net: can: xilinx_can: fix extended frame handling

commit 5793affe upstream.

Using IDR_SRR in RXFIFO_ID to test for the presence of data is only
valid for standard frames. For extended frames the bit is always 1 and
IDR_RTR should be used instead. This patch switches the check to use
CAN_RTR_FLAG which is correctly set when reading the ID.

The patch also changes the DW1/DW2 to be read unconditionally, since
this is necessary to remove the frame from the RXFIFO.
Signed-off-by: Jeppe Ledet-Pedersen <jlp@gomspace.com>
Acked-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/net/can/xilinx_can.c

@@ -508,10 +508,11 @@ static int xcan_rx(struct net_device *ndev)
 			cf->can_id |= CAN_RTR_FLAG;
 	}
 
-	if (!(id_xcan & XCAN_IDR_SRR_MASK)) {
+	/* DW1/DW2 must always be read to remove message from RXFIFO */
 	data[0] = priv->read_reg(priv, XCAN_RXFIFO_DW1_OFFSET);
 	data[1] = priv->read_reg(priv, XCAN_RXFIFO_DW2_OFFSET);
 
+	if (!(cf->can_id & CAN_RTR_FLAG)) {
 		/* Change Xilinx CAN data format to socketCAN data format */
 		if (cf->can_dlc > 0)
 			*(__be32 *)(cf->data) = cpu_to_be32(data[0]);