usb gadget: composite gadget core

Add <linux/usb/composite.h> interfaces for composite gadget drivers, and
basic implementation support behind it:

  - struct usb_function ... groups one or more interfaces into a function
    managed as one unit within a configuration, to which it's added by
    usb_add_function().

  - struct usb_configuration ... groups one or more such functions into
    a configuration managed as one unit by a driver, to which it's added
    by usb_add_config().  These operate at either high or full/low speeds
    and at a given bMaxPower.

  - struct usb_composite_driver ... groups one or more such configurations
    into a gadget driver, which may be registered or unregistered.

  - struct usb_composite_dev ... a usb_composite_driver manages this; it
    wraps the usb_gadget exposed by the controller driver.

This also includes some basic kerneldoc.

How to use it (the short version):  provide a usb_composite_driver with a
bind() that calls usb_add_config() for each of the needed configurations.
The configurations in turn have bind() calls, which will usb_add_function()
for each function required.  Each function's bind() allocates resources
needed to perform its tasks, like endpoints; sometimes configurations will
allocate resources too.

Separate patches will convert most gadget drivers to this infrastructure.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/DocBook/gadget.tmpl

@@ -524,6 +524,41 @@ These utilities include endpoint autoconfiguration.
 <!-- !Edrivers/usb/gadget/epautoconf.c -->
 </sect1>
 
+<sect1 id="composite"><title>Composite Device Framework</title>
+
+<para>The core API is sufficient for writing drivers for composite
+USB devices (with more than one function in a given configuration),
+and also multi-configuration devices (also more than one function,
+but not necessarily sharing a given configuration).
+There is however an optional framework which makes it easier to
+reuse and combine functions.
+</para>
+
+<para>Devices using this framework provide a <emphasis>struct
+usb_composite_driver</emphasis>, which in turn provides one or
+more <emphasis>struct usb_configuration</emphasis> instances.
+Each such configuration includes at least one
+<emphasis>struct usb_function</emphasis>, which packages a user
+visible role such as "network link" or "mass storage device".
+Management functions may also exist, such as "Device Firmware
+Upgrade".
+</para>
+
+!Iinclude/linux/usb/composite.h
+!Edrivers/usb/gadget/composite.c
+
+</sect1>
+
+<sect1 id="functions"><title>Composite Device Functions</title>
+
+<para>At this writing, a few of the current gadget drivers have
+been converted to this framework.
+Near-term plans include converting all of them, except for "gadgetfs".
+</para>
+
+</sect1>
+
+
 </chapter>
 
 <chapter id="controllers"><title>Peripheral Controller Drivers</title>