[MTD] add get and put methods

This patch adds get_device() and put_device() methods to the MTD description
structure (struct mtd_info). These methods are called by MTD whenever the MTD
device is get or put. They are needed when the underlying driver is something
smarter then just flash chip driver, for example UBI.
Signed-off-by: Artem Bityutskiy <dedekind@infradead.org>

drivers/mtd/mtdcore.c

@@ -214,12 +214,23 @@ struct mtd_info *get_mtd_device(struct mtd_info *mtd, int num)
 			ret = NULL;
 	}
 
-	if (ret && !try_module_get(ret->owner))
+	if (!ret)
+		goto out_unlock;
+
+	if (!try_module_get(ret->owner)) {
+		ret = NULL;
+		goto out_unlock;
+	}
+
+	if (ret->get_device && ret->get_device(ret)) {
+		module_put(ret->owner);
 		ret = NULL;
+		goto out_unlock;
+	}
 
-	if (ret)
 	ret->usecount++;
 
+out_unlock:
 	mutex_unlock(&mtd_table_mutex);
 	return ret;
 }
@@ -235,8 +246,8 @@ struct mtd_info *get_mtd_device(struct mtd_info *mtd, int num)
 
 struct mtd_info *get_mtd_device_nm(const char *name)
 {
-	int i;
-	struct mtd_info *mtd = ERR_PTR(-ENODEV);
+	int i, err = -ENODEV;
+	struct mtd_info *mtd = NULL;
 
 	mutex_lock(&mtd_table_mutex);
 
@@ -247,17 +258,27 @@ struct mtd_info *get_mtd_device_nm(const char *name)
 		}
 	}
 
-	if (i == MAX_MTD_DEVICES)
+	if (!mtd)
 		goto out_unlock;
 
 	if (!try_module_get(mtd->owner))
 		goto out_unlock;
 
+	if (mtd->get_device) {
+		err = mtd->get_device(mtd);
+		if (err)
+			goto out_put;
+	}
+
 	mtd->usecount++;
+	mutex_unlock(&mtd_table_mutex);
+	return mtd;
 
+out_put:
+	module_put(mtd->owner);
 out_unlock:
 	mutex_unlock(&mtd_table_mutex);
-	return mtd;
+	return ERR_PTR(err);
 }
 
 void put_mtd_device(struct mtd_info *mtd)
@@ -266,6 +287,8 @@ void put_mtd_device(struct mtd_info *mtd)
 
 	mutex_lock(&mtd_table_mutex);
 	c = --mtd->usecount;
+	if (mtd->put_device)
+		mtd->put_device(mtd);
 	mutex_unlock(&mtd_table_mutex);
 	BUG_ON(c < 0);
 

include/linux/mtd/mtd.h

@@ -207,6 +207,13 @@ struct mtd_info {
 
 	struct module *owner;
 	int usecount;
+
+	/* If the driver is something smart, like UBI, it may need to maintain
+	 * its own reference counting. The below functions are only for driver.
+	 * The driver may register its callbacks. These callbacks are not
+	 * supposed to be called by MTD users */
+	int (*get_device) (struct mtd_info *mtd);
+	void (*put_device) (struct mtd_info *mtd);
 };