iwlagn: fix RX skb alignment

So I dug deeper into the DMA problems I had with iwlagn and a kind soul
helped me in that he said something about pci-e alignment and mentioned
the iwl_rx_allocate function to check for crossing 4KB boundaries. Since
there's 8KB A-MPDU support, crossing 4k boundaries didn't seem like
something the device would fail with, but when I looked into the
function for a minute anyway I stumbled over this little gem:

BUG_ON(rxb->dma_addr & (~DMA_BIT_MASK(36) & 0xff));

Clearly, that is a totally bogus check, one would hope the compiler
removes it entirely. (Think about it)

After fixing it, I obviously ran into it, nothing guarantees the
alignment the way you want it, because of the way skbs and their
headroom are allocated. I won't explain that here nor double-check that
I'm right, that goes beyond what most of the CC'ed people care about.

So then I came up with the patch below, and so far my system has
survived minutes with 64K pages, when it would previously fail in
seconds. And I haven't seen a single instance of the TX bug either. But
when you see the patch it'll be pretty obvious to you why.

This should fix the following reported kernel bugs:

http://bugzilla.kernel.org/show_bug.cgi?id=11596
http://bugzilla.kernel.org/show_bug.cgi?id=11393
http://bugzilla.kernel.org/show_bug.cgi?id=11983

I haven't checked if there are any elsewhere, but I suppose RHBZ will
have a few instances too...

I'd like to ask anyone who is CC'ed (those are people I know ran into
the bug) to try this patch.

I am convinced that this patch is correct in spirit, but I haven't
understood why, for example, there are so many unmap calls. I'm not
entirely convinced that this is the only bug leading to the TX reply
errors.

Signed-off-by: Johannes Berg <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/iwlwifi/iwl-agn.c

···
 
 		rxq->queue[i] = NULL;
 
-		pci_dma_sync_single_for_cpu(priv->pci_dev, rxb->dma_addr,
+		pci_dma_sync_single_for_cpu(priv->pci_dev, rxb->aligned_dma_addr,
 					    priv->hw_params.rx_buf_size,
 					    PCI_DMA_FROMDEVICE);
 		pkt = (struct iwl_rx_packet *)rxb->skb->data;
···
 			rxb->skb = NULL;
 		}
 
-		pci_unmap_single(priv->pci_dev, rxb->dma_addr,
-				 priv->hw_params.rx_buf_size,
+		pci_unmap_single(priv->pci_dev, rxb->real_dma_addr,
+				 priv->hw_params.rx_buf_size + 256,
 				 PCI_DMA_FROMDEVICE);
 		spin_lock_irqsave(&rxq->lock, flags);
 		list_add_tail(&rxb->list, &priv->rxq.rx_used);

drivers/net/wireless/iwlwifi/iwl-dev.h

···
 #define	DEFAULT_LONG_RETRY_LIMIT  4U
 
 struct iwl_rx_mem_buffer {
-	dma_addr_t dma_addr;
+	dma_addr_t real_dma_addr;
+	dma_addr_t aligned_dma_addr;
 	struct sk_buff *skb;
 	struct list_head list;
 };

drivers/net/wireless/iwlwifi/iwl-rx.c

···
 		list_del(element);
 
 		/* Point to Rx buffer via next RBD in circular buffer */
-		rxq->bd[rxq->write] = iwl_dma_addr2rbd_ptr(priv, rxb->dma_addr);
+		rxq->bd[rxq->write] = iwl_dma_addr2rbd_ptr(priv, rxb->aligned_dma_addr);
 		rxq->queue[rxq->write] = rxb;
 		rxq->write = (rxq->write + 1) & RX_QUEUE_MASK;
 		rxq->free_count--;
···
 		rxb = list_entry(element, struct iwl_rx_mem_buffer, list);
 
 		/* Alloc a new receive buffer */
-		rxb->skb = alloc_skb(priv->hw_params.rx_buf_size,
+		rxb->skb = alloc_skb(priv->hw_params.rx_buf_size + 256,
 				__GFP_NOWARN | GFP_ATOMIC);
 		if (!rxb->skb) {
 			if (net_ratelimit())
···
 		list_del(element);
 
 		/* Get physical address of RB/SKB */
-		rxb->dma_addr =
-		    pci_map_single(priv->pci_dev, rxb->skb->data,
-			   priv->hw_params.rx_buf_size, PCI_DMA_FROMDEVICE);
+		rxb->real_dma_addr = pci_map_single(
+					priv->pci_dev,
+					rxb->skb->data,
+					priv->hw_params.rx_buf_size + 256,
+					PCI_DMA_FROMDEVICE);
+		/* dma address must be no more than 36 bits */
+		BUG_ON(rxb->real_dma_addr & ~DMA_BIT_MASK(36));
+		/* and also 256 byte aligned! */
+		rxb->aligned_dma_addr = ALIGN(rxb->real_dma_addr, 256);
+		skb_reserve(rxb->skb, rxb->aligned_dma_addr - rxb->real_dma_addr);
+
 		list_add_tail(&rxb->list, &rxq->rx_free);
 		rxq->free_count++;
 	}
···
 	for (i = 0; i < RX_QUEUE_SIZE + RX_FREE_BUFFERS; i++) {
 		if (rxq->pool[i].skb != NULL) {
 			pci_unmap_single(priv->pci_dev,
-					 rxq->pool[i].dma_addr,
-					 priv->hw_params.rx_buf_size,
+					 rxq->pool[i].real_dma_addr,
+					 priv->hw_params.rx_buf_size + 256,
 					 PCI_DMA_FROMDEVICE);
 			dev_kfree_skb(rxq->pool[i].skb);
 		}
···
 		 * to an SKB, so we need to unmap and free potential storage */
 		if (rxq->pool[i].skb != NULL) {
 			pci_unmap_single(priv->pci_dev,
-					 rxq->pool[i].dma_addr,
-					 priv->hw_params.rx_buf_size,
+					 rxq->pool[i].real_dma_addr,
+					 priv->hw_params.rx_buf_size + 256,
 					 PCI_DMA_FROMDEVICE);
 			priv->alloc_rxb_skb--;
 			dev_kfree_skb(rxq->pool[i].skb);