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

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 27.10.2015 [17:02:16 +1100], Alexey Kardashevskiy wrote:
> 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,
> >otherwise.
> >
> >Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
> >---
> >  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)
> >  #define HAVE_ARCH_DMA_SET_MASK 1
> >  extern int dma_set_mask(struct device *dev, u64 dma_mask);
> >
> >+#define HAVE_ARCH_DMA_GET_PAGE_SHIFT 1
> >+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)
> >  }
> >  EXPORT_SYMBOL(dma_set_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?

Right, so the logic of my series, goes like this:

a) We currently are assuming DMA_PAGE_SHIFT (conceptual constant) is
PAGE_SHIFT everywhere, including Power.

b) After 2/7, the Power code will return either the IOMMU table's shift
value, if set, or PAGE_SHIFT (I guess this would be the case if
get_dma_ops(dev) == &dma_direct_ops, as you said). That is no different
than we have now, except we can return the accurate IOMMU value if
available.

3) After 3/7, the platform can override the generic Power
get_dma_page_shift().

4) After 4/7, pseries will return the DDW value, if available, then
fallback to the IOMMU table's value. I think in the case of
get_dma_ops(dev)==&dma_direct_ops, the only way that can happen is if we
are using DDW, right?

-Nish