eCos USB Slave Support
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# Data Endpoints

## Name

Data Endpoints -- Data endpoint data structures

## Synopsis

    #include <cyg/io/usb/usbs.h>

    typedef struct usbs_rx_endpoint {
        void                 (*start_rx_fn)(struct usbs_rx_endpoint*);
        void                 (*set_halted_fn)(struct usbs_rx_endpoint*, cyg_bool);
        void                 (*complete_fn)(void*, int);
        void*                complete_data;
        unsigned char*       buffer;
        int                  buffer_size;
        cyg_bool             halted;
    } usbs_rx_endpoint;

    typedef struct usbs_tx_endpoint {
        void                 (*start_tx_fn)(struct usbs_tx_endpoint*);
        void                 (*set_halted_fn)(struct usbs_tx_endpoint*, cyg_bool);
        void                 (*complete_fn)(void*, int);
        void*                complete_data;
        const unsigned char* buffer;
        int                  buffer_size;
        cyg_bool             halted;
    } usbs_tx_endpoint;

## Receive and Transmit Data Structures

In addition to a single usbs_control_endpoint data structure per USB slave device, the USB device driver should also provide receive and transmit data structures corresponding to the other endpoints. The names of these are determined by the device driver. For example, the SA1110 USB device driver package provides `usbs_sa11x0_ep1` for receives and `usbs_sa11x0_ep2` for transmits.

Unlike control endpoints, the common USB slave package does provide a number of utility routines to manipulate data endpoints. For example `usbs_start_rx_buffer` can be used to receive data from the host into a buffer. In addition the USB device driver can provide devtab entries such as `/dev/usbs1r` and `/dev/usbs2w`, so higher-level code can `open` these devices and then perform blocking `read` and `write` operations.

However, the operation of data endpoints and the various endpoint-related functions is relatively straightforward. First consider a usbs_rx_endpoint structure. The device driver will provide the members `start_rx_fn` and `set_halted_fn`, and it will maintain the `halted` field. To receive data, higher-level code sets the `buffer`, `buffer_size`, `complete_fn` and optionally the `complete_data` fields. Next the `start_rx_fn` member should be called. When the transfer has finished the device driver will invoke the completion function, using `complete_data` as the first argument and a size field for the second argument. A negative size indicates an error of some sort: `-EGAIN` indicates that the endpoint has been halted, usually at the request of the host; `-EPIPE` indicates that the connection between the host and the peripheral has been broken. Certain device drivers may generate other error codes.

If higher-level code needs to halt or unhalt an endpoint then it can invoke the `set_halted_fn` member. When an endpoint is halted, invoking `start_rx_fn` wit `buffer_size` set to 0 indicates that higher-level code wants to block until the endpoint is no longer halted; at that point the completion function will be invoked.

USB device drivers are allowed to assume that higher-level protocols ensure that host and peripheral agree on the amount of data that will be transferred, or at least on an upper bound. Therefore there is no need for the device driver to maintain its own buffers, and copy operations are avoided. If the host sends more data than expected then the resulting behaviour is undefined.

Transmit endpoints work in essentially the same way as receive endpoints. Higher-level code should set the `buffer` and `buffer_size` fields to point at the data to be transferred, then call `start_tx_fn`, and the device driver will invoked the completion function when the transfer has completed.

USB device drivers are not expected to perform any locking. If at any time there are two concurrent receive operations for a given endpoint, or two concurrent transmit operations, then the resulting behaviour is undefined. It is the responsibility of higher-level code to perform any synchronisation that may be necessary. In practice, conflicts are unlikely because typically a given endpoint will only be accessed sequentially by just one part of the overall system.

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