Subject: Re: [PATCH v3 13/16] can: m_can: Introduce a tx_fifo_in_flight counter

From: Simon Horman <[email-masked]>
Date: Fri, 17 Mar 2023 17:02:44 +0100

On Wed, Mar 15, 2023 at 12:05:43PM +0100, Markus Schneider-Pargmann wrote:
> 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 <[email-masked]>

Nit, assuming the values are always positive, I think
that unsigned might be a more appropriate type than int
for the tx_fifo_in_flight field, and associated function
parameters and local variables.

That notwithstanding,

Reviewed-by: Simon Horman <[email-masked]>

> ---
>  drivers/net/can/m_can/m_can.c | 41 ++++++++++++++++++++++++++++++++++-
>  drivers/net/can/m_can/m_can.h |  4 ++++
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index 27d36bcc094c..4ad8f08f8284 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -442,6 +442,7 @@ 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;
>  
>  	for (int i = 0; i != cdev->tx_fifo_size; ++i) {
>  		if (!cdev->tx_ops[i].skb)
> @@ -453,6 +454,10 @@ 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
> @@ -1023,6 +1028,24 @@ 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;
> @@ -1032,6 +1055,7 @@ 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);
>  
> @@ -1061,12 +1085,15 @@ 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;
>  }
>  
> @@ -1161,6 +1188,7 @@ 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)) {
> @@ -1846,11 +1874,22 @@ static netdev_tx_t m_can_start_peripheral_xmit(struct m_can_classdev *cdev,
>  	}
>  
>  	netif_stop_queue(cdev->net);
> +
> +	m_can_start_tx(cdev);
> +
>  	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)
> +{
> +	m_can_start_tx(cdev);
> +
> +	return m_can_tx_handler(cdev, skb);
> +}
> +
>  static netdev_tx_t m_can_start_xmit(struct sk_buff *skb,
>  				    struct net_device *dev)
>  {
> @@ -1862,7 +1901,7 @@ 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 2e1a52980a18..e230cf320a6c 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 tx_fifo_size;
>  	int next_tx_op;
> -- 
> 2.39.2
>