ixl(4): Fix queue MSI and legacy IRQ rearming

When MSI or legacy interrupt is used driver controls wheter
queues can trigger an interrupt with the Interrupt Linked List.
While processing traffic first index of the list is set to EOL
value to stop queues from triggering interrupts. This index was
not reset to the correct value when driver attempted to re-enable
interrupts from queues, what prevented driver from processing any
traffic. Fix that by setting correct first index in the
ixl_if_enable_intr function.

While at that fix the comments style and make ixl_if_enable_intr
and ixl_if_disable_intr more consistent.

Signed-off-by: Krzysztof Galazka <krzysztof.galazka@intel.com>

PR:		288077
Suggested by:	Mike Belanger <mibelanger@qnx.com>
Approved by:	kbowling (mentor), erj (mentor)
Tested by:	gowtham.kumar.ks_intel.com,
Sponsored by:	Intel Corporation
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D51331

Author: Krzysztof Galazka

sys/dev/ixl/if_ixl.c

@@ -1151,13 +1151,20 @@ ixl_if_enable_intr(if_ctx_t ctx)
 	struct ixl_pf *pf = iflib_get_softc(ctx);
 	struct ixl_vsi *vsi = &pf->vsi;
 	struct i40e_hw		*hw = vsi->hw;
-	struct ixl_rx_queue	*que = vsi->rx_queues;
+	struct ixl_rx_queue	*rx_que = vsi->rx_queues;
 
 	ixl_enable_intr0(hw);
 	/* Enable queue interrupts */
-	for (int i = 0; i < vsi->num_rx_queues; i++, que++)
-		/* TODO: Queue index parameter is probably wrong */
-		ixl_enable_queue(hw, que->rxr.me);
+	if (vsi->shared->isc_intr == IFLIB_INTR_MSIX) {
+		for (int i = 0; i < vsi->num_rx_queues; i++, rx_que++)
+			ixl_enable_queue(hw, rx_que->rxr.me);
+	} else {
+		/*
+		 * Set PFINT_LNKLST0 FIRSTQ_INDX to 0x0 to enable
+		 * triggering interrupts by queues.
+		 */
+		wr32(hw, I40E_PFINT_LNKLST0, 0x0);
+	}
 }
 
 /*
@@ -1175,11 +1182,13 @@ ixl_if_disable_intr(if_ctx_t ctx)
 
 	if (vsi->shared->isc_intr == IFLIB_INTR_MSIX) {
 		for (int i = 0; i < vsi->num_rx_queues; i++, rx_que++)
-			ixl_disable_queue(hw, rx_que->msix - 1);
+			ixl_disable_queue(hw, rx_que->rxr.me);
 	} else {
-		// Set PFINT_LNKLST0 FIRSTQ_INDX to 0x7FF
-		// stops queues from triggering interrupts
-		wr32(hw, I40E_PFINT_LNKLST0, 0x7FF);
+		/*
+		 * Set PFINT_LNKLST0 FIRSTQ_INDX to End of List (0x7FF)
+		 * to stop queues from triggering interrupts.
+		 */
+		wr32(hw, I40E_PFINT_LNKLST0, IXL_QUEUE_EOL);
 	}
 }