USB host controller driver
##########################

This driver is capable to serve different controller types, which a platform
offers, like OHCI, UHCI, EHCI, and XHCI.

It can report the configuration it currently has, as well as a report
of all USB devices it has currently recognized. Both reports can be enabled
separatly via the config node of the driver:

!+ start usb_host | ram: 10M
!  + binary pc_usb_host
!  + provides | + service Usb
!  + config
!    + report | config: yes | devices: yes
!-

The devices report contains most relevant information about an USB device,
beside of its product/vendor information, it also contains information
about configurations, interface, and endpoints available.

Example report:

!+ devices
!  + device usb-2-2 | speed: full | vendor_id: 0x4e6 | product_id: 0x5116 | class: 0x0
!    + config | active: true | value: 0x1
!      + interface
!        | active:      true
!        | number:      0x0
!        | alt_setting: 0x0
!        | class:       0xb
!        | subclass:    0x0
!        | protocol:    0x0
!        + endpoint | address: 0x83 | attributes: 0x3 | max_packet_size: 0x8
!        + endpoint | address: 0x5  | attributes: 0x2 | max_packet_size: 0x40
!        + endpoint | address: 0x84 | attributes: 0x2 | max_packet_size: 0x40
!  + device usb-3-1 | speed: super | vendor_id: 0x1d6b | product_id: 0x3 | class: 0x9
!    + config | active: true | value: 0x1
!      + interface
!        | active:      true
!        | number:      0x0
!        | alt_setting: 0x0
!        | class:       0x9
!        | subclass:    0x0
!        | protocol:    0x0
!        + endpoint | address: 0x81 | attributes: 0x3 | max_packet_size: 0x4
!  + device usb-2-1 | speed: high | vendor_id: 0x1d6b | product_id: 0x2 | class: 0x9
!    + config | active: true | value: 0x1
!      + interface
!        | active:      true
!        | number:      0x0
!        | alt_setting: 0x0
!        | class:       0x9
!        | subclass:    0x0
!        | protocol:    0x0
!        + endpoint | address: 0x81 | attributes: 0x3 | max_packet_size: 0x4
!  + device usb-1-1 | speed: high | vendor_id: 0x1d6b | product_id: 0x2 | class: 0x9
!    + config | active: true | value: 0x1
!      + interface
!        | active:      true
!        | number:      0x0
!        | alt_setting: 0x0
!        | class:       0x9
!        | subclass:    0x0
!        | protocol:    0x0
!        + endpoint | address: 0x81 | attributes: 0x3 | max_packet_size: 0x4
!-

An USB device can get assigned to a client request via policy nodes
within the configuration of the USB host controller driver. One can
assign devices based on their name, a vendor-product-tuple, or by
defining the class-value for all USB device interfaces that matches
that class. Here are some examples:

!+ start usb_host | ram: 10M
!  + binary pc_usb_host
!  + provides | + service Usb
!  + config
!    + policy | label: usb_hid ->
!      + device | class: 0x3
!    + policy | label: usb_block ->
!      + device usb-1-2
!    + policy | label: usb_net ->
!      + device | vendor_id: 0x0b95 | product_id: 0x1790
!-

Currently, if an USB device is assigned to more than one client, only the first client
that acquires it gains access to the device. Once the device got acquired, it vanishs
from the devices ROM of the other clients.


BIOS Handoff
~~~~~~~~~~~~

Per default the driver performs a hand off of the USB controller from the
BIOS, since it still may access the controller when booting, for example, from
an USB device. The BIOS hand off induces the execution of BIOS/SMM USB driver
code and potentially DMA operations. Unfortunately, some ACPI tables report
wrong RMRR information, which implicates IOMMU faults on illegal DMA
operations and consequently the hand off may fail after noticeably long
timeouts. Therefore, the hand off can be disabled in the USB driver
configuration like follows.

!+ config | bios_handoff: no -
