Re: [PATCH RFC] fix problems with NETIF_F_HIGHDMA in networking drivers v2

FUJITA Tomonori
2010-03-04T04:58:17Z

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;
}