Known Limitations

Maximal jumbo frames

The Ethernet MAC supports a maximum MTU of 16,383 bytes on all Ethernet types. However, the DMA Medusa engine cannot always transfer frames of the full MAC MTU. The maximum frame length supported by DMA RX depends on the size of the user DMA header (see Header metadata format in NDK Application), which is prepended to the Ethernet packet and becomes part of the transferred data:

max Ethernet frame (RX) = 16,383 − HDR_LEN
max DMA frame size (RX) = 16,379 − HDR_LEN

where HDR_LEN is the user header size in bytes. For example:

  • MINIMAL application (HDR_LEN = 0): max Ethernet frame = 16,383, max DMA frame = 16,379 (= MAC MTU − 4 B CRC).

  • NIC application (HDR_LEN = 4): max Ethernet frame = 16,379, max DMA frame = 16,375.

Exceeding the DMA limit causes an internal counter overflow — the DMA reports an incorrect byte count (approximately 232 minus the actual length) or stalls completely. NDP applications may also show incorrect speed for frames near the limit.

In the TX direction, DMA Medusa supports up to 16,383 bytes (the frame is sent with +4B CRC = 16,387 B on the wire).

DMA Calypte supports frames up to 4,096 bytes only.

For proper MTU functionality during DMA transfers, you may need to configure the software buffer sizes using the nfb-dma tool. Also configure the MTU in the Ethernet MAC using the nfb-eth tool; the default value is 1,526 bytes.

DPDK settings

The DPDK native DMA drivers require the --iova-mode pa setting. This is set by default on most machines, but on some AMD systems it can cause issues, potentially leading to host system shutdown. The recommended way to run a DPDK application with DPDK native drivers is:

dpdk-testpmd --iova-mode pa -a 0000:17:00.0,queue_driver=native