mlxsw: pci: Ring RDQ and CQ doorbells once per several completions

mlxsw: pci: Ring RDQ and CQ doorbells once per several completions

Currently, for each CQE in CQ, we ring CQ doorbell, then handle RDQ and
ring RDQ doorbell. Finally we ring CQ arm doorbell - once per CQ tasklet.

The idea of ringing CQ doorbell before RDQ doorbell, is to be sure that
when we post new WQE (after RDQ is handled), there is an available CQE.
This was done because of a hardware bug as part of
commit c9ebea04cb1b ("mlxsw: pci: Ring CQ's doorbell before RDQ's").

There is no real reason to ring RDQ and CQ doorbells for each completion,
it is better to handle several completions and reduce number of ringings,
as access to hardware is expensive (time wise) and might take time because
of memory barriers.

A previous patch changed CQ tasklet to handle up to 64 Rx packets. With
this limitation, we can ring CQ and RDQ doorbells once per CQ tasklet.
The counters of the doorbells are increased by the amount of packets
that we handled, then the device will know for which completion to send
an additional event.

To avoid reordering CQ and RDQ doorbells' ring, let the tasklet to ring
also RDQ doorbell, mlxsw_pci_cqe_rdq_handle() handles the counter but
does not ring the doorbell.

Note that with this change there is no need to copy the CQE, as we ring CQ
doorbell only after Rx packet processing (which uses the CQE) is done.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>