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

[Boris Brezillon <boris.brezillon@collabora.com>]

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>
---
 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