can: m_can: Batch acknowledge rx fifo

Instead of acknowledging every item of the fifo, only acknowledge the
last item read. This behavior is documented in the datasheet. The new
getindex will be the acknowledged item + 1.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-8-msp@baylibre.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.c

@@ -530,9 +530,6 @@ static int m_can_read_fifo(struct net_device *dev, u32 fgi)
 	}
 	stats->rx_packets++;
 
-	/* acknowledge rx fifo 0 */
-	m_can_write(cdev, M_CAN_RXF0A, fgi);
-
 	timestamp = FIELD_GET(RX_BUF_RXTS_MASK, fifo_header.dlc) << 16;
 
 	m_can_receive_skb(cdev, skb, timestamp);
@@ -553,8 +550,9 @@ static int m_can_do_rx_poll(struct net_device *dev, int quota)
 	u32 rxfs;
 	u32 rx_count;
 	u32 fgi;
+	int ack_fgi = -1;
 	int i;
-	int err;
+	int err = 0;
 
 	rxfs = m_can_read(cdev, M_CAN_RXF0S);
 	if (!(rxfs & RXFS_FFL_MASK)) {
@@ -568,13 +566,20 @@ static int m_can_do_rx_poll(struct net_device *dev, int quota)
 	for (i = 0; i < rx_count && quota > 0; ++i) {
 		err = m_can_read_fifo(dev, fgi);
 		if (err)
-			return err;
+			break;
 
 		quota--;
 		pkts++;
+		ack_fgi = fgi;
 		fgi = (++fgi >= cdev->mcfg[MRAM_RXF0].num ? 0 : fgi);
 	}
 
+	if (ack_fgi != -1)
+		m_can_write(cdev, M_CAN_RXF0A, ack_fgi);
+
+	if (err)
+		return err;
+
 	return pkts;
 }