netdev - Re: Do I need to skb_put() Ethernet frames to a minimum of 60 bytes?

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Date:	Tue, 21 Aug 2012 19:42:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arvid Brodin <Arvid.Brodin@...n.com>
Cc:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Do I need to skb_put() Ethernet frames to a minimum of 60
 bytes?

On Tue, 2012-08-21 at 17:34 +0000, Arvid Brodin wrote:

> Nicolas, can you take a look at this? At the moment I'm using the following change
> in macb.c to avoid TX underruns on short packages:
> 
> --- a/drivers/net/ethernet/cadence/macb.c	2012-05-04 19:14:41.927719667 +0200
> +++ b/drivers/net/ethernet/cadence/macb.c	2012-08-21 19:22:40.063739049 +0200
> @@ -618,6 +618,7 @@ static void macb_poll_controller(struct
>  }
>  #endif
> 
> +#define MIN_ETHFRAME_LEN	60
>  static int macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct macb *bp = netdev_priv(dev);
> @@ -635,6 +636,12 @@ static int macb_start_xmit(struct sk_buf
>  	printk("\n");
>  #endif
> 
> +	if (skb->len < MIN_ETHFRAME_LEN) {
> +		/* Pad skb to minium Ethernet frame size */
> +		if (skb_tailroom(skb) >= MIN_ETHFRAME_LEN - skb->len)
> +			memset(skb_put(skb, MIN_ETHFRAME_LEN - skb->len), 0,
> +						MIN_ETHFRAME_LEN - skb->len);
> +	}
>  	len = skb->len;
>  	spin_lock_irqsave(&bp->lock, flags);
> 
> 
> ... but as you can see this is limited to linear skbs which has been allocated with
> enough tailroom. Perhaps there are better ways to fix the problem? (Maybe the hardware
> is actually doing the padding already and the problem has to do with the way the DMA
> transfer is set up?)
> 

other net drivers use skb_padto() for this ...




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