Chapter 13. USB Devices

13.1. Introduction

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.

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.

  • Devices connect to ports on the computer directly or on devices called hubs, forming a treelike device structure.

  • The devices can be connected and disconnected at run time.

  • Devices can suspend themselves and trigger resumes of the host system

  • As the devices can be powered from the bus, the host software has to keep track of power budgets for each hub.

  • 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)

  • Devices are intelligent and contain easily accessible information about themselves

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 USB 2.0 implementation stack in FreeBSD/NetBSD. It is recommended however to read it together with the relevant 2.0 specifications and other developer resources:

13.1.1. Structure of the USB Stack

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.

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.

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 or userland. They use the USB driver interface (USBDI) exposed by the services layer.

13.2. Host Controllers

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.

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 USB 2.0 Specification 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.

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.

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. The virtual 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: Universal Host Controller Interface (UHCI) from Intel and Open Host Controller Interface (OHCI) from Compaq, Microsoft, and National Semiconductor. The UHCI specification has been designed to reduce hardware complexity by requiring 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 a lot of work themselves.

13.2.1. UHCI

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.

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:

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 generated by 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.

Refer to the UHCI Specification for a more elaborate description.

13.2.2. OHCI

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 a queue 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 (interrupt, isochronous, control, and bulk) 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.

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.

See the UHCI Specification for a more elaborate description. The middle layer provides access to the device in a controlled way and maintains resources in use by the different drivers and the services layer. The layer takes care of the following aspects:

  • The device configuration information

  • The pipes to communicate with a device

  • Probing and attaching and detaching form a device.

13.3. USB Device Information

13.3.1. Device Configuration Information

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 are active 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.

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.

This hierarchical configuration information is described in the device by a standard set of descriptors (see section 9.6 of the USB specification). They can be requested through the Get Descriptor Request. The services layer caches these descriptors to avoid unnecessary transfers on the USB bus. Access to the descriptors is provided through function calls.

  • 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.

  • Configuration descriptors: The number of interfaces in this configuration, suspend and resume functionality supported and power requirements.

  • Interface descriptors: interface class, subclass and protocol if applicable, number of alternate settings for the interface and the number of endpoints.

  • Endpoint descriptors: Endpoint address, direction and type, maximum packet size supported and polling frequency if type is interrupt endpoint. There is no descriptor for the default endpoint (endpoint 0) and it is never counted in an interface descriptor.

  • 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.

Class specifications can add their own descriptor types that are available through the GetDescriptor Request.

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.

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 packet size.

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.

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 the transfers is described in the USB specification. 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. The commands supported on this pipe can be extended by a device class specification to support additional functionality.

  • Bulk pipe: This is the USB equivalent to a raw transmission medium.

  • 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 specified when creating the pipe.

  • Isochronous pipe: These pipes are intended for isochronous data, for example video or audio 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.

The availability of the necessary bandwidth is calculated during the creation of the pipe. Transfers are 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 are allowed 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.

More information on scheduling of transfers and bandwidth reclamation can be found in chapter 5 of the USB specification, section 1.3 of the UHCI specification, and section 3.4.2 of the OHCI specification.

13.4. Device Probe and Attach

After the notification by the hub that a new device has been connected, the service layer switches on 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 be able 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 interfaces within one configuration. Each driver only needs to support a subset of the interfaces.

The probing for a driver for a newly attached device checks first for device specific drivers. If not found, 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 iterates over 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, even though the device might still be attached.

13.4.1. Device Disconnect and Detach

A device driver should expect to receive errors during any transaction with the device. The design of 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.

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.

The disconnection of a device is signaled 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.

13.5. USB Drivers Protocol Information

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. 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 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 section on the USB home pages.

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.

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.

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.

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 key presses 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.

Example: Firmware download Many devices that have been developed are based on a general purpose processor with an additional USB core added to it. Since 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.

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 device 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.

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.

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.

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 the existing 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.

The support for the USB floppy from Y-E Data is again less straightforward as a new command set has been designed.


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