Intel i40e rx burst API non-conformance?

[Bruce Richardson]

On Wed, Jul 09, 2025 at 04:59:51PM +0200, Morten Brørup wrote:
> Looking at the i40e driver source code, I think it doesn't conform to the API when requesting small bursts.
> 
> Let's say the hardware has received 31 packets.
> rte_eth_rx_burst(...,16) will return 16 packets and leave 15 in the driver's staging buffer (which has capacity for 32 packets).
> 
> Time passes, and more packets are received.
> 
> Now, rte_eth_rx_burst(...,16) will only return the 15 packets from the staging buffer, and not proceed to check for more [1].
> It thereby (according to my interpretation of the API description) informs the application that no more packets are ready to be received, which is incorrect.
> 

Again, the spec is not entirely clear. I would partially agree with you and
partially disagree, but overall, it's probably not the best behaviour here.

Rather than informing the app that there are no more packets, I would say
instead that it incorrectly informs the app that the descriptor ring is not
potentially overloaded. The reason for the difference is that, even in the
case of a driver correctly returning any number of packets, by the time the
function returns there could always be more in the ring. Where the real
risk, IMHO, is that you could miss out on a ring filling up without being
aware of it, because a full burst is a warning sign of overload.

/Bruce

> [1]: https://elixir.bootlin.com/dpdk/v25.07-rc2/source/drivers/net/intel/i40e/i40e_rxtx.c#L640
> 
> PS: I stumbled across this while reviewing how the very old 32 packet limit bugs (where it truncated large rx/tx requests to 32 packets) had been fixed.
>