selftests: mlxsw: qos_pfc: Adjust the test to support 8 lanes

selftests: mlxsw: qos_pfc: Adjust the test to support 8 lanes

'qos_pfc' test checks PFC behavior. The idea is to limit the traffic
using a shaper somewhere in the flow of the packets. In this area, the
buffer is smaller than the buffer at the beginning of the flow, so it fills
up until there is no more space left. The test configures there PFC
which is supposed to notice that the headroom is filling up and send PFC
Xoff to indicate the transmitter to stop sending traffic for the priorities
sharing this PG.

The Xon/Xoff threshold is auto-configured and always equal to
2*(MTU rounded up to cell size). Even after sending the PFC Xoff packet,
traffic will keep arriving until the transmitter receives and processes
the PFC packet. This amount of traffic is known as the PFC delay allowance.

Currently the buffer for the delay traffic is configured as 100KB. The
MTU in the test is 10KB, therefore the threshold for Xoff is about 20KB.
This allows 80KB extra to be stored in this buffer.

8-lane ports use two buffers among which the configured buffer is split,
the Xoff threshold then applies to each buffer in parallel.

The test does not take into account the behavior of 8-lane ports, when the
ports are configured to 400Gbps with 8 lanes or 800Gbps with 8 lanes,
packets are dropped and the test fails.

Check if the relevant ports use 8 lanes, in such case double the size of
the buffer, as the headroom is split half-half.

Cc: Shuah Khan <shuah@kernel.org>
Fixes: bfa804784e32 ("selftests: mlxsw: Add a PFC test")
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/23ff11b7dff031eb04a41c0f5254a2b636cd8ebb.1705502064.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>