kernel: solos-pci: Increase headroom on received packets

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r46993

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46995 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/generic/patches-3.18/083-solos-pci-Increase-headroom-on-received-packets.patch

@@ -0,0 +1,54 @@
+From: David Woodhouse <dwmw2@infradead.org>
+Date: Thu, 17 Sep 2015 11:19:53 +0100
+Subject: [PATCH] solos-pci: Increase headroom on received packets
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+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:
+http://marc.info/?l=linux-netdev&m=144236093519172&w=2
+
+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@intel.com>
+---
+
+--- a/drivers/atm/solos-pci.c
++++ b/drivers/atm/solos-pci.c
+@@ -805,7 +805,12 @@ static void solos_bh(unsigned long card_
+ 					continue;
+ 				}
+ 
+-				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_
+ 		/* 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 =
+ 					pci_map_single(card->dev, skb->data,