netdev - Re: [PATCH net-next] tg3: Add skb->xmit

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Message-ID: <20141013173806.GB26105@breakpoint.cc>
Date:	Mon, 13 Oct 2014 19:38:06 +0200
From:	Florian Westphal <fw@...len.de>
To:	Prashant Sreedharan <prashant@...adcom.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, dborkman@...hat.com,
	mchan@...adcom.com
Subject: Re: [PATCH net-next] tg3: Add skb->xmit_more support

Prashant Sreedharan <prashant@...adcom.com> wrote:
> Ring TX doorbell only if xmit_more is not set or the queue is stopped.
> 
> Suggested-by: Daniel Borkmann <dborkman@...hat.com>
> Signed-off-by: Prashant Sreedharan <prashant@...adcom.com>
> Signed-off-by: Michael Chan <mchan@...adcom.com>
> ---
>  drivers/net/ethernet/broadcom/tg3.c |   10 ++++++----
>  1 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/broadcom/tg3.c b/drivers/net/ethernet/broadcom/tg3.c
> index ba49948..dbb41c1 100644
> --- a/drivers/net/ethernet/broadcom/tg3.c
> +++ b/drivers/net/ethernet/broadcom/tg3.c
> @@ -8099,9 +8099,6 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	/* Sync BD data before updating mailbox */
>  	wmb();
>  
> -	/* Packets are ready, update Tx producer idx local and on card. */
> -	tw32_tx_mbox(tnapi->prodmbox, entry);
> -
>  	tnapi->tx_prod = entry;
>  	if (unlikely(tg3_tx_avail(tnapi) <= (MAX_SKB_FRAGS + 1))) {
>  		netif_tx_stop_queue(txq);
> @@ -8116,7 +8113,12 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  			netif_tx_wake_queue(txq);
>  	}
>  
> -	mmiowb();
> +	if (!skb->xmit_more || netif_xmit_stopped(txq)) {
> +		/* Packets are ready, update Tx producer idx on card. */
> +		tw32_tx_mbox(tnapi->prodmbox, entry);

I think you need to swap the test, i.e.

netif_xmit_stopped() || xmit_more

Otherwise, hw may not be aware of further packets wainting
and queue might not be restarted?
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