Re: [PATCH v2 3/6] drm/pagemap: DMA map folios when possible

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

On Fri, Jul 25, 2025 at 05:39:27PM +0200, Francois Dugast wrote:
> If the page is part of a folio, DMA map the whole folio at once instead of
> mapping individual pages one after the other. For example if 2MB folios
> are used instead of 4KB pages, this reduces the number of DMA mappings by
> 512.
> 
> The folio order (and consequently, the size) is persisted in the struct
> drm_pagemap_device_addr to be available at the time of unmapping.
> 
> v2:
> - Initialize order variable (Matthew Brost)
> - Set proto and dir for completeness (Matthew Brost)
> - Do not populate drm_pagemap_addr, document it (Matthew Brost)
> - Add and use macro NR_PAGES(order) (Matthew Brost)
> 
> Signed-off-by: Francois Dugast <francois.dugast@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>

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

> ---
>  drivers/gpu/drm/drm_pagemap.c | 25 ++++++++++++++++++-------
>  include/drm/drm_pagemap.h     | 10 ++++++++--
>  2 files changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> index de320aeed812..b6f5d3a8e445 100644
> --- a/drivers/gpu/drm/drm_pagemap.c
> +++ b/drivers/gpu/drm/drm_pagemap.c
> @@ -222,22 +222,30 @@ static int drm_pagemap_migrate_map_pages(struct device *dev,
>  {
>  	unsigned long i;
>  
> -	for (i = 0; i < npages; ++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)
> -			continue;
> +			goto next;
>  
>  		if (WARN_ON_ONCE(is_zone_device_page(page)))
>  			return -EFAULT;
>  
> -		dma_addr = dma_map_page(dev, page, 0, PAGE_SIZE, dir);
> +		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_SYSTEM, 0, dir);
> +			dma_addr, DRM_INTERCONNECT_SYSTEM, order, dir);
> +
> +next:
> +		i += NR_PAGES(order);
>  	}
>  
>  	return 0;
> @@ -261,11 +269,14 @@ static void drm_pagemap_migrate_unmap_pages(struct device *dev,
>  {
>  	unsigned long i;
>  
> -	for (i = 0; i < npages; ++i) {
> +	for (i = 0; i < npages;) {
>  		if (!pagemap_addr[i].addr || dma_mapping_error(dev, pagemap_addr[i].addr))
> -			continue;
> +			goto next;
> +
> +		dma_unmap_page(dev, pagemap_addr[i].addr, PAGE_SIZE << pagemap_addr[i].order, dir);
>  
> -		dma_unmap_page(dev, pagemap_addr[i].addr, PAGE_SIZE, dir);
> +next:
> +		i += NR_PAGES(pagemap_addr[i].order);
>  	}
>  }
>  
> diff --git a/include/drm/drm_pagemap.h b/include/drm/drm_pagemap.h
> index 1d5919a99139..ef18d98dbc7e 100644
> --- a/include/drm/drm_pagemap.h
> +++ b/include/drm/drm_pagemap.h
> @@ -6,6 +6,8 @@
>  #include <linux/hmm.h>
>  #include <linux/types.h>
>  
> +#define NR_PAGES(order) (1U << order)
> +
>  struct drm_pagemap;
>  struct drm_pagemap_zdd;
>  struct device;
> @@ -173,7 +175,9 @@ struct drm_pagemap_devmem_ops {
>  	 * @pagemap_addr: Pointer to array of DMA information (source)
>  	 * @npages: Number of pages to copy
>  	 *
> -	 * Copy pages to device memory.
> +	 * Copy pages to device memory. If the order of a @pagemap_addr entry
> +	 * is greater than 0, the entry is populated but subsequent entries
> +	 * within the range of that order are not populated.
>  	 *
>  	 * Return: 0 on success, a negative error code on failure.
>  	 */
> @@ -187,7 +191,9 @@ struct drm_pagemap_devmem_ops {
>  	 * @pagemap_addr: Pointer to array of DMA information (destination)
>  	 * @npages: Number of pages to copy
>  	 *
> -	 * Copy pages to system RAM.
> +	 * Copy pages to system RAM. If the order of a @pagemap_addr entry
> +	 * is greater than 0, the entry is populated but subsequent entries
> +	 * within the range of that order are not populated.
>  	 *
>  	 * Return: 0 on success, a negative error code on failure.
>  	 */
> -- 
> 2.43.0
>