can: m_can: Introduce a tx_fifo_in_flight counter

Keep track of the number of transmits in flight.

This patch prepares the driver to control the network interface queue
based on this counter. By itself this counter be
implemented with an atomic, but as we need to do other things in the
critical sections later I am using a spinlock instead.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20240207093220.2681425-12-msp@baylibre.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.c

@@ -484,6 +484,7 @@ static u32 m_can_get_timestamp(struct m_can_classdev *cdev)
 static void m_can_clean(struct net_device *net)
 {
 	struct m_can_classdev *cdev = netdev_priv(net);
+	unsigned long irqflags;
 
 	if (cdev->tx_ops) {
 		for (int i = 0; i != cdev->tx_fifo_size; ++i) {
@@ -497,6 +498,10 @@ static void m_can_clean(struct net_device *net)
 
 	for (int i = 0; i != cdev->can.echo_skb_max; ++i)
 		can_free_echo_skb(cdev->net, i, NULL);
+
+	spin_lock_irqsave(&cdev->tx_handling_spinlock, irqflags);
+	cdev->tx_fifo_in_flight = 0;
+	spin_unlock_irqrestore(&cdev->tx_handling_spinlock, irqflags);
 }
 
 /* For peripherals, pass skb to rx-offload, which will push skb from
@@ -1067,6 +1072,24 @@ static void m_can_tx_update_stats(struct m_can_classdev *cdev,
 	stats->tx_packets++;
 }
 
+static void m_can_finish_tx(struct m_can_classdev *cdev, int transmitted)
+{
+	unsigned long irqflags;
+
+	spin_lock_irqsave(&cdev->tx_handling_spinlock, irqflags);
+	cdev->tx_fifo_in_flight -= transmitted;
+	spin_unlock_irqrestore(&cdev->tx_handling_spinlock, irqflags);
+}
+
+static void m_can_start_tx(struct m_can_classdev *cdev)
+{
+	unsigned long irqflags;
+
+	spin_lock_irqsave(&cdev->tx_handling_spinlock, irqflags);
+	++cdev->tx_fifo_in_flight;
+	spin_unlock_irqrestore(&cdev->tx_handling_spinlock, irqflags);
+}
+
 static int m_can_echo_tx_event(struct net_device *dev)
 {
 	u32 txe_count = 0;
@@ -1076,6 +1099,7 @@ static int m_can_echo_tx_event(struct net_device *dev)
 	int i = 0;
 	int err = 0;
 	unsigned int msg_mark;
+	int processed = 0;
 
 	struct m_can_classdev *cdev = netdev_priv(dev);
 
@@ -1105,12 +1129,15 @@ static int m_can_echo_tx_event(struct net_device *dev)
 
 		/* update stats */
 		m_can_tx_update_stats(cdev, msg_mark, timestamp);
+		++processed;
 	}
 
 	if (ack_fgi != -1)
 		m_can_write(cdev, M_CAN_TXEFA, FIELD_PREP(TXEFA_EFAI_MASK,
 							  ack_fgi));
 
+	m_can_finish_tx(cdev, processed);
+
 	return err;
 }
 
@@ -1192,6 +1219,7 @@ static irqreturn_t m_can_isr(int irq, void *dev_id)
 				timestamp = m_can_get_timestamp(cdev);
 			m_can_tx_update_stats(cdev, 0, timestamp);
 			netif_wake_queue(dev);
+			m_can_finish_tx(cdev, 1);
 		}
 	} else  {
 		if (ir & (IR_TEFN | IR_TEFW)) {
@@ -1890,6 +1918,8 @@ static netdev_tx_t m_can_start_xmit(struct sk_buff *skb,
 		return NETDEV_TX_OK;
 	}
 
+	m_can_start_tx(cdev);
+
 	if (cdev->is_peripheral)
 		return m_can_start_peripheral_xmit(cdev, skb);
 	else

drivers/net/can/m_can/m_can.h

@@ -108,6 +108,10 @@ struct m_can_classdev {
 	// Store this internally to avoid fetch delays on peripheral chips
 	u32 tx_fifo_putidx;
 
+	/* Protects shared state between start_xmit and m_can_isr */
+	spinlock_t tx_handling_spinlock;
+	int tx_fifo_in_flight;
+
 	struct m_can_tx_op *tx_ops;
 	int tx_fifo_size;
 	int next_tx_op;