The U-Boot Driver Model Project
===============================
USB analysis
============
Marek Vasut <marek.vasut@gmail.com>
2012-02-16

I) Overview
-----------

  1) The USB Host driver
  ----------------------
  There are basically four or five USB host drivers. All such drivers currently
  provide at least the following fuctions:

    usb_lowlevel_init() ... Do the initialization of the USB controller hardware
    usb_lowlevel_stop() ... Do the shutdown of the USB controller hardware

    usb_event_poll() ...... Poll interrupt from USB device, often used by KBD

    submit_control_msg() .. Submit message via Control endpoint
    submit_int_msg() ...... Submit message via Interrupt endpoint
    submit_bulk_msg() ..... Submit message via Bulk endpoint


    This allows for the host driver to be easily abstracted.

  2) The USB hierarchy
  --------------------

  In the current implementation, the USB Host driver provides operations to
  communicate via the USB bus. This is realised by providing access to a USB
  root port to which an USB root hub is attached. The USB bus is scanned and for
  each newly found device, a struct usb_device is allocated. See common/usb.c
  and include/usb.h for details.

II) Approach
------------

  1) The USB Host driver
  ----------------------

  Converting the host driver will follow the classic driver model consideration.
  Though, the host driver will have to call a function that registers a root
  port with the USB core in it's probe() function, let's call this function

    usb_register_root_port(&ops);

  This will allow the USB core to track all available root ports. The ops
  parameter will contain structure describing operations supported by the root
  port:

  struct usb_port_ops {
    void   (*usb_event_poll)();
    int    (*submit_control_msg)();
    int    (*submit_int_msg)();
    int    (*submit_bulk_msg)();
  }

  2) The USB hierarchy and hub drivers
  ------------------------------------

  Converting the USB heirarchy should be fairy simple, considering the already
  dynamic nature of the implementation. The current usb_hub_device structure
  will have to be converted to a struct instance. Every such instance will
  contain components of struct usb_device and struct usb_hub_device in it's
  private data, providing only accessors in order to properly encapsulate the
  driver.

  By registering the root port, the USB framework will instantiate a USB hub
  driver, which is always present, the root hub. The root hub and any subsequent
  hub instance is represented by struct instance and it's private data contain
  amongst others common bits from struct usb_device.

  Note the USB hub driver is partly defying the usual method of registering a
  set of callbacks to a particular core driver. Instead, a static set of
  functions is defined and the USB hub instance is passed to those. This creates
  certain restrictions as of how the USB hub driver looks, but considering the
  specification for USB hub is given and a different type of USB hub won't ever
  exist, this approach is ok:

  - Report how many ports does this hub have:
      uint get_nr_ports(struct instance *hub);
  - Get pointer to device connected to a port:
      struct instance *(*get_child)(struct instance *hub, int port);
  - Instantiate and configure device on port:
      struct instance *(*enum_dev_on_port)(struct instance *hub, int port);

  3) USB device drivers
  ---------------------

  The USB device driver, in turn, will have to register various ops structures
  with certain cores. For example, USB disc driver will have to register it's
  ops with core handling USB discs etc.