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    SubjectRe: [PATCH RFC] fix problems with NETIF_F_HIGHDMA in networking drivers v2
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    On Wed, Mar 3, 2010 at 10:58 PM, FUJITA Tomonori
    <fujita.tomonori@lab.ntt.co.jp> wrote:
    > On Wed, 03 Mar 2010 20:55:55 -0600
    > Robert Hancock <hancockrwd@gmail.com> wrote:
    >
    >> Many networking drivers have issues with the use of the NETIF_F_HIGHDMA flag.
    >> This flag actually indicates whether or not the device/driver can handle
    >> skbs located in high memory (as opposed to lowmem). If the flag isn't set and
    >> the skb is located in highmem, it needs to be copied.
    >> There are two problems with this flag:
    >>
    >> -Many drivers only set the flag when they detect they can use 64-bit DMA,
    >> since otherwise they could receive DMA addresses that they can't handle
    >> (which on platforms without IOMMU/SWIOTLB support is fatal). This means that if
    >> 64-bit support isn't available, even buffers located below 4GB will get copied
    >> unnecessarily.
    >>
    >> -Some drivers set the flag even though they can't actually handle 64-bit DMA,
    >> which would mean that on platforms without IOMMU/SWIOTLB they would get a DMA
    >> mapping error if the memory they received happened to be located above 4GB.
    >>
    >> In order to fix this problem, the existing NETIF_F_HIGHDMA flag is split into
    >> two new flags:
    >>
    >> NETIF_F_DMA_HIGH - indicates if the driver can do DMA to highmem at all
    >> NETIF_F_DMA_64BIT - indicates the driver can do DMA to 64-bit memory
    >
    > Why can't you use dev->dma_mask here like the following?
    >
    > Then you can fix drivers that use the NETIF_F_HIGHDMA flag to indicate
    > that they don't support 64bit DMA.
    >
    > diff --git a/net/core/dev.c b/net/core/dev.c
    > index bcc490c..b15f94b 100644
    > --- a/net/core/dev.c
    > +++ b/net/core/dev.c
    > @@ -129,6 +129,7 @@
    >  #include <linux/jhash.h>
    >  #include <linux/random.h>
    >  #include <trace/events/napi.h>
    > +#include <linux/pci.h>
    >
    >  #include "net-sysfs.h"
    >
    > @@ -1787,14 +1788,21 @@ static inline int illegal_highdma(struct net_device *dev, struct sk_buff *skb)
    >  {
    >  #ifdef CONFIG_HIGHMEM
    >        int i;
    > +       if (!(dev->features & NETIF_F_HIGHDMA)) {
    > +               for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
    > +                       if (PageHighMem(skb_shinfo(skb)->frags[i].page))
    > +                               return 1;
    > +       }
    >
    > -       if (dev->features & NETIF_F_HIGHDMA)
    > -               return 0;
    > -
    > -       for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
    > -               if (PageHighMem(skb_shinfo(skb)->frags[i].page))
    > -                       return 1;
    > +       if (PCI_DMA_BUS_IS_PHYS) {
    > +               struct device *pdev = dev->dev.parent;
    >
    > +               for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
    > +                       dma_addr_t addr = page_to_phys(skb_shinfo(skb)->frags[i].page);
    > +                       if (!pdev->dma_mask || addr + PAGE_SIZE - 1 > *pdev->dma_mask)
    > +                               return 1;
    > +               }
    > +       }
    >  #endif
    >        return 0;
    >  }
    >

    This seems like it could be a reasonable approach. The only thing is
    that in this code you're returning 1 if the parent device has no DMA
    mask set. Wouldn't it make more sense to return 0 in this case? I'm
    assuming that in that situation it's a virtual device not backed by
    any hardware and there should be no DMA mask restriction...
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