[PATCH v3] dma-mapping: set default segment_boundary_mask to ULONG_MAX

[PATCH v3] dma-mapping: set default segment_boundary_mask to ULONG_MAX

[Nicolin Chen]

The default segment_boundary_mask was set to DMA_BIT_MAKS(32)
a decade ago by referencing SCSI/block subsystem, as a 32-bit
mask was good enough for most of the devices.

Now more and more drivers set dma_masks above DMA_BIT_MAKS(32)
while only a handful of them call dma_set_seg_boundary(). This
means that most drivers have a 4GB segmention boundary because
DMA API returns a 32-bit default value, though they might not
really have such a limit.

The default segment_boundary_mask should mean "no limit" since
the device doesn't explicitly set the mask. But a 32-bit mask
certainly limits those devices capable of 32+ bits addressing.

So this patch sets default segment_boundary_mask to ULONG_MAX.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
---

Changelog:
v2->v3
 * Resent -- no change
   v2 was sent 4 months ago https://lkml.org/lkml/2020/4/6/881
   but it has not got replied yet
v1->v2
 * Followed Robin's comments to revise the commit message by
   dropping one paragraph of not-entirely-true justification
   (no git-diff level change, so please ack if you tested v1)

 include/linux/dma-mapping.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 016b96b384bd..27df499aa041 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -628,7 +628,7 @@ static inline unsigned long dma_get_seg_boundary(struct device *dev)
 {
 	if (dev->dma_parms && dev->dma_parms->segment_boundary_mask)
 		return dev->dma_parms->segment_boundary_mask;
-	return DMA_BIT_MASK(32);
+	return ULONG_MAX;
 }
 
 static inline int dma_set_seg_boundary(struct device *dev, unsigned long mask)
-- 
2.17.1

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Re: [PATCH v3] dma-mapping: set default segment_boundary_mask to ULONG_MAX

[Christoph Hellwig]

Applied to the dma-mapping tree.  This should give us the whole merge
window to root out any obvious problems with drivers.
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