drm/panfrost: Check sgt to know whether pages are already mapped

In the MMU's page fault ISR for a heap object, determine whether the
faulting address belongs to a 2MiB block that was already mapped by
checking its corresponding sgt in the Panfrost BO.

This is done in preparation for a future commit in which the MMU mapping
helper might fail, but the page array is left populated, so this cannot
be used as a check for an early bail-out.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Link: https://lore.kernel.org/r/20251019145225.3621989-6-adrian.larumbe@collabora.com
Signed-off-by: Steven Price <steven.price@arm.com>

Author: larunbe

drivers/gpu/drm/panfrost/panfrost_mmu.c

@@ -602,10 +602,12 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
 		refcount_set(&bo->base.pages_use_count, 1);
 	} else {
 		pages = bo->base.pages;
-		if (pages[page_offset]) {
-			/* Pages are already mapped, bail out. */
-			goto out;
-		}
+	}
+
+	sgt = &bo->sgts[page_offset / (SZ_2M / PAGE_SIZE)];
+	if (sgt->sgl) {
+		/* Pages are already mapped, bail out. */
+		goto out;
 	}
 
 	mapping = bo->base.base.filp->f_mapping;
@@ -627,7 +629,6 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
 		}
 	}
 
-	sgt = &bo->sgts[page_offset / (SZ_2M / PAGE_SIZE)];
 	ret = sg_alloc_table_from_pages(sgt, pages + page_offset,
 					NUM_FAULT_PAGES, 0, SZ_2M, GFP_KERNEL);
 	if (ret)