Re: [PATCH 2/3] tg3: Fix tx_pending checks for tg3_tso_bug

[Michael Chan <mchan@broadcom.com>]

On Tue, 2014-08-19 at 11:52 -0700, Benjamin Poirier wrote: 
> @@ -7838,11 +7838,14 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
>                        struct netdev_queue *txq, struct sk_buff *skb)
>  {
>         struct sk_buff *segs, *nskb;
> -       u32 frag_cnt_est = skb_shinfo(skb)->gso_segs * 3;
>  
> -       /* Estimate the number of fragments in the worst case */
> -       if (unlikely(tg3_tx_avail(tnapi) <= frag_cnt_est)) {
> +       if (unlikely(tg3_tx_avail(tnapi) <= skb_shinfo(skb)->gso_segs)) {
> +               trace_printk("stopping queue, %d <= %d\n",
> +                            tg3_tx_avail(tnapi), skb_shinfo(skb)->gso_segs);
>                 netif_tx_stop_queue(txq);
> +               trace_printk("stopped queue\n");
> +               tnapi->wakeup_thresh = skb_shinfo(skb)->gso_segs;
> +               BUG_ON(tnapi->wakeup_thresh >= tnapi->tx_pending);
>  
>                 /* netif_tx_stop_queue() must be done before checking
>                  * checking tx index in tg3_tx_avail() below, because in 

I don't quite understand this logic and I must be missing something.
gso_segs is the number of TCP segments the large packet will be broken
up into.  If it exceeds dev->gso_max_segs, it means it exceeds
hardware's capabilty and it will do GSO instead of TSO.  But in this
case in tg3_tso_bug(), we are doing GSO and we may not have enough DMA
descriptors to do GSO.  Each gso_seg typically requires 2 DMA
descriptors.