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

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

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_INTERCONNECT_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)?

Matt

> 
> /Thomas