Subject: Re: [PATCH 2/2] ixgbe: support skb->xmit_more in netdev_ops->ndo_start_xmit()

From: Hannes Frederic Sowa <[email-masked]>
Date: Tue, 26 Aug 2014 17:40:57 +0200

On Di, 2014-08-26 at 08:00 -0700, Alexander Duyck wrote:
> On 08/25/2014 04:35 PM, David Miller wrote:
> > 
> > From: Daniel Borkmann <[email-masked]>
> > 
> > This implements the deferred tail pointer flush API for the ixgbe
> > driver. Similar version also proposed longer time ago by Alexander Duyck.
> > 
> > Signed-off-by: Daniel Borkmann <[email-masked]>
> > Signed-off-by: David S. Miller <[email-masked]>
> > ---
> >  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > index 87bd53f..ba9ceaa 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > @@ -6958,9 +6958,10 @@ static void ixgbe_tx_map(struct ixgbe_ring *tx_ring,
> >  
> >  	tx_ring->next_to_use = i;
> >  
> > -	/* notify HW of packet */
> > -	ixgbe_write_tail(tx_ring, i);
> > -
> > +	if (!skb->xmit_more) {
> > +		/* notify HW of packet */
> > +		ixgbe_write_tail(tx_ring, i);
> > +	}
> >  	return;
> >  dma_error:
> >  	dev_err(tx_ring->dev, "TX DMA map failed\n");
> > 
> 
> It might help to add some handling for the case where xmit_more is set,
> but the ring has become full.  This current implementation introduces
> the risk of triggering a Tx hang.

IMHO this should be done before the patch lands in the driver.

> My advice would be to pull the ixgbe_maybe_stop_tx code at the end of
> xmit_frame into the if check here, and perhaps look into adding an
> additional check to see if BQL has stopped the ring as well.

I would like to have the BQL check not in the driver but in the generic
code steering xmit_more.

Otherwise I'll like the API.

David, because of debugging and driver bugs, can we have an interface
flag for that, so we can e.g. switch xmit_more to 0 permanently? It
would also be nice to have for documentation purposes, so ethtool e.g.
can discover feature-set of a driver.

Thanks,
Hannes