On Wed, 03 Mar 2010 20:55:55 -0600
Robert Hancock [off-list ref] wrote:

quoted

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;
 }