drm/panthor: Flush shmem writes before mapping buffers CPU-uncached

The shmem layer zeroes out the new pages using cached mappings, and if
we don't CPU-flush we might leave dirty cachelines behind, leading to
potential data leaks and/or asynchronous buffer corruption when dirty
cachelines are evicted.

Fixes: 8a1cc07 ("drm/panthor: Add GEM logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251107171214.1186299-1-boris.brezillon@collabora.com

Author: bbrezillon

drivers/gpu/drm/panthor/panthor_gem.c

@@ -288,6 +288,23 @@ panthor_gem_create_with_handle(struct drm_file *file,
 
 	panthor_gem_debugfs_set_usage_flags(bo, 0);
 
+	/* If this is a write-combine mapping, we query the sgt to force a CPU
+	 * cache flush (dma_map_sgtable() is called when the sgt is created).
+	 * This ensures the zero-ing is visible to any uncached mapping created
+	 * by vmap/mmap.
+	 * FIXME: Ideally this should be done when pages are allocated, not at
+	 * BO creation time.
+	 */
+	if (shmem->map_wc) {
+		struct sg_table *sgt;
+
+		sgt = drm_gem_shmem_get_pages_sgt(shmem);
+		if (IS_ERR(sgt)) {
+			ret = PTR_ERR(sgt);
+			goto out_put_gem;
+		}
+	}
+
 	/*
 	 * Allocate an id of idr table where the obj is registered
 	 * and handle has the id what user can see.
@@ -296,6 +313,7 @@ panthor_gem_create_with_handle(struct drm_file *file,
 	if (!ret)
 		*size = bo->base.base.size;
 
+out_put_gem:
 	/* drop reference from allocate - handle holds it now. */
 	drm_gem_object_put(&shmem->base);