netdev - Re: [net PATCH] atl1c: Fix misuse of netdev_alloc

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Date:	Sun, 28 Jul 2013 16:20:54 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	David Miller <davem@...emloft.net>,
	luis.henriques@...onical.com, netdev@...r.kernel.org,
	jcliburn@...il.com, stable@...r.kernel.org
Subject: Re: [net PATCH] atl1c: Fix misuse of netdev_alloc_skb in refilling
 rx ring

On Sun, 2013-07-28 at 16:01 -0700, Eric Dumazet wrote:
> On Sun, 2013-07-28 at 21:22 +0100, Ben Hutchings wrote:
> 
> > 
> > Since we know lengths > 4K work, perhaps it would be worth testing with
> > the fragment cache size reduced to 16K?  The driver would never
> > previously have used RX buffers crossing 16K boundaries, except if SLOB
> > was used (and that's an unlikely combination).
> 
> Sure, please note the following maths :
> 
> NET_SKB_PAD + 1536 + sizeof(struct skb_shared_info) = 1920
> 
> 16384/1920 = 8
> 
> 32768/1920 = 17
> 
> I don't think atl1c is used in any critical host (given it doesn't even
> provide RX checksums and GRO ...), so I will provide a patch doing mere
> page allocations.
> 

Oh well, look at code around line 2530

        * The atl1c chip can DMA to 64-bit addresses, but it uses a single
         * shared register for the high 32 bits, so only a single, aligned,
         * 4 GB physical address range can be used at a time.
         *
         * Supporting 64-bit DMA on this hardware is more trouble than it's
         * worth.  It is far easier to limit to 32-bit DMA than update
         * various kernel subsystems to support the mechanics required by a
         * fixed-high-32-bit system.
         */
        if ((pci_set_dma_mask(pdev, DMA_BIT_MASK(32)) != 0) ||
            (pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32)) != 0)) {
                dev_err(&pdev->dev, "No usable DMA configuration,aborting\n");
                goto err_dma;
        }

It looks like we have a winner !

This $@!? really needs DMA32 allocations.

Currently only tested on TX patch, it needs same care on RX

diff --git a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
index 786a874..e2ee962 100644
--- a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
+++ b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
@@ -1660,7 +1660,8 @@ static int atl1c_alloc_rx_buffer(struct atl1c_adapter *adapter)
 	while (next_info->flags & ATL1C_BUFFER_FREE) {
 		rfd_desc = ATL1C_RFD_DESC(rfd_ring, rfd_next_to_use);
 
-		skb = netdev_alloc_skb(adapter->netdev, adapter->rx_buffer_len);
+		skb = __netdev_alloc_skb(adapter->netdev, adapter->rx_buffer_len,
+					 GFP_ATOMIC | GFP_DMA32);
 		if (unlikely(!skb)) {
 			if (netif_msg_rx_err(adapter))
 				dev_warn(&pdev->dev, "alloc rx buffer failed\n");


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