7.3.2 Packet Data Transmission

Packet data transmission takes place inside the linkoutput callback registered with the lwIP stack. This callback is invoked whenever the lwIP stack receives a packet for transmission from the application layer. The pbuf can contain a chain of packet buffers and, hence, the DMA descriptors are properly updated (chained if necessary), with SOP, EOP and length fields. The first DMA descriptor is marked with the EOP and OWNER flags, while only the last is set with the EOP flag. After filling the BD's with the pbuf information, the BD, which corresponds to the SOP, is written to the HEAD descriptor pointer register to start the transmission. Once a packet is transmitted, the EMAC Control Core generates a transmit interrupt. This interrupt is cleared only if the completion pointer is written with the last BD processed. In the interrupt handler, the next BD to process is taken and traversed to reach the BD that corresponds to the end of the packet. This BD, which corresponds to the end of the packet, is written to the completion pointer. After this, the pbuf that corresponds to this packet is freed. Thus, it is made sure that the freeing of pbuf is done only after the packet transmission is complete.