mlxsw: pci: Use NAPI for event processing

Spectrum ASICs only support a single interrupt, that means that all the
events are handled by one IRQ (interrupt request) handler. Once an
interrupt is received, we schedule tasklet to handle events from EQ and
then schedule tasklets to handle completions from CQs. Tasklet runs in
softIRQ (software IRQ) context, and will be run on the same CPU which
scheduled it. That means that today we use only one CPU to handle all the
packets (both network packets and EMADs) from hardware.

This can be improved using NAPI. The idea is to use NAPI instance per
CQ, which is mapped 1:1 to DQ (RDQ or SDQ). NAPI poll method can be run
in kernel thread, so then the driver will be able to handle WQEs in several
CPUs. Convert the existing code to use NAPI APIs.

Add NAPI instance as part of 'struct mlxsw_pci_queue' and initialize it
as part of CQs initialization. Set the appropriate poll method and dummy
net device, according to queue number, similar to tasklet setup. For CQs
which are used for completions of RDQ, use Rx poll method and
'napi_dev_rx', which is set as 'threaded'. It means that Rx poll method
will run in kernel context, so several RDQs will be handled in parallel.
For CQs which are used for completions of SDQ, use Tx poll method and
'napi_dev_tx', this method will run in softIRQ context, as it is
recommended in NAPI documentation, as Tx packets' processing is short task.

Convert mlxsw_pci_cq_{rx,tx}_tasklet() to poll methods. Handle 'budget'
argument - ignore it in Tx poll method, as it is recommended to not limit
Tx processing. For Rx processing, handle up to 'budget' completions.
Return 'work_done' which is the amount of completions that were handled.

Handle the following cases:
1. After processing 'budget' completions, the driver still has work to do:
   Return work-done = budget. In that case, the NAPI instance will be
   polled again (without the need to be rescheduled). Do not re-arm the
   queue, as NAPI will handle the reschedule, so we do not have to involve
   hardware to send an additional interrupt for the completions that should
   be processed.

2. Event processing has been completed:
   Call napi_complete_done() to mark NAPI processing as completed, which
   means that the poll method will not be rescheduled. Re-arm the queue,
   as all completions were handled.

   In case that poll method handled exactly 'budget' completions, return
   work-done = budget -1, to distinguish from the case that driver still
   has completions to handle. Otherwise, return the amount of completions
   that were handled.

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>