netdev - RE: [PATCH net-next v4 2/2] net: systemport: pad packets to a minimum of 64 bytes

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Date:	Mon, 12 May 2014 12:45:32 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Florian Fainelli' <f.fainelli@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net-next v4 2/2] net: systemport: pad packets to a
 minimum of 64 bytes

From: Florian Fainelli
> The switch fabric which is used behind the Broadcom SYSTEMPORT Ethernet
> controller will discard any incoming packet that is not 64 bytes or
> more. Since the UniMAC hardware transmits up to the specified size, just
> make sure that we properly pad the end of the packet with zeroes using
> skb_padto() and set a minimum packet length of 64 bytes.

Surely the above should just be:
    "Pad transmit packets to the minimum length (64 bytes)."
All ethernet packets should be padded to 64 bytes (including the CRC).

...
> @@ -848,10 +849,22 @@ static netdev_tx_t bcm_sysport_xmit(struct sk_buff *skb,
>  		}
>  	}
> 
> -	mapping = dma_map_single(kdev, skb->data, skb->len, DMA_TO_DEVICE);
> +	/* The Ethernet switch we are interfaced with needs packets to be at
> +	 * least 64 bytes otherwise they will be discarded when they enter
> +	 * the switch port logic. The UniMAC hardware transmits up to the
> +	 * specified length.
> +	 */
> +	if (skb_padto(skb, 64)) {

Shouldn't that be 60?
I presume there is an appropriate constant defined for it as well.

> +		ret = NETDEV_TX_OK;
> +		goto out;
> +	}
> +
> +	skb_len = skb->len < 64 ? 64 : skb->len;
> +
> +	mapping = dma_map_single(kdev, skb->data, skb_len, DMA_TO_DEVICE);
>  	if (dma_mapping_error(kdev, mapping)) {
>  		netif_err(priv, tx_err, dev, "DMA map failed at %p (len=%d)\n",
> -				skb->data, skb->len);
> +				skb->data, skb_len);
>  		ret = NETDEV_TX_OK;
>  		goto out;
>  	}
...

	david


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