Re: [net PATCH] atl1c: Fix misuse of netdev_alloc_skb in refilling rx ring

Ben Hutchings
2013-07-29T00:07:18Z

On Sun, 2013-07-28 at 16:20 -0700, Eric Dumazet wrote:
> 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.
[...]

Just like many older PCI/PCIe devices. So what? pci_map_single() takes
care of that, just so long as the driver sets the DMA mask correctly.
In fact, PCI devices have a 32-bit DMA mask by default.

Ben.

--
Ben Hutchings, Staff Engineer, Solarflare
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