Re: large DMA segments vs SWIOTLB

[Christoph Hellwig <hch@lst.de>]

On Thu, Aug 01, 2019 at 10:35:02AM +0200, Lucas Stach wrote:
> Hi Christoph,
> 
> Am Donnerstag, den 01.08.2019, 09:29 +0200 schrieb Christoph Hellwig:
> > Hi Lukas,
> > 
> > have you tried the latest 5.3-rc kernel, where we limited the NVMe
> > I/O size based on the swiotlb buffer size?
> 
> Yes, the issue was reproduced on 5.3-rc2. I now see your commit
> limiting the request size, so I guess I need to dig in to see why I'm
> still getting requests larger than the SWIOTLB max segment size. Thanks
> for the pointer!

a similar setup to yours the
dma_addressing_limited doesn't work, but if we changed it to a <=
it does.  The result is counter to what I'd expect, but because I'm on
vacation I didn't have time to look into why it works.  This is his
patch, let me know if this works for you:


diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index f7d1eea32c78..89ac1cf754cc 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -689,7 +689,7 @@ static inline int dma_coerce_mask_and_coherent(struct device *dev, u64 mask)
  */
 static inline bool dma_addressing_limited(struct device *dev)
 {
-	return min_not_zero(dma_get_mask(dev), dev->bus_dma_mask) <
+	return min_not_zero(dma_get_mask(dev), dev->bus_dma_mask) <=
 			    dma_get_required_mask(dev);
 }
 
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