[Acked] [PATCH 1/2 Xenial SRU] e1000: Double Tx descriptors needed check for 82544

[Andy Whitcroft]

On Mon, May 16, 2016 at 11:19:11AM -0600, Tim Gardner wrote:
> From: Alexander Duyck <aduyck at mirantis.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1582328
> 
> The 82544 has code that adds one additional descriptor per data buffer.
> However we weren't taking that into account when determining the descriptors
> needed for the next transmit at the end of the xmit_frame path.
> 
> This change takes that into account by doubling the number of descriptors
> needed for the 82544 so that we can avoid a potential issue where we could
> hang the Tx ring by loading frames with xmit_more enabled and then stopping
> the ring without writing the tail.
> 
> In addition it adds a few more descriptors to account for some additional
> workarounds that have been added over time.
> 
> Signed-off-by: Alexander Duyck <aduyck at mirantis.com>
> Tested-by: Aaron Brown <aaron.f.brown at intel.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher at intel.com>
> (cherry picked from commit a4605fef7132f19afded76ee025c957558271a7d)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  drivers/net/ethernet/intel/e1000/e1000_main.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
> index 3fc7bde..0b81d93 100644
> --- a/drivers/net/ethernet/intel/e1000/e1000_main.c
> +++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
> @@ -3256,12 +3256,29 @@ static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
>  			     nr_frags, mss);
>  
>  	if (count) {
> +		/* The descriptors needed is higher than other Intel drivers
> +		 * due to a number of workarounds.  The breakdown is below:
> +		 * Data descriptors: MAX_SKB_FRAGS + 1
> +		 * Context Descriptor: 1
> +		 * Keep head from touching tail: 2
> +		 * Workarounds: 3
> +		 */
> +		int desc_needed = MAX_SKB_FRAGS + 7;
> +
>  		netdev_sent_queue(netdev, skb->len);
>  		skb_tx_timestamp(skb);
>  
>  		e1000_tx_queue(adapter, tx_ring, tx_flags, count);
> +
> +		/* 82544 potentially requires twice as many data descriptors
> +		 * in order to guarantee buffers don't end on evenly-aligned
> +		 * dwords
> +		 */
> +		if (adapter->pcix_82544)
> +			desc_needed += MAX_SKB_FRAGS + 1;
> +
>  		/* Make sure there is space in the ring for the next send. */
> -		e1000_maybe_stop_tx(netdev, tx_ring, MAX_SKB_FRAGS + 2);
> +		e1000_maybe_stop_tx(netdev, tx_ring, desc_needed);
>  
>  		if (!skb->xmit_more ||
>  		    netif_xmit_stopped(netdev_get_tx_queue(netdev, 0))) {

This seems to do what is claimed.  Simple enough.

Acked-by: Andy Whitcroft <apw at canonical.com>

-apw