Re: [PATCH 4/4] drm/panthor: Actually check huge-page mapping on sparse regions

[Liviu Dudau <liviu.dudau@arm.com>]

On Thu, Sep 17, 2026 at 02:33:46PM +0200, Boris Brezillon wrote:
> With the recent changes to iova_mapped_as_huge_page(), the check for
> huge-page mapping of sparse BOs is actually simple:
> 
> - for a sparse mapping, we know the BO offset any VA in this regions is
>   va & (SZ_2M - 1)
> - the VA we're searching the BO offset for is the 2M-aligned
>   aligned_va value
> 
> This guarantees that the BO offset to check is always zero in that case.
> 
> This is simple enough to let the code check if page 0 is a huge page
> and save the unmap+map dance when the dummy BO is not backed by a
> a huge page. So let's do that and kill the comment that says it's too
> complicated.
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>

Best regards,
Liviu

> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index d2897099763e..01564d250adf 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -2337,18 +2337,18 @@ iova_mapped_as_huge_page(struct drm_gpuva *mapping, u64 va)
>  
>  		return false;
>  	} else {
> -		const struct page *pg = bo->backing.pages[bo_offset >> PAGE_SHIFT];
>  		struct panthor_vma *vma = container_of(mapping, struct panthor_vma, base);
>  		bool is_sparse = vma->flags & DRM_PANTHOR_VM_BIND_OP_MAP_SPARSE;
> +		const struct page *pg;
>  
> -		/* If the unmapped VMA stands for a sparse mapping, always
> -		 * assume the backing storage is a THP, since the overhead of
> -		 * unmapping 2MiB worth of 4KiB pages and remapping some of
> -		 * them is offset by the logic of working out whether it's
> -		 * the opposite case right below.
> +		/* BO offset on a sparse mapping is chosen so that 2M-aligned
> +		 * VAs point to the start of the BO. Since aligned_va (the
> +		 * address we check huge-page against) is 2M-aligned, the BO
> +		 * offset is guaranteed to be zero.
> +		 * Check panthor_fix_sparse_map_offset() for more details.
>  		 */
>  		if (is_sparse)
> -			return true;
> +			bo_offset = 0;
>  
>  		/* In case of shmem backing, we know we can only have a huge
>  		 * mapping if the bo_offset is 2M aligned, meaning we can skip
> @@ -2357,6 +2357,7 @@ iova_mapped_as_huge_page(struct drm_gpuva *mapping, u64 va)
>  		if (!IS_ALIGNED(bo_offset, SZ_2M))
>  			return false;
>  
> +		pg = bo->backing.pages[bo_offset >> PAGE_SHIFT];
>  		return folio_size(page_folio(pg)) >= SZ_2M;
>  	}
>  }
> 
> -- 
> 2.55.0
> 

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯