Re: [PATCH net-next 5/5] net/mlx4: flush pending doorbell when dropping a TX skb

[Daniel Machon]

> If mlx4_en_xmit drops an skb (for example because DMA mapping fails) and
> that skb happens to be the last one in the batch, the doorbell for the
> WQEs posted earlier in the batch isn't written.
> 
> The queue stays awake, though, so those WQEs are eventually announced to
> the HW after the next TX, but that depends on traffic and could induce
> latency.
> 
> Avoid all of that by writing the pending doorbell in the drop path.
> 
> Signed-off-by: Joe Damato <joe@xxxxxxx>
> ---
>  drivers/net/ethernet/mellanox/mlx4/en_tx.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> index 123071a47bed..3166837711be 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> @@ -1135,6 +1135,9 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
>  
>  tx_drop_count:
>  	ring->tx_dropped++;
> +	/* an earlier skb in this batch may have deferred the doorbell */
> +	if (ring->doorbell_pending)

I think checking !netdev_xmit_more() here would let you ditch the new ring
state, but it seems bnxt does it the same way, so fine by me.

> +		mlx4_en_xmit_doorbell(ring);
>  tx_drop:
>  	dev_kfree_skb_any(skb);
>  	return NETDEV_TX_OK;
> -- 
> 2.53.0-Meta
>

Reviewed-by: Daniel Machon <daniel.machon@xxxxxxxxxxxxx>