Comtrol eCos - Manual (Page 708)

Control Endpoints
4. When buffer_size is 0 and fill_buffer_fn is NULL, no more data is available and the transfer has
completed.
5. Optionally a completion function can be installed. This will be invoked with 0 if the transfer completes suc-
cessfully, or with an error code if the transfer is cancelled because of another control messsage.
If the requested data is contiguous then the only fields that need to be manipulated are buffer and
buffer_size , and optionally complete_fn . If the requested data is not contiguous then the initial control
message handler should update fill_buffer_fn and some or all of the other fields, as required. An example
of this is the handling of the standard get-descriptor control message by usbs_handle_standard_control .
Polling Support
typedef struct usbs_control_endpoint {
void (*poll_fn)(struct usbs_control_endpoint*);
int interrupt_vector;
...
} usbs_control_endpoint;
In nearly all circumstances USB I/O should be interrupt-driven. However, there are special environments such as
RedBoot where polled operation may be appropriate. If the device driver can operate in polled mode then it will
provide a suitable function via the
poll_fn field, and higher-level code can invoke this regularly. This polling
function will take care of all endpoints associated with the device, not just the control endpoint. If the USB hardware
involves a single interrupt vector then this will be identified in the data structure as well.
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