[OpenWrt-Devel] [PATCH v2] solos-pci: Increase headroom on received packets

David Woodhouse dwmw2 at infradead.org
Thu Sep 17 06:19:53 EDT 2015

A comment in include/linux/skbuff.h says that:

 * Various parts of the networking layer expect at least 32 bytes of
 * headroom, you should not reduce this.

This was demonstrated by a panic when handling fragmented IPv6 packets:

It's not entirely clear if that comment is still valid — and if it is,
perhaps netif_rx() ought to be enforcing it with a warning.

But either way, it is rather stupid from a performance point of view
for us to be receiving packets into a buffer which doesn't have enough
room to prepend an Ethernet header — it means that *every* incoming
packet is going to be need to be reallocated. So let's fix that.

Signed-off-by: David Woodhouse <David.Woodhouse at intel.com>
For CC and trunk please. Related to Trac #20532. This works around the
panic seen there, and potentially others that aren't yet fixed. But
really it *ought* to be just a performance fix to avoid buffer

diff --git a/drivers/atm/solos-pci.c b/drivers/atm/solos-pci.c
index 74e18b0..3d7fb65 100644
--- a/drivers/atm/solos-pci.c
+++ b/drivers/atm/solos-pci.c
@@ -805,7 +805,12 @@ static void solos_bh(unsigned long card_arg)
-				skb = alloc_skb(size + 1, GFP_ATOMIC);
+				/* Use netdev_alloc_skb() because it adds NET_SKB_PAD of
+				 * headroom, and ensures we can route packets back out an
+				 * Ethernet interface (for example) without having to
+				 * reallocate. Adding NET_IP_ALIGN also ensures that both
+				 * PPPoATM and PPPoEoBR2684 packets end up aligned. */
+				skb = netdev_alloc_skb_ip_align(NULL, size + 1);
 				if (!skb) {
 					if (net_ratelimit())
 						dev_warn(&card->dev->dev, "Failed to allocate sk_buff for RX\n");
@@ -869,7 +874,10 @@ static void solos_bh(unsigned long card_arg)
 		/* Allocate RX skbs for any ports which need them */
 		if (card->using_dma && card->atmdev[port] &&
 		    !card->rx_skb[port]) {
-			struct sk_buff *skb = alloc_skb(RX_DMA_SIZE, GFP_ATOMIC);
+			/* Unlike the MMIO case (qv) we can't add NET_IP_ALIGN
+			 * here; the FPGA can only DMA to addresses which are
+			 * aligned to 4 bytes. */
+			struct sk_buff *skb = dev_alloc_skb(RX_DMA_SIZE);
 			if (skb) {
 				SKB_CB(skb)->dma_addr =
 					dma_map_single(&card->dev->dev, skb->data,

David Woodhouse                            Open Source Technology Centre
David.Woodhouse at intel.com                              Intel Corporation

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