Re: [PATCH v2 14/17] drm/xe: Support pcie p2p dma as a fast interconnect

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Nov 11, 2025 at 05:44:04PM +0100, Thomas Hellström wrote:
> Mimic the dma-buf method using dma_[map|unmap]_resource to map
> for pcie-p2p dma.
> 
> There's an ongoing area of work upstream to sort out how this best
> should be done. One method proposed is to add an additional
> pci_p2p_dma_pagemap aliasing the device_private pagemap and use
> the corresponding pci_p2p_dma_pagemap page as input for
> dma_map_page(). However, that would incur double the amount of
> memory and latency to set up the drm_pagemap and given the huge
> amount of memory present on modern GPUs, that would really not work.
> Hence the simple approach used in this patch.
> 
> v2:
> - Simplify xe_page_to_pcie(). (Matt Brost)
> 
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> ---
>  drivers/gpu/drm/xe/xe_svm.c | 34 +++++++++++++++++++++++++++++++---
>  drivers/gpu/drm/xe/xe_svm.h |  1 +
>  2 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
> index c0b17b548a00..86d7b0882b60 100644
> --- a/drivers/gpu/drm/xe/xe_svm.c
> +++ b/drivers/gpu/drm/xe/xe_svm.c
> @@ -3,6 +3,8 @@
>   * Copyright © 2024 Intel Corporation
>   */
>  
> +#include <linux/pci-p2pdma.h>
> +
>  #include <drm/drm_drv.h>
>  #include <drm/drm_managed.h>
>  #include <drm/drm_pagemap.h>
> @@ -441,6 +443,14 @@ static u64 xe_page_to_dpa(struct page *page)
>  	return dpa;
>  }
>  
> +static u64 xe_page_to_pcie(struct page *page)
> +{
> +	struct xe_pagemap *xpagemap = xe_page_to_pagemap(page);
> +	struct xe_vram_region *vr = xe_pagemap_to_vr(xpagemap);
> +
> +	return xe_page_to_dpa(page) - vr->dpa_base + vr->io_start;
> +}
> +
>  enum xe_svm_copy_dir {
>  	XE_SVM_COPY_TO_VRAM,
>  	XE_SVM_COPY_TO_SRAM,
> @@ -792,7 +802,10 @@ static bool xe_has_interconnect(struct drm_pagemap_peer *peer1,
>  	struct device *dev1 = xe_peer_to_dev(peer1);
>  	struct device *dev2 = xe_peer_to_dev(peer2);
>  
> -	return dev1 == dev2;
> +	if (dev1 == dev2)
> +		return true;
> +
> +	return pci_p2pdma_distance(to_pci_dev(dev1), dev2, true) >= 0;
>  }
>  
>  static DRM_PAGEMAP_OWNER_LIST_DEFINE(xe_owner_list);
> @@ -1552,13 +1565,27 @@ xe_drm_pagemap_device_map(struct drm_pagemap *dpagemap,
>  		addr = xe_page_to_dpa(page);
>  		prot = XE_INTERCONNECT_VRAM;
>  	} else {
> -		addr = DMA_MAPPING_ERROR;
> -		prot = 0;
> +		addr = dma_map_resource(dev,
> +					xe_page_to_pcie(page),
> +					PAGE_SIZE << order, dir,
> +					DMA_ATTR_SKIP_CPU_SYNC);
> +		prot = XE_INTERCONNECT_P2P;
>  	}
>  
>  	return drm_pagemap_addr_encode(addr, prot, order, dir);
>  }
>  
> +static void xe_drm_pagemap_device_unmap(struct drm_pagemap *dpagemap,
> +					struct device *dev,
> +					struct drm_pagemap_addr addr)
> +{
> +	if (addr.proto != XE_INTERCONNECT_P2P)
> +		return;
> +
> +	dma_unmap_resource(dev, addr.addr, PAGE_SIZE << addr.order,
> +			   addr.dir, DMA_ATTR_SKIP_CPU_SYNC);
> +}
> +
>  static void xe_pagemap_destroy_work(struct work_struct *work)
>  {
>  	struct xe_pagemap *xpagemap = container_of(work, typeof(*xpagemap), destroy_work);
> @@ -1595,6 +1622,7 @@ static void xe_pagemap_destroy(struct drm_pagemap *dpagemap, bool from_atomic_or
>  
>  static const struct drm_pagemap_ops xe_drm_pagemap_ops = {
>  	.device_map = xe_drm_pagemap_device_map,
> +	.device_unmap = xe_drm_pagemap_device_unmap,
>  	.populate_mm = xe_drm_pagemap_populate_mm,
>  	.destroy = xe_pagemap_destroy,
>  };
> diff --git a/drivers/gpu/drm/xe/xe_svm.h b/drivers/gpu/drm/xe/xe_svm.h
> index 60eae01a4220..64971c9b2a1a 100644
> --- a/drivers/gpu/drm/xe/xe_svm.h
> +++ b/drivers/gpu/drm/xe/xe_svm.h
> @@ -13,6 +13,7 @@
>  #include <drm/drm_pagemap_util.h>
>  
>  #define XE_INTERCONNECT_VRAM DRM_INTERCONNECT_DRIVER
> +#define XE_INTERCONNECT_P2P (XE_INTERCONNECT_VRAM + 1)
>  
>  struct drm_device;
>  struct drm_file;
> -- 
> 2.51.1
>