drm/nouveau: allow creation of zero-sized mm

Useful for places where a given chipset may or may not have a given
resource, and we want to avoid having to spray checks for the mm's
existance around everywhere.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/core/mm.c

@@ -218,13 +218,16 @@ nouveau_mm_init(struct nouveau_mm *mm, u32 offset, u32 length, u32 block)
 	node = kzalloc(sizeof(*node), GFP_KERNEL);
 	if (!node)
 		return -ENOMEM;
+
+	if (length) {
 		node->offset  = roundup(offset, mm->block_size);
-	node->length = rounddown(offset + length, mm->block_size) - node->offset;
+		node->length  = rounddown(offset + length, mm->block_size);
+		node->length -= node->offset;
+	}
 
 	list_add_tail(&node->nl_entry, &mm->nodes);
 	list_add_tail(&node->fl_entry, &mm->free);
 	mm->heap_nodes++;
-	mm->heap_size += length;
 	return 0;
 }
 

drivers/gpu/drm/nouveau/core/include/core/mm.h

@@ -19,7 +19,6 @@ struct nouveau_mm {
 
 	u32 block_size;
 	int heap_nodes;
-	u32 heap_size;
 };
 
 int  nouveau_mm_init(struct nouveau_mm *, u32 offset, u32 length, u32 block);