[igb] add DROP_EN to each RX queue config if TX flow control is disabled

[Eric Joyner]

This looks good to me. The only comment I have is that according to the
I350 datasheet, DROP_EN is already set on all the queues but 0 by default,
but I haven't checked for the other adapters covered by igb.

---
- Eric Joyner

On Mon, Sep 8, 2014 at 10:12 PM, Adrian Chadd <adrian at freebsd.org> wrote:

> Hi,
>
> This patch enables the DROP_EN flag to each RX queue if TX flow
> control is disabled. It (mostly) mirrors what the ixgbe driver does.
>
> This prevents a single full RX ring from stalling the rest of the RX rings.
>
> How's it look? (indenting and such aside, thanks google.)
>
>
>
> -a
>
> Index: sys/dev/e1000/if_igb.c
> ===================================================================
> --- sys/dev/e1000/if_igb.c (revision 271290)
> +++ sys/dev/e1000/if_igb.c (working copy)
> @@ -4712,6 +4712,18 @@
>   rctl |= E1000_RCTL_SZ_2048;
>   }
>
> + /*
> + * If TX flow control is disabled and there's >1 queue defined,
> + * enable DROP.
> + *
> + * This drops frames rather than hanging the RX MAC for all queues.
> + */
> + if ((adapter->num_queues > 1) &&
> +     (adapter->fc == e1000_fc_none ||
> +     adapter->fc == e1000_fc_rx_pause)) {
> +        srrctl |= E1000_SRRCTL_DROP_EN;
> + }
> +
>   /* Setup the Base and Length of the Rx Descriptor Rings */
>   for (int i = 0; i < adapter->num_queues; i++, rxr++) {
>   u64 bus_addr = rxr->rxdma.dma_paddr;
> @@ -6255,6 +6267,7 @@
>
>   adapter->hw.fc.current_mode = adapter->hw.fc.requested_mode;
>   e1000_force_mac_fc(&adapter->hw);
> + /* XXX TODO: update DROP_EN on each RX queue if appropriate */
>   return (error);
>  }
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