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<chapter id="usb">
<title>USB Devices</title>
<para><emphasis>This chapter was written by &a.nhibma;. Modifications made for
the handbook by &a.murray;.</emphasis></para>
<sect1>
<title>Introduction</title>
<para>The Universal Serial Bus (USB) is a new way of attaching
devices to personal computers. The bus architecture features
two-way communication and has been developed as a response to
devices becoming smarter and requiring more interaction with the
host. USB support is included in all current PC chipsets and is
therefore available in all recently built PCs. Apple's
introduction of the USB-only iMac has been a major incentive for
hardware manufacturers to produce USB versions of their devices.
The future PC specifications specify that all legacy connectors
on PCs should be replaced by one or more USB connectors,
providing generic plug and play capabilities. Support for USB
hardware was available at a very early stage in NetBSD and was
developed by Lennart Augustsson for the NetBSD project. The
code has been ported to FreeBSD and we are currently maintaining
a shared code base. For the implementation of the USB subsystem
a number of features of USB are important.</para>
<para><emphasis>Lennart Augustsson has done most of the implementation of
the USB support for the NetBSD project. Many thanks for this
incredible amount of work. Many thanks also to Ardy and Dirk for
their comments and proofreading of this paper.</emphasis></para>
<itemizedlist>
<listitem><para>Devices connect to ports on the computer
directly or on devices called hubs, forming a treelike device
structure.</para></listitem>
<listitem><para>The devices can be connected and disconnected at
run time.</para></listitem>
<listitem><para>Devices can suspend themselves and trigger
resumes of the host system</para></listitem>
<listitem><para>As the devices can be powered from the bus, the
host software has to keep track of power budgets for each
hub.</para></listitem>
<listitem><para>Different quality of service requirements by the
different device types together with the maximum of 126
devices that can be connected to the same bus, require proper
scheduling of transfers on the shared bus to take full
advantage of the 12Mbps bandwidth available. (over 400Mbps
with USB 2.0)</para></listitem>
<listitem><para>Devices are intelligent and contain easily
accessible information about themselves</para></listitem>
</itemizedlist>
<para>The development of drivers for the USB subsystem and devices
connected to it is supported by the specifications that have
been developed and will be developed. These specifications are
publicly available from the USB home pages. Apple has been very
strong in pushing for standards based drivers, by making drivers
for the generic classes available in their operating system
MacOS and discouraging the use of separate drivers for each new
device. This chapter tries to collate essential information for a
basic understanding of the present implementation of the USB
stack in FreeBSD/NetBSD. It is recommended however to read it
together with the relevant specifications mentioned in the
references below.</para>
<sect2>
<title>Structure of the USB Stack</title>
<para>The USB support in FreeBSD can be split into three
layers. The lowest layer contains the host controller driver,
providing a generic interface to the hardware and its scheduling
facilities. It supports initialisation of the hardware,
scheduling of transfers and handling of completed and/or failed
transfers. Each host controller driver implements a virtual hub
providing hardware independent access to the registers
controlling the root ports on the back of the machine.</para>
<para>The middle layer handles the device connection and
disconnection, basic initialisation of the device, driver
selection, the communication channels (pipes) and does
resource management. This services layer also controls the
default pipes and the device requests transferred over
them.</para>
<para>The top layer contains the individual drivers supporting
specific (classes of) devices. These drivers implement the
protocol that is used over the pipes other than the default
pipe. They also implement additional functionality to make the
device available to other parts of the kernel oruserland. They
use the USB driver interface (USBDI) exposed by the services
layer.</para>
</sect2>
</sect1>
<sect1 id="usb-hc">
<title>Host Controllers</title>
<para>The host controller (HC) controls the transmission of
packets on the bus. Frames of 1 millisecond are used. At the
start of each frame the host controller generates a Start of
Frame (SOF) packet.</para>
<para>The SOF packet is used to synchronise to the start of the
frame and to keep track of the frame number. Within each frame
packets are transferred, either from host to device (out) or
from device to host (in). Transfers are always initiated by the
host (polled transfers). Therefore there can only be one host
per USB bus. Each transfer of a packet has a status stage in
which the recipient of the data can return either ACK
(acknowledge reception), NAK (retry), STALL (error condition) or
nothing (garbled data stage, device not available or
disconnected). Section 8.5 of the <ulink
url="http://www.usb.org/developers/docs.html">USB
specification</ulink> explains the details of packets in more
detail. Four different types of transfers can occur on a USB
bus: control, bulk, interrupt and isochronous. The types of
transfers and their characteristics are described below (`Pipes'
subsection).</para>
<para>Large transfers between the device on the USB bus and the
device driver are split up into multiple packets by the host
controller or the HC driver.</para>
<para>Device requests (control transfers) to the default endpoints
are special. They consist of two or three phases: SETUP, DATA
(optional) and STATUS. The set-up packet is sent to the
device. If there is a data phase, the direction of the data
packet(s) is given in the set-up packet. The direction in the
status phase is the opposite of the direction during the data
phase, or IN if there was no data phase. The host controller
hardware also provides registers with the current status of the
root ports and the changes that have occurred since the last
reset of the status change register. Access to these registers
is provided through a virtualised hub as suggested in the USB
specification [ 2]. Thevirtual hub must comply with the hub
device class given in chapter 11 of that specification. It must
provide a default pipe through which device requests can be sent
to it. It returns the standard andhub class specific set of
descriptors. It should also provide an interrupt pipe that
reports changes happening at its ports. There are currently two
specifications for host controllers available: <ulink
url="http://developer.intel.com/design/USB/UHCI11D.htm">Universal
Host Controller Interface</ulink> (UHCI; Intel) and <ulink
url="http://www.compaq.com/productinfo/development/openhci.html">Open
Host Controller Interface</ulink> (OHCI; Compaq, Microsoft,
National Semiconductor). The UHCI specification has been
designed to reduce hardware complexity byrequiring the host
controller driver to supply a complete schedule of the transfers
for each frame. OHCI type controllers are much more independent
by providing a more abstract interface doing alot of work
themselves. </para>
<sect2>
<title>UHCI</title>
<para>The UHCI host controller maintains a framelist with 1024
pointers to per frame data structures. It understands two
different data types: transfer descriptors (TD) and queue
heads (QH). Each TD represents a packet to be communicated to
or from a device endpoint. QHs are a means to groupTDs (and
QHs) together.</para>
<para>Each transfer consists of one or more packets. The UHCI
driver splits large transfers into multiple packets. For every
transfer, apart from isochronous transfers, a QH is
allocated. For every type of transfer these QHs are collected
at a QH for that type. Isochronous transfers have to be
executed first because of the fixed latency requirement and
are directly referred to by the pointer in the framelist. The
last isochronous TD refers to the QH for interrupt transfers
for that frame. All QHs for interrupt transfers point at the
QH for control transfers, which in turn points at the QH for
bulk transfers. The following diagram gives a graphical
overview of this:</para>
<para>This results in the following schedule being run in each
frame. After fetching the pointer for the current frame from
the framelist the controller first executes the TDs for all
the isochronous packets in that frame. The last of these TDs
refers to the QH for the interrupt transfers for
thatframe. The host controller will then descend from that QH
to the QHs for the individual interrupt transfers. After
finishing that queue, the QH for the interrupt transfers will
refer the controller to the QH for all control transfers. It
will execute all the subqueues scheduled there, followed by
all the transfers queued at the bulk QH. To facilitate the
handling of finished or failed transfers different types of
interrupts are generatedby the hardware at the end of each
frame. In the last TD for a transfer the Interrupt-On
Completion bit is set by the HC driver to flag an interrupt
when the transfer has completed. An error interrupt is flagged
if a TD reaches its maximum error count. If the short packet
detect bit is set in a TD and less than the set packet length
is transferred this interrupt is flagged to notify
the controller driver of the completed transfer. It is the host
controller driver's task to find out which transfer has
completed or produced an error. When called the interrupt
service routine will locate all the finished transfers and
call their callbacks.</para>
<para>See for a more elaborate description the <ulink
url="http://developer.intel.com/design/USB/UHCI11D.htm">UHCI
specification.</ulink></para>
</sect2>
<sect2>
<title>OHCI</title>
<para>Programming an OHCI host controller is much simpler. The
controller assumes that a set of endpoints is available, and
is aware of scheduling priorities and the ordering of the
types of transfers in a frame. The main data structure used by
the host controller is the endpoint descriptor (ED) to which
aqueue of transfer descriptors (TDs) is attached. The ED
contains the maximum packet size allowed for an endpoint and
the controller hardware does the splitting into packets. The
pointers to the data buffers are updated after each transfer
and when the start and end pointer are equal, the TD is
retired to the done-queue. The four types of endpoints have
their own queues. Control and bulk endpoints are queued each at
their own queue. Interrupt EDs are queued in a tree, with the
level in the tree defining the frequency at which they
run.</para>
<para>framelist interruptisochronous control bulk</para>
<para>The schedule being run by the host controller in each
frame looks as follows. The controller will first run the
non-periodic control and bulk queues, up to a time limit set
by the HC driver. Then the interrupt transfers for that frame
number are run, by using the lower five bits of the frame
number as an index into level 0 of the tree of interrupts
EDs. At the end of this tree the isochronous EDs are connected
and these are traversed subsequently. The isochronous TDs
contain the frame number of the first frame the transfer
should be run in. After all the periodic transfers have been
run, the control and bulk queues are traversed
again. Periodically the interrupt service routine is called to
process the done queue and call the callbacks for each
transfer and reschedule interrupt and isochronous
endpoints.</para>
<para>See for a more elaborate description the <ulink
url="http://www.compaq.com/productinfo/development/openhci.html">
OHCI specification</ulink>. Services layer The middle layer
provides access to the device in a controlled way and
maintains resources inuse by the different drivers and the
services layer. The layer takes care of the following
aspects:</para>
<itemizedlist>
<listitem><para>The device configuration
information</para></listitem>
<listitem><para>The pipes to communicate with a
device</para></listitem>
<listitem><para>Probing and attaching and detaching form a
device.</para></listitem>
</itemizedlist>
</sect2>
</sect1>
<sect1 id="usb-dev">
<title>USB Device Information</title>
<sect2>
<title>Device configuration information</title>
<para>Each device provides different levels of configuration
information. Each device has one or more configurations, of
which one is selected during probe/attach. A configuration
provides power and bandwidth requirements. Within each
configuration there can be multiple interfaces. A device
interface is a collection of endpoints. For example USB
speakers can have an interface for the audio data (Audio
Class) and an interface for the knobs, dials and buttons (HID
Class). All interfaces in a configuration areactive at the
same time and can be attached to by different drivers. Each
interface can have alternates, providing different quality of
service parameters. In for example cameras this is used to
provide different frame sizes and numbers of frames per
second.</para>
<para>Within each interface 0 or more endpoints can be
specified. Endpoints are the unidirectional access points for
communicating with a device. They provide buffers to
temporarily store incoming or outgoing data from the
device. Each endpoint has a unique address within
a configuration, the endpoint's number plus its direction. The
default endpoint, endpoint 0, is not part of any interface and
available in all configurations. It is managed by the services
layer and not directly available to device drivers.</para>
<para>Level 0 Level 1 Level 2 Slot 0</para>
<para>Slot 3 Slot 2 Slot 1</para>
<para>(Only 4 out of 32 slots shown)</para>
<para>This hierarchical configuration information is described
in the device by a standard set of descriptors (see section 9.6
of the USB specification [ 2]). They can be requested through
the Get Descriptor Request. The services layer caches these
descriptors to avoid unnecessary transferson the USB
bus. Access to the descriptors is provided through function
calls.</para>
<itemizedlist>
<listitem><para>Device descriptors: General information about
the device, like Vendor, Product and Revision Id, supported
device class, subclass and protocol if applicable, maximum
packet size for the default endpoint, etc.</para></listitem>
<listitem><para>Configuration descriptors: The number of
interfaces in this configuration, suspend and resume
functionality supported and power
requirements.</para></listitem>
<listitem><para>Interface descriptors: interface class,
subclass and protocol if applicable, number of alternate
settings for the interface and the number of
endpoints.</para></listitem>
<listitem><para>Endpoint descriptors: Endpoint address,
direction and type, maximum packet size supported and
polling frequency if type is interrupt endpoint. There is no
descriptor for thedefault endpoint (endpoint 0) and it is
never counted in an interface descriptor.</para></listitem>
<listitem><para>String descriptors: In the other descriptors
string indices are supplied for some fields.These can be
used to retrieve descriptive strings, possibly in multiple
languages.</para></listitem>
</itemizedlist>
<para>Class specifications can add their own descriptor types
that are available through the GetDescriptor Request.</para>
<para>Pipes Communication to end points on a device flows
through so-called pipes. Drivers submit transfers to endpoints
to a pipe and provide a callback to be called on completion or
failure of the transfer (asynchronous transfers) or wait for
completion (synchronous transfer). Transfers to an endpoint
are serialised in the pipe. A transfer can either complete,
fail or time-out (if a time-out has been set). There are two
types of time-outs for transfers. Time-outs can happen due to
time-out on the USBbus (milliseconds). These time-outs are
seen as failures and can be due to disconnection of the
device. A second form of time-out is implemented in software
and is triggered when a transfer does not complete within a
specified amount of time (seconds). These are caused by a
device acknowledging negatively (NAK) the transferred
packets. The cause for this is the device not being ready to
receive data, buffer under- or overrun or protocol
errors.</para>
<para>If a transfer over a pipe is larger than the maximum
packet size specified in the associated endpoint descriptor,
the host controller (OHCI) or the HC driver (UHCI) will split
the transfer into packets of maximum packet size, with the
last packet possibly smaller than the maximum
packetsize.</para>
<para>Sometimes it is not a problem for a device to return less
data than requested. For example abulk-in-transfer to a modem
might request 200 bytes of data, but the modem has only 5
bytes available at that time. The driver can set the short
packet (SPD) flag. It allows the host controller to accept a
packet even if the amount of data transferred is less than
requested. This flag is only valid for in-transfers, as the
amount of data to be sent to a device is always known
beforehand. If an unrecoverable error occurs in a device
during a transfer the pipe is stalled. Before any more data is
accepted or sent the driver needs to resolve the cause of the
stall and clear the endpoint stall condition through send the
clear endpoint halt device request over the default
pipe. The default endpoint should never stall.</para>
<para>There are four different types of endpoints and
corresponding pipes: - Control pipe / default pipe: There is
one control pipe per device, connected to the default endpoint
(endpoint 0). The pipe carries the device requests and
associated data. The difference between transfers over the
default pipe and other pipes is that the protocol for
thetransfers is described in the USB specification [ 2]. These
requests are used to reset and configure the device. A basic
set of commands that must be supported by each device is
provided in chapter 9 of the USB specification [ 2]. The
commands supported on this pipe canbe extended by a device
class specification to support additional
functionality.</para>
<itemizedlist>
<listitem><para>Bulk pipe: This is the USB equivalent to a raw
transmission medium.</para></listitem>
<listitem><para>Interrupt pipe: The host sends a request for
data to the device and if the device has nothing to send, it
will NAK the data packet. Interrupt transfers are scheduled
at a frequency specifiedwhen creating the
pipe.</para></listitem>
<listitem><para>Isochronous pipe: These pipes are intended for
isochronous data, for example video oraudio streams, with
fixed latency, but no guaranteed delivery. Some support for
pipes of this type is available in the current
implementation. Packets in control, bulk and interrupt
transfers are retried if an error occurs during transmission
or the device acknowledges the packet negatively (NAK) due to
for example lack of buffer space to store the incoming
data. Isochronous packets are however not retried in case of
failed delivery or NAK of a packet as this might violate the
timing constraints.</para></listitem>
</itemizedlist>
<para>The availability of the necessary bandwidth is calculated
during the creation of the pipe. Transfersare scheduled within
frames of 1 millisecond. The bandwidth allocation within a
frame is prescribed by the USB specification, section 5.6 [
2]. Isochronous and interrupt transfers areallowed to consume
up to 90% of the bandwidth within a frame. Packets for control
and bulk transfers are scheduled after all isochronous and
interrupt packets and will consume all the remaining
bandwidth.</para>
<para>More information on scheduling of transfers and bandwidth
reclamation can be found in chapter 5of the USB specification
[ 2], section 1.3 of the UHCI specification [ 3] and section
3.4.2 of the OHCI specification [4].</para>
</sect2>
</sect1>
<sect1 id="usb-devprobe">
<title>Device probe and attach</title>
<para>After the notification by the hub that a new device has been
connected, the service layer switcheson the port, providing the
device with 100 mA of current. At this point the device is in
its default state and listening to device address 0. The
services layer will proceed to retrieve the various descriptors
through the default pipe. After that it will send a Set Address
request to move the device away from the default device address
(address 0). Multiple device drivers might be able to support
the device. For example a modem driver might beable to support
an ISDN TA through the AT compatibility interface. A driver for
that specific model of the ISDN adapter might however be able to
provide much better support for this device. To support this
flexibility, the probes return priorities indicating their level
of support. Support for a specific revision of a product ranks
the highest and the generic driver the lowest priority. It might
also be that multiple drivers could attach to one device if
there are multiple interfaceswithin one configuration. Each
driver only needs to support a subset of the interfaces.</para>
<para>The probing for a driver for a newly attached device checks
first for device specific drivers. If notfound, the probe code
iterates over all supported configurations until a driver
attaches in a configuration. To support devices with multiple
drivers on different interfaces, the probe iteratesover all
interfaces in a configuration that have not yet been claimed by
a driver. Configurations that exceed the power budget for the
hub are ignored. During attach the driver should initialise the
device to its proper state, but not reset it, as this will make
the device disconnect itself from the bus and restart the
probing process for it. To avoid consuming unnecessary bandwidth
should not claim the interrupt pipe at attach time, but
should postpone allocating the pipe until the file is opened and
the data is actually used. When the file is closed the pipe
should be closed again, eventhough the device might still be
attached.</para>
<sect2>
<title>Device disconnect and detach</title>
<para>A device driver should expect to receive errors during any
transaction with the device. The designof USB supports and
encourages the disconnection of devices at any point in
time. Drivers should make sure that they do the right thing
when the device disappears.</para>
<para>Furthermore a device that has been disconnected and
reconnected will not be reattached at the same device
instance. This might change in the future when more devices
support serial numbers (see the device descriptor) or other
means of defining an identity for a device have been
developed.</para>
<para>The disconnection of a device is signalled by a hub in the
interrupt packet delivered to the hub driver. The status
change information indicates which port has seen a connection
change. The device detach method for all device drivers for
the device connected on that port are called and the structures
cleaned up. If the port status indicates that in the mean time
a device has been connected to that port, the procedure for
probing and attaching the device will be started. A device
reset will produce a disconnect-connect sequence on the hub
and will be handled as described above.</para>
</sect2>
</sect1>
<sect1 id="usb-protocol">
<title>USB Drivers Protocol Information</title>
<para>The protocol used over pipes other than the default pipe is
undefined by the USB specification. Information on this can be
found from various sources. The most accurate source is the
developer's section on the USB home pages [ 1]. From these pages
a growing number of deviceclass specifications are
available. These specifications specify what a compliant device
should look like from a driver perspective, basic functionality
it needs to provide and the protocol that is to be used over the
communication channels. The USB specification [ 2] includes the
description of the Hub Class. A class specification for Human
Interface Devices (HID) has been created to cater for keyboards,
tablets, bar-code readers, buttons, knobs, switches, etc. A
third example is the class specification for mass storage
devices. For a full list of device classes see the developers
sectionon the USB home pages [ 1].</para>
<para>For many devices the protocol information has not yet been
published however. Information on the protocol being used might
be available from the company making the device. Some companies
will require you to sign a Non -Disclosure Agreement (NDA)
before giving you the specifications. This in most cases
precludes making the driver open source.</para>
<para>Another good source of information is the Linux driver
sources, as a number of companies have started to provide drivers
for Linux for their devices. It is always a good idea to contact
the authors of those drivers for their source of
information.</para>
<para>Example: Human Interface Devices The specification for the
Human Interface Devices like keyboards, mice, tablets, buttons,
dials,etc. is referred to in other device class specifications
and is used in many devices.</para>
<para>For example audio speakers provide endpoints to the digital
to analogue converters and possibly an extra pipe for a
microphone. They also provide a HID endpoint in a separate
interface for the buttons and dials on the front of the
device. The same is true for the monitor control class. It is
straightforward to build support for these interfaces through
the available kernel and userland libraries together with the
HID class driver or the generic driver. Another device that
serves as an example for interfaces within one configuration
driven by different device drivers is a cheap keyboard with
built-in legacy mouse port. To avoid having the cost of
including the hardware for a USB hub in the device,
manufacturers combined the mouse data received from the PS/2 port
on the back of the keyboard and the keypresses from the keyboard
into two separate interfaces in the same configuration. The
mouse and keyboard drivers each attach to the appropriate
interface and allocate the pipes to the two independent
endpoints.</para>
<para>Example: Firmware download Many devices that have been
developed are based on a general purpose processor with
anadditional USB core added to it. Because the development of
drivers and firmware for USB devices is still very new, many
devices require the downloading of the firmware after they
have been connected.</para>
<para>The procedure followed is straightforward. The device
identifies itself through a vendor and product Id. The first
driver probes and attaches to it and downloads the firmware into
it. After that the device soft resets itself and the driver is
detached. After a short pause the devicere announces its presence
on the bus. The device will have changed its
vendor/product/revision Id to reflect the fact that it has been
supplied with firmware and as a consequence a second driver will
probe it and attach to it.</para>
<para>An example of these types of devices is the ActiveWire I/O
board, based on the EZ-USB chip. For this chip a generic firmware
downloader is available. The firmware downloaded into the
ActiveWire board changes the revision Id. It will then perform a
soft reset of the USB part of the EZ-USB chip to disconnect from
the USB bus and again reconnect.</para>
<para>Example: Mass Storage Devices Support for mass storage
devices is mainly built around existing protocols. The Iomega
USB Zipdrive is based on the SCSI version of their drive. The
SCSI commands and status messages are wrapped in blocks and
transferred over the bulk pipes to and from the device,
emulating a SCSI controller over the USB wire. ATAPI and UFI
commands are supported in a similar fashion.</para>
<para>The Mass Storage Specification supports 2 different types of
wrapping of the command block.The initial attempt was based on
sending the command and status through the default pipe and
using bulk transfers for the data to be moved between the host
and the device. Based on experience a second approach was
designed that was based on wrapping the command and status
blocks and sending them over the bulk out and in endpoint. The
specification specifies exactly what has to happen when and what
has to be done in case an error condition is encountered. The
biggest challenge when writing drivers for these devices is to
fit USB based protocol into theexisting support for mass storage
devices. CAM provides hooks to do this in a fairly straight
forward way. ATAPI is less simple as historically the IDE
interface has never had many different appearances.</para>
<para>The support for the USB floppy from Y-E Data is again less
straightforward as a new command set has been designed.</para>
</sect1>
</chapter>
