netdev - Re: [PATCH RFC] fix problems with NETIF_F

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Date:	Thu, 4 Mar 2010 13:58:06 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	hancockrwd@...il.com
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org, davem@...emloft.net, bzolnier@...il.com
Subject: Re: [PATCH RFC] fix problems with NETIF_F_HIGHDMA in networking
 drivers v2

On Wed, 03 Mar 2010 20:55:55 -0600
Robert Hancock <hancockrwd@...il.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;
 }
--
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