RFC: i40e xmit path HW limitation

Hi, Konstantin, Helin,
there is a documented limitation of xl710 controllers (i40e driver) which is not handled in any way by a DPDK driver.
From the datasheet chapter 8.4.1:

"• A single transmit packet may span up to 8 buffers (up to 8 data descriptors per packet including
both the header and payload buffers).
• The total number of data descriptors for the whole TSO (explained later on in this chapter) is
unlimited as long as each segment within the TSO obeys the previous rule (up to 8 data descriptors
per segment for both the TSO header and the segment payload buffers)."

This means that, for instance, long cluster with small fragments has to be linearized before it may be placed on the HW ring.
In more standard environments like Linux or FreeBSD drivers the solution is straight forward - call skb_linearize()/m_collapse() corresponding.
In the non-conformist environment like DPDK life is not that easy - there is no easy way to collapse the cluster into a linear buffer from inside the device driver
since device driver doesn't allocate memory in a fast path and utilizes the user allocated pools only.

Here are two proposals for a solution:

1. We may provide a callback that would return a user TRUE if a give
   cluster has to be linearized and it should always be called before
   rte_eth_tx_burst(). Alternatively it may be called from inside the
   rte_eth_tx_burst() and rte_eth_tx_burst() is changed to return some
   error code for a case when one of the clusters it's given has to be
   linearized.
2. Another option is to allocate a mempool in the driver with the
   elements consuming a single page each (standard 2KB buffers would
   do). Number of elements in the pool should be as Tx ring length
   multiplied by "64KB/(linear data length of the buffer in the pool
   above)". Here I use 64KB as a maximum packet length and not taking
   into an account esoteric things like "Giant" TSO mentioned in the
   spec above. Then we may actually go and linearize the cluster if
   needed on top of the buffers from the pool above, post the buffer
   from the mempool above on the HW ring, link the original cluster to
   that new cluster (using the private data) and release it when the
   send is done.


The first is a change in the API and would require from the application some additional handling (linearization). The second would require some additional memory but would keep all dirty details inside the driver and would leave the rest of the code intact.

Pls., comment.

thanks,
vlad