Data Endpoints

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.