Chapter 3. Kernel Mode Gadget API
3.1. Driver Life Cycle
Gadget drivers make endpoint I/O requests to hardware without needing to know many details of the
hardware, but driver setup/configuration code needs to handle some differences. Use the API like this:
1. Register a driver for the particular device side usb controller hardware, such as the net2280 on PCI
(USB 2.0), sa11x0 or pxa25x as found in Linux PDAs, and so on. At this point the device is
logically in the USB ch9 initial state ("attached"), drawing no power and not usable (since it does
not yet support enumeration). Any host should not see the device, since it’s not activated the data
line pullup used by the host to detect a device, even if VBUS power is available.
2. Register a gadget driver that implements some higher level device function. That will then bind() to
a usb_gadget, which activates the data line pullup sometime after detecting VBUS.
3. The hardware driver can now start enumerating. The steps it handles are to accept USB power and
set_address requests. Other steps are handled by the gadget driver. If the gadget driver module is
unloaded before the host starts to enumerate, steps before step 7 are skipped.
4. The gadget driver’s setup() call returns usb descriptors, based both on what the bus interface
hardware provides and on the functionality being implemented. That can involve alternate settings
or configurations, unless the hardware prevents such operation. For OTG devices, each configuration
descriptor includes an OTG descriptor.
5. The gadget driver handles the last step of enumeration, when the USB host issues a
set_configuration call. It enables all endpoints used in that configuration, with all interfaces in their
default settings. That involves using a list of the hardware’s endpoints, enabling each endpoint
according to its descriptor. It may also involve using usb_gadget_vbus_draw to let more power
be drawn from VBUS, as allowed by that configuration. For OTG devices, setting a configuration
may also involve reporting HNP capabilities through a user interface.
6. 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. Queue any number
of transfer requests to each endpoint. It may be suspended and resumed several times before being
disconnected. On disconnect, the drivers go back to step 3 (above).
7. When the gadget driver module is being unloaded, the driver unbind() callback is issued. That lets
the controller driver be unloaded.
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. Note that at this
lowest level there are no policies about how ep0 configuration logic is implemented, except that it should
obey USB specifications. 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.
Note that the lifecycle above can be slightly different for OTG devices. Other than providing an
additional OTG descriptor in each configuration, only the HNP-related differences are particularly
visible to driver code. They involve reporting requirements during the SET_CONFIGURATION request,
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