Re: [PATCH v4 3/4] drm/pagemap: Split drm_pagemap_migrate_map_pages into device / system

Thomas Hellström
2026-02-09T17:09:22Z

On Mon, 2026-02-09 at 08:58 -0800, Matthew Brost wrote:
> On Mon, Feb 09, 2026 at 04:49:03PM +0100, Thomas Hellström wrote:
> > On Wed, 2026-02-04 at 20:19 -0800, Matthew Brost wrote:
> > > Split drm_pagemap_migrate_map_pages into device / system helpers
> > > clearly
> > > seperating these operations. Will help with upcoming changes to
> > > split
> > > IOVA allocation steps.
> > >
> > > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > > ---
> > >  drivers/gpu/drm/drm_pagemap.c | 146 ++++++++++++++++++++++------
> > > ----
> > > --
> > >  1 file changed, 96 insertions(+), 50 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_pagemap.c
> > > b/drivers/gpu/drm/drm_pagemap.c
> > > index fbd69f383457..29677b19bb69 100644
> > > --- a/drivers/gpu/drm/drm_pagemap.c
> > > +++ b/drivers/gpu/drm/drm_pagemap.c
> > > @@ -205,7 +205,7 @@ static void
> > > drm_pagemap_get_devmem_page(struct
> > > page *page,
> > >  }
> > >
> > >  /**
> > > - * drm_pagemap_migrate_map_pages() - Map migration pages for GPU
> > > SVM
> > > migration
> > > + * drm_pagemap_migrate_map_device_pages() - Map device migration
> > > pages for GPU SVM migration
> > >   * @dev: The device performing the migration.
> > >   * @local_dpagemap: The drm_pagemap local to the migrating
> > > device.
> > >   * @pagemap_addr: Array to store DMA information corresponding
> > > to
> > > mapped pages.
> > > @@ -221,19 +221,22 @@ static void
> > > drm_pagemap_get_devmem_page(struct
> > > page *page,
> > >   *
> > >   * Returns: 0 on success, -EFAULT if an error occurs during
> > > mapping.
> > >   */
> > > -static int drm_pagemap_migrate_map_pages(struct device *dev,
> > > - struct drm_pagemap
> > > *local_dpagemap,
> > > - struct drm_pagemap_addr
> > > *pagemap_addr,
> > > - unsigned long
> > > *migrate_pfn,
> > > - unsigned long npages,
> > > - enum dma_data_direction
> > > dir,
> > > - const struct
> > > drm_pagemap_migrate_details *mdetails)
> > > +static int
> > > +drm_pagemap_migrate_map_device_pages(struct device *dev,
> > > +      struct drm_pagemap
> > > *local_dpagemap,
> > > +      struct drm_pagemap_addr
> > > *pagemap_addr,
> > > +      unsigned long *migrate_pfn,
> > > +      unsigned long npages,
> > > +      enum dma_data_direction
> > > dir,
> > > +      const struct
> > > drm_pagemap_migrate_details *mdetails)
> >
> > We might want to call this device_private pages. Device coherent
> > pages
> > are treated like system pages here, but I figure those are known to
> > the
> > dma subsystem and can be handled by the map_system_pages callback.
> >
>
> Yes.
>
> Eventually we will have figure out we'd want to handle Device
> coherent
> pages with a high speed fabric though.
>
> > >  {
> > >   unsigned long num_peer_pages = 0, num_local_pages = 0,
> > > i;
> > >
> > >   for (i = 0; i < npages;) {
> > >   struct page *page =
> > > migrate_pfn_to_page(migrate_pfn[i]);
> > > - dma_addr_t dma_addr;
> > > + struct drm_pagemap_zdd *zdd;
> > > + struct drm_pagemap *dpagemap;
> > > + struct drm_pagemap_addr addr;
> > >   struct folio *folio;
> > >   unsigned int order = 0;
> > >
> > > @@ -243,36 +246,26 @@ static int
> > > drm_pagemap_migrate_map_pages(struct
> > > device *dev,
> > >   folio = page_folio(page);
> > >   order = folio_order(folio);
> > >
> > > - if (is_device_private_page(page)) {
> > > - struct drm_pagemap_zdd *zdd =
> > > drm_pagemap_page_zone_device_data(page);
> > > - struct drm_pagemap *dpagemap = zdd-
> > > > dpagemap;
> > > - struct drm_pagemap_addr addr;
> > > -
> > > - if (dpagemap == local_dpagemap) {
> > > - if (!mdetails-
> > > > can_migrate_same_pagemap)
> > > - goto next;
> > > + WARN_ON_ONCE(!is_device_private_page(page));
> > >
> > > - num_local_pages +=
> > > NR_PAGES(order);
> > > - } else {
> > > - num_peer_pages +=
> > > NR_PAGES(order);
> > > - }
> > > + zdd = drm_pagemap_page_zone_device_data(page);
> > > + dpagemap = zdd->dpagemap;
> > >
> > > - addr = dpagemap->ops-
> > > >device_map(dpagemap,
> > > dev, page, order, dir);
> > > - if (dma_mapping_error(dev, addr.addr))
> > > - return -EFAULT;
> > > + if (dpagemap == local_dpagemap) {
> > > + if (!mdetails->can_migrate_same_pagemap)
> > > + goto next;
> > >
> > > - pagemap_addr[i] = addr;
> > > + num_local_pages += NR_PAGES(order);
> > >   } 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,
> > > -
> > > DRM_INTE
> > > RCONNECT_SYSTEM,
> > > - order,
> > > dir);
> > > + num_peer_pages += NR_PAGES(order);
> > >   }
> > >
> > > + addr = dpagemap->ops->device_map(dpagemap, dev,
> > > page, order, dir);
> > > + if (dma_mapping_error(dev, addr.addr))
> > > + return -EFAULT;
> > > +
> > > + pagemap_addr[i] = addr;
> > > +
> > >  next:
> > >   i += NR_PAGES(order);
> > >   }
> > > @@ -287,6 +280,59 @@ static int
> > > drm_pagemap_migrate_map_pages(struct
> > > device *dev,
> > >   return 0;
> > >  }
> > >
> > > +/**
> > > + * drm_pagemap_migrate_map_system_pages() - Map system migration
> > > pages for GPU SVM migration
> > > + * @dev: The device performing the migration.
> > > + * @pagemap_addr: Array to store DMA information corresponding
> > > to
> > > mapped pages.
> > > + * @migrate_pfn: Array of page frame numbers of system pages or
> > > peer
> > > pages to map.
> >
> > system pages or device coherent pages? "Peer" pages would typically
> > be
> > device-private pages with the same owner.
> >
> > > + * @npages: Number of system pages or peer pages to map.
> >
> > Same here.
>
> Yes, copy paste error.
>
> >
> > > + * @dir: Direction of data transfer (e.g., DMA_BIDIRECTIONAL)
> > > + *
> > > + * 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
> > > + * corresponding page, and stores the DMA address in the
> > > provided
> > > @dma_addr
> > > + * array.
> > > + *
> > > + * Returns: 0 on success, -EFAULT if an error occurs during
> > > mapping.
> > > + */
> > > +static int
> > > +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)
> > > +{
> > > + unsigned long i;
> > > +
> > > + for (i = 0; i < npages;) {
> > > + struct page *page =
> > > migrate_pfn_to_page(migrate_pfn[i]);
> > > + dma_addr_t dma_addr;
> > > + struct folio *folio;
> > > + unsigned int order = 0;
> > > +
> > > + if (!page)
> > > + goto next;
> > > +
> > > + WARN_ON_ONCE(is_device_private_page(page));
> > > + 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;
> > > +
> > > + pagemap_addr[i] =
> > > + drm_pagemap_addr_encode(dma_addr,
> > > + DRM_INTERCONNECT
> > > _SYS
> > > TEM,
> > > + order, dir);
> > > +
> > > +next:
> > > + i += NR_PAGES(order);
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +
> > >  /**
> > >   * drm_pagemap_migrate_unmap_pages() - Unmap pages previously
> > > mapped
> > > for GPU SVM migration
> > >   * @dev: The device for which the pages were mapped
> > > @@ -347,9 +393,11 @@ drm_pagemap_migrate_remote_to_local(struct
> > > drm_pagemap_devmem *devmem,
> > >       const struct
> > > drm_pagemap_migrate_details *mdetails)
> > >
> > >  {
> > > - int err = drm_pagemap_migrate_map_pages(remote_device,
> > > remote_dpagemap,
> > > - pagemap_addr,
> > > local_pfns,
> > > - npages,
> > > DMA_FROM_DEVICE, mdetails);
> > > + int err =
> > > drm_pagemap_migrate_map_device_pages(remote_device,
> > > +
> > > remote_dpagemap,
> > > +
> > > pagemap_addr,
> > > local_pfns,
> > > +        npages,
> > > DMA_FROM_DEVICE,
> > > +
> > > mdetails);
> > >
> > >   if (err)
> > >   goto out;
> > > @@ -368,12 +416,11 @@ 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_migrate_details *mdetails)
> > > +        const struct
> > > drm_pagemap_devmem_ops
> > > *ops)
> > >  {
> > > - int err = drm_pagemap_migrate_map_pages(devmem->dev,
> > > devmem-
> > > > dpagemap,
> > > - pagemap_addr,
> > > sys_pfns, npages,
> > > - DMA_TO_DEVICE,
> > > mdetails);
> > > + int err = drm_pagemap_migrate_map_system_pages(devmem-
> > > >dev,
> > > +
> > > pagemap_addr,
> > > sys_pfns,
> > > +        npages,
> > > DMA_TO_DEVICE);
> >
> >
> > Unfortunately it's a bit more complicated than this. If the
> > destination
> > gpu migrates, the range to migrate could be a mix of system pages,
> > device coherent pages and also device private pages, and previously
> > drm_pagemap_migrate_map_pages() took care of that and did the
> > correct
> > thing on a per-page basis.
> >
> > You can exercise this by setting mdetails::source_peer_migrates to
> > false on xe. That typically "works" but might generate some errors
> > in
> > the atomic multi-device tests AFAICT because reading from the BAR
> > does
> > not flush the L2 caches on BMG. But should be sufficient to
> > exercise
> > this path.
>
> Ah, yes I see I missed this - this patch isn't strickly required I
> just
> didn't want drm_pagemap_migrate_map_pages to have massive cascading
> if
> statements... I can remove for now if that is preferred or should be
> just remove source_peer_migrates and assume a value of '1'.
>
> I suggest the later because looking forward source_peer_migrates == 0
> would bb difficult to support a high speed fabric, which requires a
> IOVA
> (think UAL with virtual NAs at the target device), plus multiple
> different devices being found in the migration pages. Also, with p2p,
> isn't source_peer_migrates == '1' (write over p2p) faster than
> source_peer_migrates == '0' (read over p2p)?

With source_peer_migrates == 0 we have the drawbacks of missed cache
flushing, can't be used with CCS compression, high speed fabric and
also the speed over PCIe as you say, so I don't see xe using it in the
near-term.

So I agree. Let's just let's assume source_peer_migrates == 1 for now.

Thanks,
Thomas

>
> Matt
>
> >
> > /Thomas