sfc: Survive ISR0=0 bug in the shared IRQ case

Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sfc/nic.c

@@ -1384,6 +1384,15 @@ static irqreturn_t efx_legacy_interrupt(int irq, void *dev_id)
 		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;