can: m_can: Eliminate double read of TXFQS in tx_handler

The TXFQS register is read first to check if the fifo is full and then
immediately again to get the putidx. This is unnecessary and adds
significant overhead if read requests are done over a slow bus, for
example SPI with tcan4x5x.

Add a variable to store the value of the register. Split the
m_can_tx_fifo_full function into two to avoid the hidden m_can_read call
if not needed.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-2-msp@baylibre.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.c

@@ -369,9 +369,14 @@ m_can_txe_fifo_read(struct m_can_classdev *cdev, u32 fgi, u32 offset, u32 *val)
 	return cdev->ops->read_fifo(cdev, addr_offset, val, 1);
 }
 
+static inline bool _m_can_tx_fifo_full(u32 txfqs)
+{
+	return !!(txfqs & TXFQS_TFQF);
+}
+
 static inline bool m_can_tx_fifo_full(struct m_can_classdev *cdev)
 {
-	return !!(m_can_read(cdev, M_CAN_TXFQS) & TXFQS_TFQF);
+	return _m_can_tx_fifo_full(m_can_read(cdev, M_CAN_TXFQS));
 }
 
 static void m_can_config_endisable(struct m_can_classdev *cdev, bool enable)
@@ -1609,6 +1614,7 @@ static netdev_tx_t m_can_tx_handler(struct m_can_classdev *cdev)
 	struct sk_buff *skb = cdev->tx_skb;
 	struct id_and_dlc fifo_header;
 	u32 cccr, fdflags;
+	u32 txfqs;
 	int err;
 	int putidx;
 
@@ -1665,8 +1671,10 @@ static netdev_tx_t m_can_tx_handler(struct m_can_classdev *cdev)
 	} else {
 		/* Transmit routine for version >= v3.1.x */
 
+		txfqs = m_can_read(cdev, M_CAN_TXFQS);
+
 		/* Check if FIFO full */
-		if (m_can_tx_fifo_full(cdev)) {
+		if (_m_can_tx_fifo_full(txfqs)) {
 			/* This shouldn't happen */
 			netif_stop_queue(dev);
 			netdev_warn(dev,
@@ -1682,8 +1690,7 @@ static netdev_tx_t m_can_tx_handler(struct m_can_classdev *cdev)
 		}
 
 		/* get put index for frame */
-		putidx = FIELD_GET(TXFQS_TFQPI_MASK,
-				   m_can_read(cdev, M_CAN_TXFQS));
+		putidx = FIELD_GET(TXFQS_TFQPI_MASK, txfqs);
 
 		/* Construct DLC Field, with CAN-FD configuration.
 		 * Use the put index of the fifo as the message marker,