivers in this framework for several other USB device controllers, with plans to make many of them be widely available. Such simulators can help debug subsystems where the runtime hardware is unfriendly to software development, or is not yet available. Support for other controllers is expected to be developed and contributed over time, as this driver framework evolves.

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This function is used to register drivers using the composite driver framework.

Except for the driver data, all fields linuz this structure are read-only to the gadget driver. Use the API like this: Drivers will normally be arranged so that just loading the gadget driver module or statically linking it into a Linux kernel allows the peripheral device to be enumerated, but some drivers will defer enumeration until some higher level component like a user mode daemon enables it.

USB/Linux USB Layers/GadgetFs

Slightly more complex gadgets may have more than one single-function gadgefts at a given speed; or have configurations that only work at one speed.

The return value is zero, or a negative errno value.

If the endpoint already has a descriptor assigned to it – overwrites it with currently corresponding descriptor. Most gadget drivers have an upper boundary that connects to some Linux driver or framework in Linux. Peripheral Controller Drivers implement the gadget API, and are the only layers that talk directly to hardware.


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.

Binding configurations assigns global resources including string IDs, and per-configuration resources such as interface IDs and endpoints.

Support for other controllers is expected to be developed and contributed over time, as this driver framework evolves. The Linux-USB host-side and peripheral-side driver stacks are directly analagous, except that only the host side has a mid layer.

Linux-USB Gadget API Framework

There may also be class-specific or vendor-specific requests to handle. For example, some controllers have more endpoints than others, many don’t support high speed or isochronous transfers, and sometimes endpoints have fixed configurations. Functions may have static tables of descriptors, which are used as templates and customized with identifiers for interfaces, strings, endpoints, and more as needed by a given function instance.

These generate and consume the data that the gadget driver transfers to the host through the controller driver. The core API does not expose every possible hardware feature, linud the most widely available ones.

At this writing, there is no Linux kernel implementation of PTP on either host or peripheral side. If the gadget driver module is unloaded before the host starts to enumerate, steps before step 7 are skipped. It provides two configurations: They are implemented on 2. When the gadget driver module is being unloaded, the driver unbind callback is issued. Mentor Graphics supplies a preliminary GPL’d high speed stackbuilding on the gadget API and used with chips integrating their silicon design.

The first is a layer that doesn’t gadyetfs To support dual speed operation, a function driver provides descriptors for both high and full speed operation. These are directly analagous to host side “interface drivers”, except for multi-function “composite” gadget drivers. Such gadget-side interface drivers should eventually be combined, to implement composite devices.


This function chooses the right descriptors for a given endpoint according to gadget speed and saves it in the endpoint desc field. Composite devices are, by definition, ones with configurations which include more than one function.

Because such hardware varies widely in capabilities and restrictions, and is used in embedded environments where space is at a premium, the gadget driver is often configured tadgetfs compile time to work with endpoints supported gadgwtfs one particular controller. In Linux, from the bottom up, these layers are: Many of the chips mentioned are highly integrated System-on-Chip SOC processors, typically integrating a bit CPU core with cores for USB and numerous other features such as signal processors and optimized for efficient use on battery power.

However, its main gadgetgs to fame is being able to connect directly to recent versions of Windows, using drivers that Microsoft bundles and supports, making it much simpler to network with Windows. It enables all endpoints used in that configuration, with all interfaces in their default settings. A gadget driver implements one or more “functions”, each providing a different capability to the Lihux host, such as a network link or speakers.

The lower boundary of this driver implements hardware-neutral USB functions, using calls to the controller driver.