Re: alignment faults in 3.6
flat view
From: Rob Herring <hidden>
Date: 2012-10-11 13:20:14
Also in:
linux-arm-kernel
Possibly related (same subject, not in this thread)
- 2012-10-12 · alignment faults in 3.6 · Ben Hutchings <hidden>
- 2012-10-12 · alignment faults in 3.6 · David Laight <hidden>
- 2012-10-12 · alignment faults in 3.6 · Ben Hutchings <hidden>
- 2012-10-12 · alignment faults in 3.6 · bcrl@kvack.org (Benjamin LaHaise)
- 2012-10-12 · alignment faults in 3.6 · Eric Dumazet <hidden>
On 10/11/2012 07:40 AM, Eric Dumazet wrote:
On Thu, 2012-10-11 at 12:28 +0000, Arnd Bergmann wrote:quoted
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.
This in fact does work and eliminates the unaligned traps. However, not all h/w can do IP aligned DMA (i.MX FEC for example), so I still think this is a questionable optimization by the compiler. We're saving 1 load instruction here for data that is likely already in the cache. It may be legal per the ABI, but the downside of this optimization is much greater than the upside.
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.
I'm pretty sure this was needed as the h/w writes out full bursts of data, but I'll go back and check. Rob
quoted hunk ↗ jump to hunk
(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 kernelsdiff --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);