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# Writing a USB Device Driver

## Name

Writing a USB Device Driver -- USB Device Driver Porting Guide

## 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.

> Note: At the time of writing only one USB device driver has been implemented. Hence it is possible, perhaps probable, that some portability issues have not yet been addressed. One issue involves the different types of transfer, for example the initial target hardware had no support for isochronous or interrupt transfers, so additional functionality may be needed to switch between transfer types. Another issue would be hardware where a given endpoint number, say endpoint 1, could be used for either receiving or transmitting data, but not both because a single fifo is used. Issues like these will have to be resolved as and when additional USB device drivers are written.

## 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.

> Note: Giving a buffer size of 0 bytes a special meaning is problematical because it prevents transfers of that size. Such transfers are allowed by the USB protocol, consisting of just headers and acknowledgements and an empty data phase, although rarely useful. A future modification of the device driver specification will address this issue, although care has to be taken that the functionality remains accessible through devtab entries as well as via low-level accesses.

## 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:

`endpoint_type`

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

`endpoint_number`

  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.

`endpoint_direction`

  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.

`endpoint`

  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.

`devtab_entry`

  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`.

`min_size`

  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.

`max_size`

  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.

`max_in_padding`

  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.

`alignment`

  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 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
    };

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

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