CRC_SIZE
size_t len = cmd->count - CRC_SIZE;
#define CMD_OVERHEAD_SIZE (COUNT_SIZE + CRC_SIZE)
rx_size = data_size - CRC_SIZE;
((td->len + CRC_SIZE) >= ed->max_pkt_size)))
pkt->len > td_length - CRC_SIZE) {
pkt->len = td_length - CRC_SIZE;
out_be16(&td->length, pkt->len + CRC_SIZE);