sfc: Extend the legacy interrupt workarounds

Siena has two problems with legacy interrupts:
  1. There is no synchronisation between the ISR read completion,
     and the interrupt deassert message.
  2. A downstream read at the "wrong" moment can return 0, and
     suppress generating the next interrupt.

Falcon should suffer from both of these, and it appears it does.
Enable EFX_WORKAROUND_15783 on Falcon as well.

Also, when we see queues == 0, ensure we always schedule or rearm
every event queue.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sfc/nic.c

@@ -1356,33 +1356,28 @@ static irqreturn_t efx_legacy_interrupt(int irq, void *dev_id)
 		}
 		result = IRQ_HANDLED;
 
-	} else if (EFX_WORKAROUND_15783(efx) &&
-		   efx->irq_zero_count++ == 0) {
+	} else if (EFX_WORKAROUND_15783(efx)) {
 		efx_qword_t *event;
 
-		/* Ensure we rearm all event queues */
+		/* We can't return IRQ_HANDLED more than once on seeing ISR=0
+		 * because this might be a shared interrupt. */
+		if (efx->irq_zero_count++ == 0)
+			result = IRQ_HANDLED;
+
+		/* Ensure we schedule or rearm all event queues */
 		efx_for_each_channel(channel, efx) {
 			event = efx_event(channel, channel->eventq_read_ptr);
 			if (efx_event_present(event))
 				efx_schedule_channel(channel);
+			else
+				efx_nic_eventq_read_ack(channel);
 		}
-
-		result = IRQ_HANDLED;
 	}
 
 	if (result == IRQ_HANDLED) {
 		efx->last_irq_cpu = raw_smp_processor_id();
 		EFX_TRACE(efx, "IRQ %d on CPU %d status " EFX_DWORD_FMT "\n",
 			  irq, raw_smp_processor_id(), EFX_DWORD_VAL(reg));
-	} else if (EFX_WORKAROUND_15783(efx)) {
-		/* We can't return IRQ_HANDLED more than once on seeing ISR0=0
-		 * because this might be a shared interrupt, but we do need to
-		 * check the channel every time and preemptively rearm it if
-		 * it's idle. */
-		efx_for_each_channel(channel, efx) {
-			if (!channel->work_pending)
-				efx_nic_eventq_read_ack(channel);
-		}
 	}
 
 	return result;

drivers/net/sfc/workarounds.h

@@ -37,7 +37,7 @@
 /* Truncated IPv4 packets can confuse the TX packet parser */
 #define EFX_WORKAROUND_15592 EFX_WORKAROUND_FALCON_AB
 /* Legacy ISR read can return zero once */
-#define EFX_WORKAROUND_15783 EFX_WORKAROUND_SIENA
+#define EFX_WORKAROUND_15783 EFX_WORKAROUND_ALWAYS
 /* Legacy interrupt storm when interrupt fifo fills */
 #define EFX_WORKAROUND_17213 EFX_WORKAROUND_SIENA