RX Descriptor Processing

Versal ACAP Technical Reference Manual (AM011)

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The start location of the RX buffer descriptors must be written with the receive-buffer queue base address before reception is enabled (receive enable in the network control register). After reception is enabled, any writes to the receive-buffer queue base address register are ignored.

When read, it returns the current pointer position in the descriptor list, though this is only valid and stable when receive is disabled.

If the filter block indicates that a frame should be copied to memory, the receive data DMA operation starts writing data into the receive buffer. If an error occurs, the buffer is recovered.

An internal counter represents the receive-buffer queue pointer and it is not visible through the CPU interface. The receive-buffer queue pointer increments by two words after using each buffer. It re-initializes to the receive-buffer queue base address when any descriptor has its wrap bit set.

As receive AXI buffers are used, the receive AXI buffer manager sets bit zero of the first word of the descriptor to logic one, to indicate that the AXI buffer was used.

Software should search through the used bits in the AXI buffer descriptors to determine how many frames are received by checking the start of frame and end of frame bits.

By default, partial store and forward is not enabled; that is, the controller waits for the full packet to be available before forwarding. If the DMA is configured in the packet buffer partial store and forward mode, received frames are written out to the AHB/AXI buffers as soon as enough frame data exists in the packet buffer, which means several full buffers are used before some error conditions can be detected. If a receive error is detected, the receive buffer currently being written is recovered. Previous buffers are not recovered. For example, when receiving frames with CRC errors or excessive length, it is possible that a frame fragment might be stored in a sequence of receive buffers. Software can detect these fragment by looking for start-of-frame bit set in a buffer following a buffer with no-end-of frame bit set.

A properly working 10/100/1000 Ethernet system does not have excessive length frames or frames greater than 128 bytes with CRC errors. When using a default value of 128 bytes for the receive buffer, it is rare to find a frame fragment in a receive AXI buffer because collision fragments are less than 128 bytes long.

Only good received frames are written out of the DMA and no fragments exist in the AXI buffers due to MAC receiver errors. However, there is still the possibility of fragments due to DMA errors. For example, when a used bit is read on the second buffer of a multi-buffer frame.

If bit zero of the receive buffer descriptor is already set when the receive buffer manager reads the location of the receive AXI buffer, the buffer is already used and cannot be used again until the software has processed the frame and cleared bit zero. In this case, the buffer not available bit in the receive status register is set and an interrupt is triggered. The receive resource error statistics register is also incremented.

There is an option to automatically discard received frames when no AXI buffer resource is available. This option is controlled by the DMA_Config [force_discard_on_err] register bit. By default, the received frames are not automatically discarded. When this feature is off, the received packets remain stored in the packet buffer until an AXI buffer resource becomes available. This can lead to an eventual packet buffer overflow occurs when packets continue to be received because the [0, used] bit of the receive-buffer descriptor is still set.

After a used bit is read, the receive-buffer manager re-reads the location of the receive buffer descriptor every time a new packet is received.

When the DMA is configured in the packet buffer full store and forward mode, a receive overrun condition occurs when the receive packet buffer is full, or if an AXI error occurred.

For a receive overrun condition, the receive overrun interrupt is asserted and the buffer currently being written is recovered. The next frame that is received whose address is recognized reuses the buffer.

To force a flush of the packet from the receive packet buffer, write a 1 to the Network_Control [flush_rx_pkt_pclk] register bit. This only occurs when the RX DMA is not currently writing packet data out to the AXI (that is, it is in an IDLE state). If the RX DMA is active, a write to this bit is ignored.