Testing scatter support for PMDs using testpmd

[Jeremy Spewock <jspewock@iol.unh.edu>]

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Hello maintainers,

In porting over the first ethdev suite to the new DTS framework, there was
an inconsistency that we found and we were wondering if anyone would be
able to shed some light on this. In general the inconsistency pertains to
Intel and Mellanox NICs, where one will throw an error and not start
testpmd while the other will work as expected.

In the original DTS suite for testing scattered packets, testpmd is started
with the flags --max-packet-len=9000 and --mbuf-size=2048. This starts and
works fine on Intel NICs, but when you use the same flags on a Mellanox
NIC, it will produce the error seen below. There is a flag that exists for
testpmd called --enable-scatter and when this flag is provided, the
Mellanox NIC seems to accept the flags and start without error.

Our assumption is that this should be consistent between NICs, is there a
reason that one NIC would allow for starting testpmd without explicitly
enabling scatter and the other wouldn't? Should this flag always be
present, and is it an error that testpmd can start without it in the first
place?

Here is the error provided when attempting to run on a Mellanox NIC:

mlx5_net: port 0 Rx queue 0: Scatter offload is not configured and no
enough mbuf space(2048) to contain the maximum RX packet length(9000) with
head-room(128)
mlx5_net: port 0 unable to allocate rx queue index 0
Fail to configure port 0 rx queues
Start ports failed

Thank you for any insight,
Jeremy

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