x86: Disable DAC on VIA bridges

Several reports that VIA bridges don't support DAC and corrupt
data.  I don't know if it's fixed, but let's just blacklist
them all for now.

It can be overwritten with iommu=usedac

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-i386/dma-mapping.h

@@ -123,6 +123,8 @@ dma_mapping_error(dma_addr_t dma_addr)
 	return 0;
 }
 
+extern int forbid_dac;
+
 static inline int
 dma_supported(struct device *dev, u64 mask)
 {
@@ -134,6 +136,10 @@ dma_supported(struct device *dev, u64 mask)
         if(mask < 0x00ffffff)
                 return 0;
 
+	/* Work around chipset bugs */
+	if (forbid_dac > 0 && mask > 0xffffffffULL)
+		return 0;
+
 	return 1;
 }