Re: [PATCH 2/7 v2] powerpc/dma-mapping: override dma_get_page_shift

From: Alexey Kardashevskiy
Date: Tue Oct 27 2015 - 02:02:29 EST

On 10/24/2015 07:57 AM, Nishanth Aravamudan wrote:
On Power, the kernel's page size can differ from the IOMMU's page size,
so we need to override the generic implementation, which always returns
the kernel's page size. Lookup the IOMMU's page size from struct
iommu_table, if available. Fallback to the kernel's page size,

Signed-off-by: Nishanth Aravamudan <nacc@xxxxxxxxxxxxxxxxxx>
arch/powerpc/include/asm/dma-mapping.h | 3 +++
arch/powerpc/kernel/dma.c | 9 +++++++++
2 files changed, 12 insertions(+)

diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h
index 7f522c0..c5638f4 100644
--- a/arch/powerpc/include/asm/dma-mapping.h
+++ b/arch/powerpc/include/asm/dma-mapping.h
@@ -125,6 +125,9 @@ static inline void set_dma_offset(struct device *dev, dma_addr_t off)
extern int dma_set_mask(struct device *dev, u64 dma_mask);

+extern unsigned long dma_get_page_shift(struct device *dev);
#include <asm-generic/dma-mapping-common.h>

extern int __dma_set_mask(struct device *dev, u64 dma_mask);
diff --git a/arch/powerpc/kernel/dma.c b/arch/powerpc/kernel/dma.c
index 59503ed..e805af2 100644
--- a/arch/powerpc/kernel/dma.c
+++ b/arch/powerpc/kernel/dma.c
@@ -335,6 +335,15 @@ int dma_set_mask(struct device *dev, u64 dma_mask)

+unsigned long dma_get_page_shift(struct device *dev)
+ struct iommu_table *tbl = get_iommu_table_base(dev);
+ if (tbl)
+ return tbl->it_page_shift;

All PCI devices have this initialized on POWER (at least, our, IBM's POWER) so 4K will always be returned here while in the case of (get_dma_ops(dev)==&dma_direct_ops) it could actually return PAGE_SHIFT. Is 4K still preferred value to return here?

+ return PAGE_SHIFT;
u64 __dma_get_required_mask(struct device *dev)
struct dma_map_ops *dma_ops = get_dma_ops(dev);

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