docs: Add USB spec for version and control proxy interfaces.

Author: zyp

docs/source/developers/index.rst

@@ -1,4 +1,6 @@
 .. toctree::
    usb_trace_interface
    usb_power_interface
+   usb_version_interface
+   usb_control_proxy_interface
    writing_documentation

docs/source/developers/usb_control_proxy_interface.rst

@@ -0,0 +1,23 @@
+USB Control Proxy Interface
+===========================
+
+The control proxy interface is identified by ``bInterfaceClass = 0xff`` and ``bInterfaceSubclass = 0x58 ('X')``.
+
+Certain operating systems (e.g. Windows) disallows issuing control requests to an interface that's already claimed for bulk transfer by another process.
+To allow e.g. configuring the :ref:`usb_trace_interface` while it's already opened for capture, this interface is provided as a workaround.
+
+Control Requests
+----------------
+
+Control requests are vendor-specific interface-directed, i.e. with ``bmRequestType = 0x41 or 0xc1``
+and the lower half of ``wIndex`` containing the ``bInterfaceNumber`` of this interface.
+
+``bRequest`` has a range for each supported target interface with an associated offset.
+When a request is handled, the offset is subtracted from ``bRequest`` and the request is forwarded to the target interface's handler.
+
+
+==============  ======  ==========================
+bRequest range  Offset  Target interface
+==============  ======  ==========================
+0x01 - 0x0f     0x00    :ref:`usb_trace_interface`
+==============  ======  ==========================

docs/source/developers/usb_power_interface.rst

@@ -1,7 +1,7 @@
 USB Power Interface
 ===================
 
-The trace interface is identified by ``bInterfaceClass = 0xff`` and ``bInterfaceSubclass = 0x50 ('P')``.
+The power interface is identified by ``bInterfaceClass = 0xff`` and ``bInterfaceSubclass = 0x50 ('P')``.
 
 Control Requests
 ----------------

docs/source/developers/usb_trace_interface.rst

@@ -1,3 +1,5 @@
+.. _usb_trace_interface:
+
 USB Trace Interface
 ===================
 

docs/source/developers/usb_version_interface.rst

@@ -0,0 +1,13 @@
+USB Version Interface
+=====================
+
+The version interface is identified by ``bInterfaceClass = 0xff`` and ``bInterfaceSubclass = 0x56 ('V')``.
+
+The interface string of the version interface contains the version of the current gateware build, as per ``git describe --always --long --dirty``.
+
+Example: ``Version: v1.0.0-0-g3ad3fa4``
+
+Control Requests
+----------------
+
+The version interface currently has no defined control requests.