igb: Fix the transmission mode of queue 0 for Qav mode

When Qav mode is enabled, queue 0 should be kept on Stream Reservation
mode. From the i210 datasheet, section 8.12.19:

"Note: Queue0 QueueMode must be set to 1b when TransmitMode is set to
Qav." ("QueueMode 1b" represents the Stream Reservation mode)

The solution is to give queue 0 the all the credits it might need, so
it has priority over queue 1.

A situation where this can happen is when cbs is "installed" only on
queue 1, leaving queue 0 alone. For example:

$ tc qdisc replace dev enp2s0 handle 100: parent root mqprio num_tc 3 \
     	   map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 queues 1@0 1@1 2@2 hw 0

$ tc qdisc replace dev enp2s0 parent 100:2 cbs locredit -1470 \
     	   hicredit 30 sendslope -980000 idleslope 20000 offload 1

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -1700,7 +1700,22 @@ static void igb_configure_cbs(struct igb_adapter *adapter, int queue,
 	WARN_ON(hw->mac.type != e1000_i210);
 	WARN_ON(queue < 0 || queue > 1);
 
-	if (enable) {
+	if (enable || queue == 0) {
+		/* i210 does not allow the queue 0 to be in the Strict
+		 * Priority mode while the Qav mode is enabled, so,
+		 * instead of disabling strict priority mode, we give
+		 * queue 0 the maximum of credits possible.
+		 *
+		 * See section 8.12.19 of the i210 datasheet, "Note:
+		 * Queue0 QueueMode must be set to 1b when
+		 * TransmitMode is set to Qav."
+		 */
+		if (queue == 0 && !enable) {
+			/* max "linkspeed" idleslope in kbps */
+			idleslope = 1000000;
+			hicredit = ETH_FRAME_LEN;
+		}
+
 		set_tx_desc_fetch_prio(hw, queue, TX_QUEUE_PRIO_HIGH);
 		set_queue_mode(hw, queue, QUEUE_MODE_STREAM_RESERVATION);