[PATCH 15/18] can: m_can: Introduce a tx_fifo_in_flight counter

[Markus Schneider-Pargmann <msp@baylibre.com>]

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>
---
 drivers/net/can/m_can/m_can.c | 27 ++++++++++++++++++++++++++-
 drivers/net/can/m_can/m_can.h |  4 ++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
index 42cde31fc0a8..90c9ff474149 100644
--- a/drivers/net/can/m_can/m_can.c
+++ b/drivers/net/can/m_can/m_can.c
@@ -452,6 +452,10 @@ 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(&cdev->tx_handling_spinlock);
+	cdev->tx_fifo_in_flight = 0;
+	spin_unlock(&cdev->tx_handling_spinlock);
 }
 
 /* For peripherals, pass skb to rx-offload, which will push skb from
@@ -1022,6 +1026,7 @@ 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);
 
@@ -1051,12 +1056,17 @@ 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));
 
+	spin_lock(&cdev->tx_handling_spinlock);
+	cdev->tx_fifo_in_flight -= processed;
+	spin_unlock(&cdev->tx_handling_spinlock);
+
 	return err;
 }
 
@@ -1821,11 +1831,26 @@ static netdev_tx_t m_can_start_peripheral_xmit(struct m_can_classdev *cdev,
 	}
 
 	netif_stop_queue(cdev->net);
+
+	spin_lock(&cdev->tx_handling_spinlock);
+	++cdev->tx_fifo_in_flight;
+	spin_unlock(&cdev->tx_handling_spinlock);
+
 	m_can_tx_queue_skb(cdev, skb);
 
 	return NETDEV_TX_OK;
 }
 
+static netdev_tx_t m_can_start_fast_xmit(struct m_can_classdev *cdev,
+					 struct sk_buff *skb)
+{
+	spin_lock(&cdev->tx_handling_spinlock);
+	++cdev->tx_fifo_in_flight;
+	spin_unlock(&cdev->tx_handling_spinlock);
+
+	return m_can_tx_handler(cdev, skb);
+}
+
 static netdev_tx_t m_can_start_xmit(struct sk_buff *skb,
 				    struct net_device *dev)
 {
@@ -1837,7 +1862,7 @@ static netdev_tx_t m_can_start_xmit(struct sk_buff *skb,
 	if (cdev->is_peripheral)
 		return m_can_start_peripheral_xmit(cdev, skb);
 	else
-		return m_can_tx_handler(cdev, skb);
+		return m_can_start_fast_xmit(cdev, skb);
 }
 
 static int m_can_open(struct net_device *dev)
diff --git a/drivers/net/can/m_can/m_can.h b/drivers/net/can/m_can/m_can.h
index bf2d710c982f..adbd4765accc 100644
--- a/drivers/net/can/m_can/m_can.h
+++ b/drivers/net/can/m_can/m_can.h
@@ -109,6 +109,10 @@ struct m_can_classdev {
 	// Store this internally to avoid fetch delays on peripheral chips
 	int 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 nr_tx_ops;
 	int next_tx_op;
-- 
2.38.1