RE: [PATCH RFC 04/30] net: fec: fix interrupt handling races

From: Russell King

While running: while :; do iperf -c <HOST> -P 4; done, transmit timeouts
are regularly reported.  With the tx ring dumping in place, we can see
that all entries are in use, and the hardware has finished transmitting
these packets.  However, the driver has not reclaimed these ring
entries.

This can occur if the interrupt handler is invoked at the wrong moment -
eg:

	CPU0				CPU1
	fec_enet_tx()
					interrupt, IEVENT = FEC_ENET_TXF
					FEC_ENET_TXF cleared
					napi_schedule_prep()
	napi_complete()

The result is that we clear the transmit interrupt, but we don't trigger
any cleaning of the transmit ring.  Instead, use a different strategy:

- When receiving a transmit or receive interrupt, disable both tx and rx
  interrupts, but do not acknowledge them.  Schedule a napi poll.  Don't
  loop.

- When we are polled, read IEVENT, acknowledging the pending transmit
  and receive interrupts, before then going on to process the
  appropriate rings.

This allows us to avoid the race, and has a number of other advantages:
- we cut down on the number of transmit interrupts we have to process.
- we only look at the rings which have pending events.
- we gain additional throughput: the iperf total bandwidth increases
  from about 180Mbps to 240Mbps:

...

It is usually possible to reduce the interrupt count even further by:
1) processing the rings.
2) clear the IRQ.
3) check the rings for new entries, if any start again.
This means that you don't take the interrupt for anything that completes
while processing the earlier ring entries.

The slight downside is that it is easy to exit the ISR before the IRQ
line actually drops - resulting in a spurious interrupt.

(patch 24 seems to be similar).

	David