nvmem: use kref

Use kref for reference counting. Use an approach similar to the one
seen in the common clock subsystem: don't actually destroy the nvmem
device until the last user puts it. This way we can get rid of the
users check from nvmem_unregister().
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/nvmem/core.c

@@ -19,6 +19,7 @@
 #include <linux/fs.h>
 #include <linux/idr.h>
 #include <linux/init.h>
+#include <linux/kref.h>
 #include <linux/module.h>
 #include <linux/nvmem-consumer.h>
 #include <linux/nvmem-provider.h>
@@ -31,7 +32,7 @@ struct nvmem_device {
 	int			stride;
 	int			word_size;
 	int			id;
-	int			users;
+	struct kref		refcnt;
 	size_t			size;
 	bool			read_only;
 	int			flags;
@@ -463,6 +464,8 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 		return ERR_PTR(rval);
 	}
 
+	kref_init(&nvmem->refcnt);
+
 	nvmem->id = rval;
 	nvmem->owner = config->owner;
 	if (!nvmem->owner && config->dev->driver)
@@ -532,6 +535,20 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 }
 EXPORT_SYMBOL_GPL(nvmem_register);
 
+static void nvmem_device_release(struct kref *kref)
+{
+	struct nvmem_device *nvmem;
+
+	nvmem = container_of(kref, struct nvmem_device, refcnt);
+
+	if (nvmem->flags & FLAG_COMPAT)
+		device_remove_bin_file(nvmem->base_dev, &nvmem->eeprom);
+
+	nvmem_device_remove_all_cells(nvmem);
+	device_del(&nvmem->dev);
+	put_device(&nvmem->dev);
+}
+
 /**
  * nvmem_unregister() - Unregister previously registered nvmem device
  *
@@ -541,19 +558,7 @@ EXPORT_SYMBOL_GPL(nvmem_register);
  */
 int nvmem_unregister(struct nvmem_device *nvmem)
 {
-	mutex_lock(&nvmem_mutex);
-	if (nvmem->users) {
-		mutex_unlock(&nvmem_mutex);
-		return -EBUSY;
-	}
-	mutex_unlock(&nvmem_mutex);
-
-	if (nvmem->flags & FLAG_COMPAT)
-		device_remove_bin_file(nvmem->base_dev, &nvmem->eeprom);
-
-	nvmem_device_remove_all_cells(nvmem);
-	device_del(&nvmem->dev);
-	put_device(&nvmem->dev);
+	kref_put(&nvmem->refcnt, nvmem_device_release);
 
 	return 0;
 }
@@ -647,7 +652,6 @@ static struct nvmem_device *__nvmem_device_get(struct device_node *np,
 		}
 	}
 
-	nvmem->users++;
 	mutex_unlock(&nvmem_mutex);
 
 	if (!try_module_get(nvmem->owner)) {
@@ -655,22 +659,18 @@ static struct nvmem_device *__nvmem_device_get(struct device_node *np,
 			"could not increase module refcount for cell %s\n",
 			nvmem_dev_name(nvmem));
 
-		mutex_lock(&nvmem_mutex);
-		nvmem->users--;
-		mutex_unlock(&nvmem_mutex);
-
 		return ERR_PTR(-EINVAL);
 	}
 
+	kref_get(&nvmem->refcnt);
+
 	return nvmem;
 }
 
 static void __nvmem_device_put(struct nvmem_device *nvmem)
 {
 	module_put(nvmem->owner);
-	mutex_lock(&nvmem_mutex);
-	nvmem->users--;
-	mutex_unlock(&nvmem_mutex);
+	kref_put(&nvmem->refcnt, nvmem_device_release);
 }
 
 static struct nvmem_device *nvmem_find(const char *name)