======================FunctionFS Descriptors====================== Some of the descriptors that can be written to the FFS gadget aredescribed below. Device and configuration descriptors are handledby the composite gadget and are not written by the user to theFFS gadget. Descriptors are written to the "ep0" file in the FFS gadgetfollowing the descriptor header. .. kernel-doc:: include/uapi/linux/usb/functionfs.h   :doc: descriptors Interface Descriptors--------------------- Standard USB interface descriptors may be written. The class/subclass of themost recent interface descriptor determines what type of class-specificdescriptors are accepted. Class-Specific Descriptors-------------------------- Class-specific descriptors are accepted only for the class/subclass of themost recent interface descriptor. The following are some of theclass-specific descriptors that are supported. DFU Functional Descriptor~~~~~~~~~~~~~~~~~~~~~~~~~ When the interface class is USB_CLASS_APP_SPEC and the interface subclassis USB_SUBCLASS_DFU, a DFU functional descriptor can be provided.The DFU functional descriptor is a described in the USB specification forDevice Firmware Upgrade (DFU), version 1.1 as of this writing. .. kernel-doc:: include/uapi/linux/usb/functionfs.h   :doc: usb_dfu_functional_descriptor