Re: [discuss] Re: 32-bit dma allocations on 64-bit platforms

[Takashi Iwai]

At Thu, 24 Jun 2004 13:29:00 +0200,
Andi Kleen wrote:
> 
> > Can't it be called with GFP_KERNEL at first, then with GFP_DMA if the
> > allocated pages are out of dma mask, just like in pci-gart.c?
> > (with ifdef x86-64)
> 
> That won't work reliable enough in extreme cases.

Well, it's not perfect, but it'd be far better than GFP_DMA only :)


BTW, we have the similar problem on i386, too.  The non-32bit DMA mask
always results in the allocation with GFP_DMA.  The patch below does
similar hack as described above, calling with GFP_DMA as fallback.


Takashi

--- linux-2.6.7/arch/i386/kernel/pci-dma.c	2004-06-24 15:56:46.017473544 +0200
+++ linux-2.6.7/arch/i386/kernel/pci-dma.c	2004-06-24 16:05:02.449803937 +0200
@@ -17,17 +17,35 @@ void *dma_alloc_coherent(struct device *
 			   dma_addr_t *dma_handle, int gfp)
 {
 	void *ret;
+	unsigned long dma_mask;
+
 	/* ignore region specifiers */
 	gfp &= ~(__GFP_DMA | __GFP_HIGHMEM);
 
-	if (dev == NULL || (dev->coherent_dma_mask < 0xffffffff))
+	if (dev == NULL) {
 		gfp |= GFP_DMA;
+		dma_mask = 0xffffffUL;
+	} else {
+		dma_mask = 0xffffffffUL;
+		if (dev->dma_mask)
+			dma_mask = *dev->dma_mask;
+		if (dev->coherent_dma_mask)
+			dma_mask &= (unsigned long)dev->coherent_dma_mask;
+	}
 
+ again:
 	ret = (void *)__get_free_pages(gfp, get_order(size));
 
 	if (ret != NULL) {
-		memset(ret, 0, size);
 		*dma_handle = virt_to_phys(ret);
+		if (((unsigned long)*dma_handle + size - 1) & ~dma_mask) {
+			free_pages((unsigned long)ret, get_order(size));
+			if (gfp & GFP_DMA)
+				return NULL;
+			gfp |= GFP_DMA;
+			goto again;
+		}
+		memset(ret, 0, size);
 	}
 	return ret;
 }
-
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