[PATCH 1/1] powerpc/pseries/iommu: Fix window size for direct mapping with pmem

As of today, if the DDW is big enough to fit (1 << MAX_PHYSMEM_BITS) it's
possible to use direct DMA mapping even with pmem region.

But, if that happens, the window size (len) is set to
(MAX_PHYSMEM_BITS - page_shift) instead of MAX_PHYSMEM_BITS, causing a
pagesize times smaller DDW to be created, being insufficient for correct
usage.

Fix this so the correct window size is used in this case.

Fixes: bf6e2d562bbc4("powerpc/dma: Fallback to dma_ops when persistent memory present")
Signed-off-by: Leonardo Bras <redacted>
---
 arch/powerpc/platforms/pseries/iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
index 9fc5217f0c8e..836cbbe0ecc5 100644
--- a/arch/powerpc/platforms/pseries/iommu.c
+++ b/arch/powerpc/platforms/pseries/iommu.c

@@ -1229,7 +1229,7 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
 	if (pmem_present) {
 		if (query.largest_available_block >=
 		    (1ULL << (MAX_PHYSMEM_BITS - page_shift)))
-			len = MAX_PHYSMEM_BITS - page_shift;
+			len = MAX_PHYSMEM_BITS;
 		else
 			dev_info(&dev->dev, "Skipping ibm,pmemory");
 	}

-- 
2.30.2

On 20/04/2021 14:54, Leonardo Bras wrote:

As of today, if the DDW is big enough to fit (1 << MAX_PHYSMEM_BITS) it's
possible to use direct DMA mapping even with pmem region.

But, if that happens, the window size (len) is set to
(MAX_PHYSMEM_BITS - page_shift) instead of MAX_PHYSMEM_BITS, causing a
pagesize times smaller DDW to be created, being insufficient for correct
usage.

Fix this so the correct window size is used in this case.

Good find indeed.

afaict this does not create a huge problem though as 
query.largest_available_block is always smaller than (MAX_PHYSMEM_BITS - 
page_shift) where it matters (phyp).


Reviewed-by: Alexey Kardashevskiy <redacted>

-- 
Alexey

On Tue, 2021-04-20 at 15:18 +1000, Alexey Kardashevskiy wrote:

On 20/04/2021 14:54, Leonardo Bras wrote:

quoted

As of today, if the DDW is big enough to fit (1 << MAX_PHYSMEM_BITS) it's
possible to use direct DMA mapping even with pmem region.

But, if that happens, the window size (len) is set to
(MAX_PHYSMEM_BITS - page_shift) instead of MAX_PHYSMEM_BITS, causing a
pagesize times smaller DDW to be created, being insufficient for correct
usage.

Fix this so the correct window size is used in this case.

Good find indeed.

afaict this does not create a huge problem though as 
query.largest_available_block is always smaller than (MAX_PHYSMEM_BITS - 
page_shift) where it matters (phyp).


Reviewed-by: Alexey Kardashevskiy <redacted>

Thanks for reviewing!

Leonardo Bras

On Tue, 20 Apr 2021 01:54:04 -0300, Leonardo Bras wrote:

As of today, if the DDW is big enough to fit (1 << MAX_PHYSMEM_BITS) it's
possible to use direct DMA mapping even with pmem region.

But, if that happens, the window size (len) is set to
(MAX_PHYSMEM_BITS - page_shift) instead of MAX_PHYSMEM_BITS, causing a
pagesize times smaller DDW to be created, being insufficient for correct
usage.

[...]

Applied to powerpc/next.

[1/1] powerpc/pseries/iommu: Fix window size for direct mapping with pmem
      https://git.kernel.org/powerpc/c/a9d2f9bb225fd2a764aef57738ab6c7f38d782ae

cheers