Comtrol eCos - Manual (Page 736)

USB-ethernet Data Transfers
Both usbs_eth_start_tx and usbs_eth_start_rx are asynchronous: the transfer is started and, some time
later, a completion function will be invoked. The third and fourth arguments to both usbs_eth_start_tx and
usbs_eth_start_rx supply the completion function and an argument to that function respectively. The comple-
tion function will be invoked with three arguments: a pointer to the usbs_eth data structure, usually usbs_eth0 ;
the supplied completion data ; and a return code field. A negative value indicates that an error occurred, for ex-
ample
-EPIPE if the connection between USB host and peripheral has been broken, or -EAGAIN if an endpoint
has been halted. A positive value indicates the total size of the transfer, which should correspond to the size in the
USB-ethernet header plus an additional two bytes for the header itself.
If the data transfer is succesful then the completion function will typically be invoked in DSR context rather than
in thread context, although this depends on the implementation of the underlying USB device driver. Therefore the
completion function is restricted in what it can do; in particular, it must not make any calls that will or may block
such as locking a mutex or allocating memory. The kernel documentation should be consulted for more details of
DSR’s and interrupt handling generally. Note that if the transfer finishes quickly then the completion function may
be invoked before
usbs_eth_start_rx or usbs_eth_start_tx returns. This is especially likely to happen if the
current thread is descheduled after starting the data transfer but before returning from these functions.
For transmit operations, it is possible for usbs_eth_start_tx to invoke the completion function immediately.
If there is no current connection between host and target then the transmit will fail immediately with -EPIPE . In
addition the USB-ethernet package will check the destination MAC address and make sure that the ethernet frame
really is intended for the host: either it must be for the address specified in the initialization call
usbs_eth_init ,
or it must be a broadcast packet, or the host must have enabled promiscuous mode.
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