62.6.3.7 Transmit Packet Buffer

The transmitter packet buffer will continue attempting to fetch frame data from the system memory until the packet buffer itself is full, at which point it will attempt to maintain its full level.

To accommodate the status and statistics associated with each frame, three words per packet (or two if the GMAC is configured in 64-bit Datapath mode) are reserved at the end of the packet data. If the packet is bad and requires to be dropped, the status and statistics are the only information held on that packet. Storing the status in the packet buffer memory is required in order to decouple the DMA interface of the buffer from the MAC interface, to update the MAC status/statistics and to generate interrupts in the order in which the packets that they represent were fetched from the system memory.

If any errors occur on the system bus while reading the transmit frame, the fetching of packet data from system memory is halted. The MAC transmitter continues to fetch packet data, thereby emptying the packet buffer and allowing any good non-errored frames to be transmitted successfully. Once these have been fully transmitted, the status/statistics for the errored frame will be updated and software will be informed via an interrupt that a system error occurred. This way, the error is reported in the correct packet order.

The transmit packet buffer will only attempt to read more frame data from the system bus when space is available in the packet buffer memory. If space is not available it must wait until the a packet fetched by the MAC completes transmission and is subsequently removed from the packet buffer memory. Note that if Full Store and Forward mode is active and if a single frame is fetched that is too large for the packet buffer memory, the frame is flushed and the DMA halted with an error status. This is because a complete frame must be written into the packet buffer before transmission can begin, and therefore the minimum packet buffer memory size should be chosen to satisfy the maximum frame to be transmitted in the application.

In Full Store and Forward mode, once the complete transmit frame is written into the packet buffer memory, a trigger is sent across to the MAC transmitter, which will then begin reading the frame from the packet buffer memory. Since the whole frame is present and stable in the packet buffer memory, an underflow of the transmitter is not possible. The frame is kept in the packet buffer until notification is received from the MAC that the frame data has either been successfully transmitted or can no longer be retransmitted (too many retries in half duplex mode). When this notification is received, the frame is flushed from memory to make room for a new frame to be fetched from the system memory.

In Partial Store and Forward mode, a trigger is sent across to the MAC transmitter as soon as sufficient packet data is available, which will then begin fetching the frame from the packet buffer memory. If, after this point, the MAC transmitter is able to fetch data from the packet buffer faster than the DMA can fill it, an underflow of the transmitter is possible. In this case, the transmission is terminated early, and the packet buffer is completely flushed. Transmission can only be restarted by writing to the transmit START bit.

In Half Duplex mode, the frame is kept in the packet buffer until notification is received from the MAC that the frame data has either been successfully transmitted or can no longer be retransmitted (too many retries in Half Duplex mode). When this notification is received, the frame is flushed from memory to make room for a new frame to be fetched from system memory.

In Full Duplex mode, the frame is removed from the packet buffer on the fly.

Other than underflow, the only MAC related errors that can occur are due to collisions during half duplex transmissions. When a collision occurs the frame still exists in the packet buffer memory so can be retried directly from there. Only once the MAC transmitter has failed to transmit after sixteen attempts is the frame finally flushed from the packet buffer.