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

[Konrad Rzeszutek Wilk <konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On Thu, Sep 17, 2015 at 02:22:38PM -0400, jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org wrote:
> From: Jérôme Glisse <jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> 
> 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?

> 
> Signed-off-by: Jérôme Glisse <jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> To: iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
> Cc: Joerg Roedel <jroedel-l3A5Bk7waGM@public.gmane.org>
> Cc: Konrad Rzeszutek Wilk <Konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> Cc: Alex Deucher <alexander.deucher-5C7GfCeVMHo@public.gmane.org>
> CC: Dave Airlie <airlied-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> CC: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> ---
>  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
>