On Sat, Oct 25, 2025 at 02:04:10PM +0200, 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.
>
> Signed-off-by: Thomas Hellström <[email protected]>
> ---
> drivers/gpu/drm/xe/xe_svm.c | 44 ++++++++++++++++++++++++++++++++++---
> drivers/gpu/drm/xe/xe_svm.h | 1 +
> 2 files changed, 42 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
> index 9dd96dad2cca..9814f95cb212 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>
> @@ -442,6 +444,24 @@ static u64 xe_page_to_dpa(struct page *page)
> return dpa;
> }
>
> +static u64 xe_page_to_pcie(struct page *page)
> +{
This function looks almost exactly the same as xe_page_to_dpa, maybe
extract out the common parts?
Everything else LGTM.
Matt
> + struct xe_pagemap *xpagemap = xe_page_to_pagemap(page);
> + struct xe_vram_region *vr = xe_pagemap_to_vr(xpagemap);
> + u64 hpa_base = xpagemap->hpa_base;
> + u64 ioaddr;
> + u64 pfn = page_to_pfn(page);
> + u64 offset;
> +
> + xe_assert(vr->xe, is_device_private_page(page));
> + xe_assert(vr->xe, (pfn << PAGE_SHIFT) >= hpa_base);
> +
> + offset = (pfn << PAGE_SHIFT) - hpa_base;
> + ioaddr = vr->io_start + offset;
> +
> + return ioaddr;
> +}
> +
> enum xe_svm_copy_dir {
> XE_SVM_COPY_TO_VRAM,
> XE_SVM_COPY_TO_SRAM,
> @@ -793,7 +813,10 @@ 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);
> @@ -1530,13 +1553,27 @@ 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);
> @@ -1573,6 +1610,7 @@ 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 7cd7932f56c8..f5ed48993b6d 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;
> --
> 2.51.0
>
