Re: [dpdk-dev] RFC: i40e xmit path HW limitation

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 30 Jul 2015 17:57:33 +0300
Vlad Zolotarov <vladz@cloudius-systems.com> wrote:

> 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.

Or just silently drop heavily scattered packets (and increment oerrors)
with a PMD_TX_LOG debug message.

I think a DPDK driver doesn't have to accept all possible mbufs and do
extra work. It seems reasonable to expect caller to be well behaved
in this restricted ecosystem.