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Introduction

Often the best way to write a USB device driver will be to start withan existing one and modify it as necessary. The information given hereis intended primarily as an outline rather than as a complete guide.

Note: At the time of writing only one USB device driver has beenimplemented. Hence it is possible, perhaps probable, that someportability issues have not yet been addressed. One issueinvolves the different types of transfer, for example the initialtarget hardware had no support for isochronous or interrupt transfers,so additional functionality may be needed to switch between transfertypes. Another issue would be hardware where a given endpoint number,say endpoint 1, could be used for either receiving or transmittingdata, but not both because a single fifo is used. Issues like thesewill have to be resolved as and when additional USB device drivers arewritten.

The Control Endpoint

A USB device driver should provide a single usbs_control_endpointdata structure for every USB device. Typical peripherals will haveonly one USB port so there will be just one such data structure in theentire system, but theoretically it is possible to have multiple USBdevices. These may all involve the same chip, in which case a singledevice driver should support multiple device instances, or they mayinvolve different chips. The name or names of these data structuresare determined by the device driver, but appropriate care should betaken to avoid name clashes.

A USB device cannot be used unless the control endpoint data structureexists. However, the presence of USB hardware in the target processoror board does not guarantee that the application will necessarily wantto use that hardware. To avoid unwanted code or data overheads, thedevice driver can provide a configuration option to determine whetheror not the endpoint 0 data structure is actually provided. A defaultvalue of CYGINT_IO_USB_SLAVE_CLIENTS ensures thatthe USB driver will be enabled automatically if higher-level code doesrequire USB support, while leaving ultimate control to the user.

The USB device driver is responsible for filling in thestart_fn,poll_fn andinterrupt_vector fields. Usually this canbe achieved by static initialization. The driver is also largelyresponsible for maintaining the statefield. The control_buffer array should beused to hold the first packet of a control message. Thebuffer and other fields related to datatransfers will be managed jointly by higher-level code andthe device driver. The remaining fields are generally filled in byhigher-level code, although the driver should initialize them to NULLvalues.

Hardware permitting, the USB device should be inactive until thestart_fn is invoked, for example bytristating the appropriate pins. This prevents the host frominteracting with the peripheral before all other parts of the systemhave initialized. It is expected that thestart_fn will only be invoked once, shortlyafter power-up.

Where possible the device driver should detect state changes, such aswhen the connection between host and peripheral is established, andreport these to higher-levelcode via the state_change_fn callback, ifany. The state change to and from configured state cannot easily behandled by the device driver itself, instead higher-level code such asthe common USB slave package will take care of this.

Once the connection between host and peripheral has been established,the peripheral must be ready to accept control messages at all times,and must respond to these within certain time constraints. Forexample, the standard set-address control message must be handledwithin 50ms. The USB specification provides more information on theseconstraints. The device driver is responsible for receiving theinitial packet of a control message. This packet will always be eightbytes and should be stored in thecontrol_buffer field. Certain standardcontrol messages should be detected and handled by the device driveritself. The most important is set-address, but usually the get-status,set-feature and clear-feature requests when applied to haltedendpoints should also be handled by the driver. Other standard controlmessages should first be passed on to thestandard_control_fn callback (if any), andfinally to the default handlerusbs_handle_standard_control provided by thecommon USB slave package. Class, vendor and reserved control messagesshould always be dispatched to the appropriate callback and there isno default handler for these.

Some control messages will involve further data transfer, not just theinitial packet. The device driver must handle this in accordance withthe USB specification and the buffer management strategy. Thedriver is also responsible for keeping track of whether or not thecontrol operation has succeeded and generating an ACK or STALLhandshake.

The polling support is optional and may not be feasible on allhardware. It is only used in certain specialised environments such asRedBoot. A typical implementation of the polling function would justcheck whether or not an interrupt would have occurred and, if so, callthe same code that the interrupt handler would.

Data Endpoints

In addition to the control endpoint data structure, a USB devicedriver should also provide appropriate dataendpoint data structures. Obviously this is only relevant ifthe USB support generally is desired, that is if the control endpoint isprovided. In addition, higher-level code may not require all theendpoints, so it may be useful to provide configuration options thatcontrol the presence of each endpoint. For example, the intendedapplication might only involve a single transmit endpoint and ofcourse control messages, so supporting receive endpoints might wastememory.

Conceptually, data endpoints are much simpler than the controlendpoint. The device driver has to supply two functions, one fordata transfers and another to control the halted condition. Theseimplement the functionality forusbs_start_rx_buffer,usbs_start_tx_buffer,usbs_set_rx_endpoint_halted andusbs_set_tx_endpoint_halted.The device driver is also responsible for maintaining thehalted status.

For data transfers, higher-level code will have filled in thebuffer,buffer_size,complete_fn andcomplete_data fields. The transfer functionshould arrange for the transfer to start, allowing the host to send orreceive packets. Typically this will result in an interrupt at the endof the transfer or after each packet. Once the entire transfer hasbeen completed, the driver's interrupt handling code should invoke thecompletion function. This can happen either in DSR context or threadcontext, depending on the driver's implementation. There are a numberof special cases to consider. If the endpoint is halted when thetransfer is started then the completion function can be invokedimmediately with -EAGAIN. If the transfer cannot becompleted because the connection is broken then the completionfunction should be invoked with -EPIPE. If theendpoint is stalled during the transfer, either because of a standardcontrol message or because higher-level code calls the appropriateset_halted_fn, then again the completionfunction should be invoked with -EAGAIN. Finally,the <usbs_start_rx_endpoint_wait andusbs_start_tx_endpoint_wait functions involvecalling the device driver's data transfer function with a buffer sizeof 0 bytes.

Note: Giving a buffer size of 0 bytes a special meaning is problematicalbecause it prevents transfers of that size. Such transfers are allowedby the USB protocol, consisting of just headers and acknowledgementsand an empty data phase, although rarely useful. A future modificationof the device driver specification will address this issue, althoughcare has to be taken that the functionality remains accessible throughdevtab entries as well as via low-level accesses.

Devtab Entries

For some applications or higher-level packages it may be moreconvenient to use traditional open/read/write I/O calls rather thanthe non-blocking USB I/O calls. To support this the device driver canprovide a devtab entry for each endpoint, for example:

Again care must be taken to avoid name clashes. This can be achievedby having a configuration option to control the base name, with adefault value of e.g. /dev/usbs, and appending anendpoint-specific string. This gives the application developersufficient control to eliminate any name clashes. The common USB slavepackage provides functions usbs_devtab_cwrite andusbs_devtab_cread, which can be used in thefunction tables for transmit and receive endpoints respectively. Theprivate field priv of the devtab entryshould be a pointer to the underlying endpoint data structure.

Because devtab entries are never accessed directly, only indirectly,they would usually be eliminated by the linker. To avoid this thedevtab entries should normally be defined in a separate source filewhich ends up the special library libextras.arather than in the default library libtarget.a.

Not all applications or higher-level packages will want to use thedevtab entries and the blocking I/O facilities. It may be appropriatefor the device driver to provide additional configuration options thatcontrol whether or not any or all of the devtab entries should beprovided, to avoid unnecessary memory overheads.

Interrupt Handling

A typical USB device driver will need to service interrupts for all ofthe endpoints and possibly for additional USB events such as enteringor leaving suspended mode. Usually these interrupts need not beserviced directly by the ISR. Instead, they can be left to a DSR. Ifthe peripheral is not able to accept or send another packet just yet,the hardware will generate a NAK and the host will just retry a littlebit later. If high throughput is required then it may be desirable tohandle the bulk transfer protocol largely at ISR level, that is takecare of each packet in the ISR and only activate the DSR once thewhole transfer has completed.

Control messages may involve invoking arbitrary callback functions inhigher-level code. This should normally happen at DSR level. Doing itat ISR level could seriously affect the system's interrupt latency andimpose unacceptable constraints on what operations can be performed bythose callbacks. If the device driver requires a thread anyway then itmay be appropriate to use this thread for invoking the callbacks, butusually it is not worthwhile to add a new thread to the system justfor this; higher-level code is expected to write callbacks thatfunction sensibly at DSR level. Much the same applies to thecompletion functions associated with data transfers. These should alsobe invoked at DSR or thread level.

Support for USB Testing

Optionally a USB device driver can provide support for theUSB test software. This requiresdefining a number of additional data structures, allowing thegeneric test code to work out just what the hardware is capable of andhence what testing can be performed.

The key data structure isusbs_testing_endpoint, defined in cyg/io/usb/usbs.h. In addition somecommonly required constants are provided by the common USB package incyg/io/usb/usb.h. Oneusbs_testing_endpoint structure should bedefined for each supported endpoint. The following fields need to befilled in:

This specifies the type of endpoint and should be one of USB_ENDPOINT_DESCRIPTOR_ATTR_CONTROL, BULK, ISOCHRONOUS or INTERRUPT.

This identifies the number that should be used by the host to address this endpoint. For a control endpoint it should be 0. For other types of endpoints it should be between 1 and 15.

For control endpoints this field is irrelevant. For other types of endpoint it should be either USB_ENDPOINT_DESCRIPTOR_ENDPOINT_IN or USB_ENDPOINT_DESCRIPTOR_ENDPOINT_OUT. If a given endpoint number can be used for traffic in both directions then there should be two entries in the array, one for each direction.

This should be a pointer to the appropriate usbs_control_endpoint, usbs_rx_endpoint or usbs_tx_endpoint structure, allowing the generic testing code to perform low-level I/O.

If the endpoint also has an entry in the system's device table then this field should give the corresponding string, for example '/dev/usbs1r'. This allows the generic testing code to access the device via higher-level calls like open and read.

This indicates the smallest transfer size that the hardware can support on this endpoint. Typically this will be one.

Note: Strictly speaking a minimum size of one is not quite right since it is valid for a USB transfer to involve zero bytes, in other words a transfer that involves just headers and acknowledgements and an empty data phase, and that should be tested as well. However current device drivers interpret a transfer size of 0 as special, so that would have to be resolved first.

Similarly, this specifies the largest transfer size. For control endpoints the USB protocol uses only two bytes to hold the transfer length, so there is an upper bound of 65535 bytes. In practice it is very unlikely that any control transfers would ever need to be this large, and in fact such transfers would take a long time and probably violate timing constraints. For other types of endpoint any of the protocol, the hardware, or the device driver may impose size limits. For example a given device driver might be unable to cope with transfers larger than 65535 bytes. If it should be possible to transfer arbitrary amounts of data then a value of -1 indicates no upper limit, and transfer sizes will be limited by available memory and by the capabilities of the host machine.

Wideusb USB Devices Driver

This field is needed on some hardware where it is impossible to send packets of a certain size. For example the hardware may be incapable of sending an empty bulk packet to terminate a transfer that is an exact multiple of the 64-byte bulk packet size. Instead the driver has to do some padding and send an extra byte, and the host has to be prepared to receive this extra byte. Such a driver should specify a value of 1 for the padding field. For most drivers this field should be set to 0.

A better solution would be for the device driver to supply a fragment of Tcl code that would adjust the receive buffer size only when necessary, rather than for every transfer. Forcing receive padding on all transfers when only certain transfers will actually be padded reduces the accuracy of certain tests.

On some hardware data transfers may need to be aligned to certain boundaries, for example a word boundary or a cacheline boundary. Although in theory device drivers could hide such alignment restrictions from higher-level code by having their own buffers and performing appropriate copying, that would be expensive in terms of both memory and cpu cycles. Instead the generic testing code will align any buffers passed to the device driver to the specified boundary. For example, if the driver requires that buffers be aligned to a word boundary then it should specify an alignment value of 4.

The device driver should provide an array of these structuresusbs_testing_endpoints[]. The USB testing codeexamines this array and uses the information to perform appropriatetests. Because different USB devices support different numbers ofendpoints the number of entries in the array is not known in advance,so instead the testing code looks for a special terminatorUSBS_TESTING_ENDPOINTS_TERMINATOR. An examplearray, showing just the control endpoint and the terminator, mightlook like this:

Wideusb Usb Devices Driver Vga

Note: The use of a single array usbs_testing_endpointslimits USB testing to platforms with a single USB device: if therewere multiple devices, each defining their own instance of this array,then there would a collision at link time. In practice this should notbe a major problem since typical USB peripherals only interact with asingle host machine via a single slave port. In addition, even if aperipheral did have multiple slave ports the current USB testing codewould not support this since it would not know which port to use.

Usb C To Usb Adapter