Re: [RFC PATCH] dma/swiotlb: Add helper for device driver to opt-out from swiotlb.

[Konrad Rzeszutek Wilk]

On Thu, Sep 17, 2015 at 03:02:51PM -0400, Konrad Rzeszutek Wilk wrote:
> On Thu, Sep 17, 2015 at 02:22:38PM -0400, jglisse@xxxxxxxxxx wrote:
> > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > 
> > The swiotlb dma backend is not appropriate for some devices like
> > GPU where bounce buffer or slow dma page allocations is just not
> > acceptable. With that helper device drivers can opt-out from the
> > swiotlb and just do sane things without wasting CPU cycles inside
> > the swiotlb code.
> 
> What if SWIOTLB is the only one available?
> 
> And what can't the devices use the TTM DMA backend which sets up
> buffers which don't need bounce buffer or slow dma page allocations?

And then the followup question. If it opts out - how can it do
sane things without an DMA API available? It would assume physical
addresses match the bus addresses which is not always the sane
thing.
> 
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > To: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > Cc: Joerg Roedel <jroedel@xxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <Konrad.wilk@xxxxxxxxxx>
> > Cc: Alex Deucher <alexander.deucher@xxxxxxx>
> > CC: Dave Airlie <airlied@xxxxxxxxxx>
> > CC: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >  arch/x86/include/asm/dma-mapping.h       |  3 +++
> >  arch/x86/kernel/pci-swiotlb.c            | 18 ++++++++++++++++++
> >  include/asm-generic/dma-mapping-common.h |  7 +++++++
> >  3 files changed, 28 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/dma-mapping.h b/arch/x86/include/asm/dma-mapping.h
> > index 953b726..b50745f 100644
> > --- a/arch/x86/include/asm/dma-mapping.h
> > +++ b/arch/x86/include/asm/dma-mapping.h
> > @@ -46,6 +46,9 @@ bool arch_dma_alloc_attrs(struct device **dev, gfp_t *gfp);
> >  #define HAVE_ARCH_DMA_SUPPORTED 1
> >  extern int dma_supported(struct device *hwdev, u64 mask);
> >  
> > +#define HAVE_ARCH_DMA_OVERRIDE_SWIOTLB 1
> > +int dma_override_swiotlb(struct device *dev);
> > +
> >  #include <asm-generic/dma-mapping-common.h>
> >  
> >  extern void *dma_generic_alloc_coherent(struct device *dev, size_t size,
> > diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> > index adf0392..6a9efab 100644
> > --- a/arch/x86/kernel/pci-swiotlb.c
> > +++ b/arch/x86/kernel/pci-swiotlb.c
> > @@ -117,3 +117,21 @@ void __init pci_swiotlb_late_init(void)
> >  		swiotlb_print_info();
> >  	}
> >  }
> > +
> > +/* dma_override_swiotlb() -  Override swiotlb with nommu.
> > + *
> > + * @device: Device for which to disable swiotlb.
> > + *
> > + * The swiotlb infrastructure just get in the way for some devices like GPU,
> > + * where things like bounce pages can not work properly or for which we do not
> > + * want to take slow page allocation code path. This function allows device
> > + * driver opportunity to opt-out from swiotlb.
> > + */
> > +int dma_override_swiotlb(struct device *dev)
> > +{
> > +	if (dev->archdata.dma_ops != &swiotlb_dma_ops)
> > +		return 1;
> > +	dev->archdata.dma_ops = &nommu_dma_ops;
> > +	return 1;
> > +}
> > +EXPORT_SYMBOL(dma_override_swiotlb);
> > diff --git a/include/asm-generic/dma-mapping-common.h b/include/asm-generic/dma-mapping-common.h
> > index b1bc954..452d947 100644
> > --- a/include/asm-generic/dma-mapping-common.h
> > +++ b/include/asm-generic/dma-mapping-common.h
> > @@ -355,4 +355,11 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
> >  }
> >  #endif
> >  
> > +#ifndef HAVE_ARCH_DMA_OVERRIDE_SWIOTLB
> > +static inline int dma_override_swiotlb(struct device *dev)
> > +{
> > +	return 0;
> > +}
> > +#endif
> > +
> >  #endif
> > -- 
> > 2.1.0
> > 
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