I found out that the AT91SAM7X emac driver from the 5.4.1 FreeRTOS distribution has a problem. If the pbuf passed to low_level_output contains more pbufs than the lEMACSend function has available transmit buffers all buffers will be filled but the transmission will not be started since the last buffer element can not be added to the transmit queue.
this renders the EMAC useless since it can no longer transmit any pakets. I would suggest one of the following options:
=> the pbuf should be reassembled in case its size exceeds the number of transmit buffers.
=> furthermore we should check upfront if it can fit into the buffers before allocating them.
The problem is not artifical and we have encountered it during unit testing of lwIP and FreeRTOS sicne we are using this combination commercially.
PS: Hi Richard - I have seen you have done great work on the coldfire - Thanks for making this one official.