Re: [PATCH v4 4/4] drm/pagemap: Use dma-map IOVA alloc, link, and sync API for DRM pagemap

Matthew Brost
2026-02-11T15:37:12Z

On Wed, Feb 11, 2026 at 12:34:12PM +0100, Thomas Hellström wrote:
> On Wed, 2026-02-04 at 20:19 -0800, Matthew Brost wrote:
> > The dma-map IOVA alloc, link, and sync APIs perform significantly
> > better
> > than dma-map / dma-unmap, as they avoid costly IOMMU
> > synchronizations.
> > This difference is especially noticeable when mapping a 2MB region in
> > 4KB pages.
> >
> > Use the IOVA alloc, link, and sync APIs for DRM pagemap, which create
> > DMA
> > mappings between the CPU and GPU for copying data.
> >
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> > v4:
> >  - Pack IOVA and drop dummy page (Jason)
> >
> >  drivers/gpu/drm/drm_pagemap.c | 84 +++++++++++++++++++++++++++++----
> > --
> >  1 file changed, 70 insertions(+), 14 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_pagemap.c
> > b/drivers/gpu/drm/drm_pagemap.c
> > index 29677b19bb69..52a196bc8459 100644
> > --- a/drivers/gpu/drm/drm_pagemap.c
> > +++ b/drivers/gpu/drm/drm_pagemap.c
> > @@ -280,6 +280,20 @@ drm_pagemap_migrate_map_device_pages(struct
> > device *dev,
> >   return 0;
> >  }
> >
> > +/**
> > + * struct drm_pagemap_iova_state - DRM pagemap IOVA state
> > + *
>
> No newline
>

+1

> > + * @dma_state: DMA IOVA state.
> > + * @offset: Current offset in IOVA.
> > + *
> > + * This structure acts as an iterator for packing all IOVA addresses
> > within a
> > + * contiguous range.
> > + */
> > +struct drm_pagemap_iova_state {
> > + struct dma_iova_state dma_state;
> > + unsigned long offset;
> > +};
> > +
> >  /**
> >   * drm_pagemap_migrate_map_system_pages() - Map system migration
> > pages for GPU SVM migration
> >   * @dev: The device performing the migration.
> > @@ -287,6 +301,7 @@ drm_pagemap_migrate_map_device_pages(struct
> > device *dev,
> >   * @migrate_pfn: Array of page frame numbers of system pages or peer
> > pages to map.
> >   * @npages: Number of system pages or peer pages to map.
> >   * @dir: Direction of data transfer (e.g., DMA_BIDIRECTIONAL)
> > + * @state: DMA IOVA state for mapping.
> >   *
> >   * This function maps pages of memory for migration usage in GPU
> > SVM. It
> >   * iterates over each page frame number provided in @migrate_pfn,
> > maps the
> > @@ -300,9 +315,11 @@ drm_pagemap_migrate_map_system_pages(struct
> > device *dev,
> >        struct drm_pagemap_addr
> > *pagemap_addr,
> >        unsigned long *migrate_pfn,
> >        unsigned long npages,
> > -      enum dma_data_direction dir)
> > +      enum dma_data_direction dir,
> > +      struct drm_pagemap_iova_state
> > *state)
> >  {
> >   unsigned long i;
> > + bool try_alloc = false;
> >
> >   for (i = 0; i < npages;) {
> >   struct page *page =
> > migrate_pfn_to_page(migrate_pfn[i]);
> > @@ -317,9 +334,31 @@ drm_pagemap_migrate_map_system_pages(struct
> > device *dev,
> >   folio = page_folio(page);
> >   order = folio_order(folio);
> >
> > - dma_addr = dma_map_page(dev, page, 0,
> > page_size(page), dir);
> > - if (dma_mapping_error(dev, dma_addr))
> > - return -EFAULT;
> > + if (!try_alloc) {
> > + dma_iova_try_alloc(dev, &state->dma_state,
> > +    npages * PAGE_SIZE >=
> > +    HPAGE_PMD_SIZE ?
> > +    HPAGE_PMD_SIZE : 0,
> > +    npages * PAGE_SIZE);
> > + try_alloc = true;
> > + }
>
> What happens if dma_iova_try_alloc() fails for all i < some value x and
> then suddenly succeeds for i == x? While the below code looks correct,

We only try to alloc on the first valid page - 'i' may be any value
based on the first page found or we may never alloc if the number of
pages found == 0 (possible, hence why it is inside the loop). This step
is done at most once. If the allocation fails, we use the map_page path
for the remaining loop iterations.

> I figure we'd allocate a too large IOVA region and possibly get the
> alignment wrong?

The first and only IOVA allocation attempts an aligned allocation. What
can happen is only a subset of the IOVA is used for the copy but we pack
in the pages starting at IOVA[0] and end at IOVA[number valid pages - 1].

Matt

>
> Otherwise LGTM.
>
>
> > +
> > + if (dma_use_iova(&state->dma_state)) {
> > + int err = dma_iova_link(dev, &state-
> > >dma_state,
> > + page_to_phys(page),
> > + state->offset,
> > page_size(page),
> > + dir, 0);
> > + if (err)
> > + return err;
> > +
> > + dma_addr = state->dma_state.addr + state-
> > >offset;
> > + state->offset += page_size(page);
> > + } else {
> > + dma_addr = dma_map_page(dev, page, 0,
> > page_size(page),
> > + dir);
> > + if (dma_mapping_error(dev, dma_addr))
> > + return -EFAULT;
> > + }
> >
> >   pagemap_addr[i] =
> >   drm_pagemap_addr_encode(dma_addr,
> > @@ -330,6 +369,9 @@ drm_pagemap_migrate_map_system_pages(struct
> > device *dev,
> >   i += NR_PAGES(order);
> >   }
> >
> > + if (dma_use_iova(&state->dma_state))
> > + return dma_iova_sync(dev, &state->dma_state, 0,
> > state->offset);
> > +
> >   return 0;
> >  }
> >
> > @@ -341,6 +383,7 @@ drm_pagemap_migrate_map_system_pages(struct
> > device *dev,
> >   * @pagemap_addr: Array of DMA information corresponding to mapped
> > pages
> >   * @npages: Number of pages to unmap
> >   * @dir: Direction of data transfer (e.g., DMA_BIDIRECTIONAL)
> > + * @state: DMA IOVA state for mapping.
> >   *
> >   * This function unmaps previously mapped pages of memory for GPU
> > Shared Virtual
> >   * Memory (SVM). It iterates over each DMA address provided in
> > @dma_addr, checks
> > @@ -350,10 +393,17 @@ static void
> > drm_pagemap_migrate_unmap_pages(struct device *dev,
> >       struct drm_pagemap_addr
> > *pagemap_addr,
> >       unsigned long
> > *migrate_pfn,
> >       unsigned long npages,
> > -     enum dma_data_direction
> > dir)
> > +     enum dma_data_direction
> > dir,
> > +     struct
> > drm_pagemap_iova_state *state)
> >  {
> >   unsigned long i;
> >
> > + if (state && dma_use_iova(&state->dma_state)) {
> > + dma_iova_unlink(dev, &state->dma_state, 0, state-
> > >offset, dir, 0);
> > + dma_iova_free(dev, &state->dma_state);
> > + return;
> > + }
> > +
> >   for (i = 0; i < npages;) {
> >   struct page *page =
> > migrate_pfn_to_page(migrate_pfn[i]);
> >
> > @@ -406,7 +456,7 @@ drm_pagemap_migrate_remote_to_local(struct
> > drm_pagemap_devmem *devmem,
> >          devmem->pre_migrate_fence);
> >  out:
> >   drm_pagemap_migrate_unmap_pages(remote_device, pagemap_addr,
> > local_pfns,
> > - npages, DMA_FROM_DEVICE);
> > + npages, DMA_FROM_DEVICE,
> > NULL);
> >   return err;
> >  }
> >
> > @@ -416,11 +466,13 @@ drm_pagemap_migrate_sys_to_dev(struct
> > drm_pagemap_devmem *devmem,
> >          struct page *local_pages[],
> >          struct drm_pagemap_addr
> > pagemap_addr[],
> >          unsigned long npages,
> > -        const struct drm_pagemap_devmem_ops
> > *ops)
> > +        const struct drm_pagemap_devmem_ops
> > *ops,
> > +        struct drm_pagemap_iova_state *state)
> >  {
> >   int err = drm_pagemap_migrate_map_system_pages(devmem->dev,
> >          pagemap_addr,
> > sys_pfns,
> > -        npages,
> > DMA_TO_DEVICE);
> > +        npages,
> > DMA_TO_DEVICE,
> > +        state);
> >
> >   if (err)
> >   goto out;
> > @@ -429,7 +481,7 @@ drm_pagemap_migrate_sys_to_dev(struct
> > drm_pagemap_devmem *devmem,
> >     devmem->pre_migrate_fence);
> >  out:
> >   drm_pagemap_migrate_unmap_pages(devmem->dev, pagemap_addr,
> > sys_pfns, npages,
> > - DMA_TO_DEVICE);
> > + DMA_TO_DEVICE, state);
> >   return err;
> >  }
> >
> > @@ -457,6 +509,7 @@ static int drm_pagemap_migrate_range(struct
> > drm_pagemap_devmem *devmem,
> >        const struct migrate_range_loc
> > *cur,
> >        const struct
> > drm_pagemap_migrate_details *mdetails)
> >  {
> > + struct drm_pagemap_iova_state state = {};
> >   int ret = 0;
> >
> >   if (cur->start == 0)
> > @@ -484,7 +537,7 @@ static int drm_pagemap_migrate_range(struct
> > drm_pagemap_devmem *devmem,
> >        &pages[last-
> > >start],
> >
> > &pagemap_addr[last->start],
> >        cur->start -
> > last->start,
> > -      last->ops);
> > +      last->ops,
> > &state);
> >
> >  out:
> >   *last = *cur;
> > @@ -1001,6 +1054,7 @@ EXPORT_SYMBOL(drm_pagemap_put);
> >  int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem
> > *devmem_allocation)
> >  {
> >   const struct drm_pagemap_devmem_ops *ops =
> > devmem_allocation->ops;
> > + struct drm_pagemap_iova_state state = {};
> >   unsigned long npages, mpages = 0;
> >   struct page **pages;
> >   unsigned long *src, *dst;
> > @@ -1042,7 +1096,7 @@ int drm_pagemap_evict_to_ram(struct
> > drm_pagemap_devmem *devmem_allocation)
> >   err =
> > drm_pagemap_migrate_map_system_pages(devmem_allocation->dev,
> >      pagemap_addr,
> >      dst, npages,
> > -    DMA_FROM_DEVICE);
> > +    DMA_FROM_DEVICE,
> > &state);
> >   if (err)
> >   goto err_finalize;
> >
> > @@ -1059,7 +1113,7 @@ int drm_pagemap_evict_to_ram(struct
> > drm_pagemap_devmem *devmem_allocation)
> >   migrate_device_pages(src, dst, npages);
> >   migrate_device_finalize(src, dst, npages);
> >   drm_pagemap_migrate_unmap_pages(devmem_allocation->dev,
> > pagemap_addr, dst, npages,
> > - DMA_FROM_DEVICE);
> > + DMA_FROM_DEVICE, &state);
> >
> >  err_free:
> >   kvfree(buf);
> > @@ -1103,6 +1157,7 @@ static int __drm_pagemap_migrate_to_ram(struct
> > vm_area_struct *vas,
> >   MIGRATE_VMA_SELECT_DEVICE_COHERENT,
> >   .fault_page = page,
> >   };
> > + struct drm_pagemap_iova_state state = {};
> >   struct drm_pagemap_zdd *zdd;
> >   const struct drm_pagemap_devmem_ops *ops;
> >   struct device *dev = NULL;
> > @@ -1162,7 +1217,7 @@ static int __drm_pagemap_migrate_to_ram(struct
> > vm_area_struct *vas,
> >
> >   err = drm_pagemap_migrate_map_system_pages(dev,
> > pagemap_addr,
> >      migrate.dst,
> > npages,
> > -    DMA_FROM_DEVICE);
> > +    DMA_FROM_DEVICE,
> > &state);
> >   if (err)
> >   goto err_finalize;
> >
> > @@ -1180,7 +1235,8 @@ static int __drm_pagemap_migrate_to_ram(struct
> > vm_area_struct *vas,
> >   migrate_vma_finalize(&migrate);
> >   if (dev)
> >   drm_pagemap_migrate_unmap_pages(dev, pagemap_addr,
> > migrate.dst,
> > - npages,
> > DMA_FROM_DEVICE);
> > + npages,
> > DMA_FROM_DEVICE,
> > + &state);
> >  err_free:
> >   kvfree(buf);