net/mlx5e: Make tx_port_ts logic resilient to out-of-order CQEs

Use a map structure for associating CQEs containing port timestamping
information with the appropriate skb. Track order of WQEs submitted using a
FIFO. Check if the corresponding port timestamping CQEs from the lookup
values in the FIFO are considered dropped due to time elapsed. Return the
lookup value to a freelist after consuming the skb. Reuse the freed lookup
in future WQE submission iterations.

The map structure uses an integer identifier for the key and returns an skb
corresponding to that identifier. Embed the integer identifier in the WQE
submitted to the WQ for the transmit path when the SQ is a PTP (port
timestamping) SQ. The embedded identifier can then be queried using a field
in the CQE of the corresponding port timestamping CQ. In the port
timestamping napi_poll context, the identifier is queried from the CQE
polled from CQ and used to lookup the corresponding skb from the WQE submit
path. The skb reference is removed from map and then embedded with the port
HW timestamp information from the CQE and eventually consumed.

The metadata freelist FIFO is an array containing integer identifiers that
can be pushed and popped in the FIFO. The purpose of this structure is
bookkeeping what identifier values can safely be used in a subsequent WQE
submission and should not contain identifiers that have still not been
reaped by processing a corresponding CQE completion on the port
timestamping CQ.

The ts_cqe_pending_list structure is a combination of an array and linked
list. The array is pre-populated with the nodes that will be added and
removed from the head of the linked list. Each node contains the unique
identifier value associated with the values submitted in the WQEs and
retrieved in the port timestamping CQEs. When a WQE is submitted, the node
in the array corresponding to the identifier popped from the metadata
freelist is added to the end of the CQE pending list and is marked as
"in-use". The node is removed from the linked list under two conditions.
The first condition is that the corresponding port timestamping CQE is
polled in the PTP napi_poll context. The second condition is that more than
a second has elapsed since the DMA timestamp value corresponding to the WQE
submission. When the first condition occurs, the "in-use" bit in the linked
list node is cleared, and the resources corresponding to the WQE submission
are then released. The second condition, however, indicates that the port
timestamping CQE will likely never be delivered. It's not impossible for
the device to post a CQE after an infinite amount of time though highly
improbable. In order to be resilient to this improbable case, resources
related to the corresponding WQE submission are still kept, the identifier
value is not returned to the freelist, and the "in-use" bit is cleared on
the node to indicate that it's no longer part of the linked list of "likely
to be delivered" port timestamping CQE identifiers. A count for the number
of port timestamping CQEs considered highly likely to never be delivered by
the device is maintained. This count gets decremented in the unlikely event
a port timestamping CQE considered unlikely to ever be delivered is polled
in the PTP napi_poll context.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>