Writing a USB Device Driver

Introduction

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

The Control Endpoint

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

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

The USB device driver is responsible for filling in the start_fn, poll_fn and interrupt_vector fields. Usually this can be achieved by static initialization. The driver is also largely responsible for maintaining the state field. The control_buffer array should be used to hold the first packet of a control message. The buffer and other fields related to data transfers will be managed jointly by higher-level code and the device driver. The remaining fields are generally filled in by higher-level code, although the driver should initialize them to NULL values.

Hardware permitting, the USB device should be inactive until the start_fn is invoked, for example by tristating the appropriate pins. This prevents the host from interacting with the peripheral before all other parts of the system have initialized. It is expected that the start_fn will only be invoked once, shortly after power-up.

Where possible the device driver should detect state changes, such as when the connection between host and peripheral is established, and report these to higher-level code via the state_change_fn callback, if any. The state change to and from configured state cannot easily be handled by the device driver itself, instead higher-level code such as the 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. For example, the standard set-address control message must be handled within 50ms. The USB specification provides more information on these constraints. The device driver is responsible for receiving the initial packet of a control message. This packet will always be eight bytes and should be stored in the control_buffer field. Certain standard control messages should be detected and handled by the device driver itself. The most important is set-address, but usually the get-status, set-feature and clear-feature requests when applied to halted endpoints should also be handled by the driver. Other standard control messages should first be passed on to the standard_control_fn callback (if any), and finally to the default handler usbs_handle_standard_control provided by the common USB slave package. Class, vendor and reserved control messages should always be dispatched to the appropriate callback and there is no default handler for these.

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

The polling support is optional and may not be feasible on all hardware. It is only used in certain specialised environments such as RedBoot. A typical implementation of the polling function would just check whether or not an interrupt would have occurred and, if so, call the same code that the interrupt handler would.

Data Endpoints

In addition to the control endpoint data structure, a USB device driver should also provide appropriate data endpoint data structures. Obviously this is only relevant if the USB support generally is desired, that is if the control endpoint is provided. In addition, higher-level code may not require all the endpoints, so it may be useful to provide configuration options that control the presence of each endpoint. For example, the intended application might only involve a single transmit endpoint and of course control messages, so supporting receive endpoints might waste memory.

Conceptually, data endpoints are much simpler than the control endpoint. The device driver has to supply two functions, one for data transfers and another to control the halted condition. These implement the functionality for usbs_start_rx_buffer, usbs_start_tx_buffer, usbs_set_rx_endpoint_halted and usbs_set_tx_endpoint_halted. The device driver is also responsible for maintaining the halted status.

For data transfers, higher-level code will have filled in the buffer, buffer_size, complete_fn and complete_data fields. The transfer function should arrange for the transfer to start, allowing the host to send or receive packets. Typically this will result in an interrupt at the end of the transfer or after each packet. Once the entire transfer has been completed, the driver's interrupt handling code should invoke the completion function. This can happen either in DSR context or thread context, depending on the driver's implementation. There are a number of special cases to consider. If the endpoint is halted when the transfer is started then the completion function can be invoked immediately with -EAGAIN. If the transfer cannot be completed because the connection is broken then the completion function should be invoked with -EPIPE. If the endpoint is stalled during the transfer, either because of a standard control message or because higher-level code calls the appropriate set_halted_fn, then again the completion function should be invoked with -EAGAIN. Finally, the <usbs_start_rx_endpoint_wait and usbs_start_tx_endpoint_wait functions involve calling the device driver's data transfer function with a buffer size of 0 bytes.

Devtab Entries

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

#ifdef CYGVAR_DEVS_USB_SA11X0_EP1_DEVTAB_ENTRY

static CHAR_DEVIO_TABLE(usbs_sa11x0_ep1_devtab_functions,
                        &cyg_devio_cwrite,
                        &usbs_devtab_cread,
                        &cyg_devio_bwrite,
                        &cyg_devio_bread,
                        &cyg_devio_select,
                        &cyg_devio_get_config,
                        &cyg_devio_set_config);

static CHAR_DEVTAB_ENTRY(usbs_sa11x0_ep1_devtab_entry,
                         CYGDAT_DEVS_USB_SA11X0_DEVTAB_BASENAME "1r",
                         0,
                         &usbs_sa11x0_ep1_devtab_functions,
                         &usbs_sa11x0_devtab_dummy_init,
                         0,
                         (void*) &usbs_sa11x0_ep1);
#endif

Again care must be taken to avoid name clashes. This can be achieved by having a configuration option to control the base name, with a default value of e.g. /dev/usbs, and appending an endpoint-specific string. This gives the application developer sufficient control to eliminate any name clashes. The common USB slave package provides functions usbs_devtab_cwrite and usbs_devtab_cread, which can be used in the function tables for transmit and receive endpoints respectively. The private field priv of the devtab entry should 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 the devtab entries should normally be defined in a separate source file which ends up the special library libextras.a rather than in the default library libtarget.a.

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

Interrupt Handling

A typical USB device driver will need to service interrupts for all of the endpoints and possibly for additional USB events such as entering or leaving suspended mode. Usually these interrupts need not be serviced directly by the ISR. Instead, they can be left to a DSR. If the 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 little bit later. If high throughput is required then it may be desirable to handle the bulk transfer protocol largely at ISR level, that is take care of each packet in the ISR and only activate the DSR once the whole transfer has completed.

Control messages may involve invoking arbitrary callback functions in higher-level code. This should normally happen at DSR level. Doing it at ISR level could seriously affect the system's interrupt latency and impose unacceptable constraints on what operations can be performed by those callbacks. If the device driver requires a thread anyway then it may be appropriate to use this thread for invoking the callbacks, but usually it is not worthwhile to add a new thread to the system just for this; higher-level code is expected to write callbacks that function sensibly at DSR level. Much the same applies to the completion functions associated with data transfers. These should also be invoked at DSR or thread level.

Support for USB Testing

Optionally a USB device driver can provide support for the USB test software. This requires defining a number of additional data structures, allowing the generic test code to work out just what the hardware is capable of and hence what testing can be performed.

The key data structure is usbs_testing_endpoint, defined in cyg/io/usb/usbs.h. In addition some commonly required constants are provided by the common USB package in cyg/io/usb/usb.h. One usbs_testing_endpoint structure should be defined for each supported endpoint. The following fields need to be filled in:

The device driver should provide an array of these structures usbs_testing_endpoints[]. The USB testing code examines this array and uses the information to perform appropriate tests. Because different USB devices support different numbers of endpoints the number of entries in the array is not known in advance, so instead the testing code looks for a special terminator USBS_TESTING_ENDPOINTS_TERMINATOR. An example array, showing just the control endpoint and the terminator, might look like this:

usbs_testing_endpoint usbs_testing_endpoints[] = {
    {
        endpoint_type       : USB_ENDPOINT_DESCRIPTOR_ATTR_CONTROL, 
        endpoint_number     : 0,
        endpoint_direction  : USB_ENDPOINT_DESCRIPTOR_ENDPOINT_IN,
        endpoint            : (void*) &ep0.common,
        devtab_entry        : (const char*) 0,
        min_size            : 1,
        max_size            : 0x0FFFF,
        max_in_padding      : 0,
        alignment           : 0
    },
    …,
    USBS_TESTING_ENDPOINTS_TERMINATOR
};