sandbox: Implement reference counting for address mapping

An address may be mapped twice and unmapped twice. Delete the mapping
only when the last user unmaps it.

Fix a missing comment while here.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/cpu.c

@@ -111,6 +111,7 @@ void *phys_to_virt(phys_addr_t paddr)
 		if (mentry->tag == paddr) {
 			log_debug("Used map from %lx to %p\n", (ulong)paddr,
 				  mentry->ptr);
+			mentry->refcnt++;
 			return mentry->ptr;
 		}
 	}
@@ -200,10 +201,12 @@ void unmap_physmem(const void *ptr, unsigned long flags)
 
 	mentry = find_tag(ptr);
 	if (mentry) {
-		list_del(&mentry->sibling_node);
-		log_debug("Removed map from %p to %lx\n", ptr,
-			  (ulong)mentry->tag);
-		free(mentry);
+		if (!--mentry->refcnt) {
+			list_del(&mentry->sibling_node);
+			log_debug("Removed map from %p to %lx\n", ptr,
+				  (ulong)mentry->tag);
+			free(mentry);
+		}
 	} else {
 		log_warning("Address not mapped: %p\n", ptr);
 	}
@@ -235,11 +238,14 @@ phys_addr_t map_to_sysmem(const void *ptr)
 		}
 		mentry->tag = state->next_tag++;
 		mentry->ptr = (void *)ptr;
+		mentry->refcnt = 0;
 		list_add_tail(&mentry->sibling_node, &state->mapmem_head);
 		log_debug("Added map from %p to %lx\n", ptr,
 			  (ulong)mentry->tag);
 	}
 
+	mentry->refcnt++;
+
 	/*
 	 * Return the tag as the address to use. A later call to map_sysmem()
 	 * will return ptr

arch/sandbox/include/asm/state.h

@@ -53,10 +53,13 @@ struct sandbox_wdt_info {
  * be returned, just as it would for a normal sandbox address.
  *
  * @tag: Address tag (a value which U-Boot uses to refer to the address)
+ * @refcnt: Number of references to this tag
  * @ptr: Associated pointer for that tag
+ * @sibling_node: Next node
  */
 struct sandbox_mapmem_entry {
 	ulong tag;
+	uint refcnt;
 	void *ptr;
 	struct list_head sibling_node;
 };