On Thu, 28 Jan 2010 11:43:16 -0500
Michael Breuer [off-list ref] wrote:

Update: I played with dma-debug. Was being disabled due to lack of 
memory. I forced it back on while pumping traffic through and got this:
Jan 28 11:39:30 mail kernel: ------------[ cut here ]------------
Jan 28 11:39:30 mail kernel: WARNING: at lib/dma-debug.c:902 
check_sync+0xc1/0x43f()
Jan 28 11:39:30 mail kernel: Hardware name: System Product Name
Jan 28 11:39:30 mail kernel: sky2 0000:06:00.0: DMA-API: device driver 
tries to sync DMA memory it has not allocated [device 

This test in dma-debug is bogus. Because the debug code matches
dma based on address and size; and is perfectly valid to sync a value
less than size. This is the patch I sent earlier, it isn't 100%
correct but it will let you keep testing
....................................


This should fix the dma-debug API code (and documentation), to
avoid false positives when sync is done on a partial map.

Signed-off-by: Stephen Hemminger <redacted>

--- a/Documentation/DMA-API.txt	2010-01-20 15:17:01.390143729 -0800
+++ b/Documentation/DMA-API.txt	2010-01-20 15:18:48.967875255 -0800

@@ -377,9 +377,10 @@ void
 pci_dma_sync_sg(struct pci_dev *hwdev, struct scatterlist *sg,
 		       int nelems, int direction)
 
-Synchronise a single contiguous or scatter/gather mapping.  All the
-parameters must be the same as those passed into the single mapping
-API.
+Synchronise a single contiguous or scatter/gather mapping.  The
+device and handle must be the same as those passed into the single mapping
+API. The size can be less than the original mapping if only part
+of the mapping needs to be accessed.
 
 Notes:  You must do this:
 
--- a/lib/dma-debug.c	2010-01-20 15:22:55.919519883 -0800
+++ b/lib/dma-debug.c	2010-01-20 15:26:31.648895638 -0800

@@ -285,11 +285,9 @@ static struct dma_debug_entry *hash_buck
 	}
 
 	/*
-	 * If we have multiple matches but no perfect-fit, just return
-	 * NULL.
+	 * If we have multiple matches but no perfect-fit
+	 * return best value and let caller deal with it.
 	 */
-	ret = (matches == 1) ? ret : NULL;
-
 	return ret;
 }