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

[Alexey Kardashevskiy]

On 10/28/2015 09:27 AM, Nishanth Aravamudan wrote:
> 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@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)
> > >   #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.

If it is not available, then something went wrong and BUG_ON(!tbl || 
!tbl->it_page_shift) make more sense here than pretending that this 
function can ever return PAGE_SHIFT. imho.


> 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?

This is for pseries guests; for the powernv host it is a "bypass" mode 
which does 64bit direct DMA mapping and there is no additional window for 
that (i.e. DIRECT64_PROPNAME, etc).



--
Alexey
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