netdev - Re: alignment faults in 3.6

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Date:	Thu, 11 Oct 2012 14:40:48 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Jon Masters <jonathan@...masters.org>, netdev@...r.kernel.org,
	Måns Rullgård <mans@...sr.com>,
	David Laight <David.Laight@...lab.com>,
	Rob Herring <robherring2@...il.com>
Subject: Re: alignment faults in 3.6

On Thu, 2012-10-11 at 12:28 +0000, Arnd Bergmann wrote:

> 
> Rob Herring as the original reporter has dropped off the Cc list, adding
> him back.
> 
> I assume that the calxeda xgmac driver is the culprit then. It uses
> netdev_alloc_skb() rather than netdev_alloc_skb_ip_align() in
> xgmac_rx_refill but it is not clear whether it does so intentionally
> or by accident.

Thanks Arnd

It seems an accident, since driver doesnt check skb->data alignment at
all (this can change with SLAB debug on/off)

It also incorrectly adds 64 bytes to bfsize, there is no need for this.

(or if its needed, a comment would be nice, because on prior kernels,
this makes skb->head allocations uses kmalloc-4096 instead of
kmalloc-2048 slab cache... With 3.7 its less an issue now we use order-3
pages to deliver fragments for rx skbs

So the following patch should fix the alignment, and makes driver uses
half memory than before for stable kernels

diff --git a/drivers/net/ethernet/calxeda/xgmac.c b/drivers/net/ethernet/calxeda/xgmac.c
index 16814b3..a895e18 100644
--- a/drivers/net/ethernet/calxeda/xgmac.c
+++ b/drivers/net/ethernet/calxeda/xgmac.c
@@ -671,7 +671,8 @@ static void xgmac_rx_refill(struct xgmac_priv *priv)
 		p = priv->dma_rx + entry;
 
 		if (priv->rx_skbuff[entry] == NULL) {
-			skb = netdev_alloc_skb(priv->dev, priv->dma_buf_sz);
+			skb = netdev_alloc_skb_ip_align(priv->dev,
+							priv->dma_buf_sz);
 			if (unlikely(skb == NULL))
 				break;
 
@@ -703,7 +704,7 @@ static int xgmac_dma_desc_rings_init(struct net_device *dev)
 	/* Set the Buffer size according to the MTU;
 	 * indeed, in case of jumbo we need to bump-up the buffer sizes.
 	 */
-	bfsize = ALIGN(dev->mtu + ETH_HLEN + ETH_FCS_LEN + NET_IP_ALIGN + 64,
+	bfsize = ALIGN(dev->mtu + ETH_HLEN + ETH_FCS_LEN,
 		       64);
 
 	netdev_dbg(priv->dev, "mtu [%d] bfsize [%d]\n", dev->mtu, bfsize);


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