driver core: clean up the logic to determine which /sys/dev/ directory to use

When a dev_t is set in a struct device, an symlink in /sys/dev/ is
created for it either under /sys/dev/block/ or /sys/dev/char/ depending
on the device type.

The logic to determine this would trigger off of the class of the
object, and the kobj_type set in that location.  But it turns out that
this deep nesting isn't needed at all, as it's either a choice of block
or "everything else" which is a char device.  So make the logic a lot
more simple and obvious, and remove the incorrect comments in the code
that tried to document something that was not happening at all (it is
impossible to set class->dev_kobj to NULL as the class core prevented
that from happening.

This removes the only place that class->dev_kobj was being used, so
after this, it can be removed entirely.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-4-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/base.h

@@ -209,6 +209,16 @@ int devtmpfs_init(void);
 static inline int devtmpfs_init(void) { return 0; }
 #endif
 
+#ifdef CONFIG_BLOCK
+extern struct class block_class;
+static inline bool is_blockdev(struct device *dev)
+{
+	return dev->class == &block_class;
+}
+#else
+static inline bool is_blockdev(struct device *dev) { return false; }
+#endif
+
 /* Device links support */
 int device_links_read_lock(void);
 void device_links_read_unlock(int idx);

drivers/base/core.c

@@ -3430,27 +3430,13 @@ int dev_set_name(struct device *dev, const char *fmt, ...)
 }
 EXPORT_SYMBOL_GPL(dev_set_name);
 
-/**
- * device_to_dev_kobj - select a /sys/dev/ directory for the device
- * @dev: device
- *
- * By default we select char/ for new entries.  Setting class->dev_obj
- * to NULL prevents an entry from being created.  class->dev_kobj must
- * be set (or cleared) before any devices are registered to the class
- * otherwise device_create_sys_dev_entry() and
- * device_remove_sys_dev_entry() will disagree about the presence of
- * the link.
- */
+/* select a /sys/dev/ directory for the device */
 static struct kobject *device_to_dev_kobj(struct device *dev)
 {
-	struct kobject *kobj;
-
-	if (dev->class)
-		kobj = dev->class->dev_kobj;
+	if (is_blockdev(dev))
+		return sysfs_dev_block_kobj;
 	else
-		kobj = sysfs_dev_char_kobj;
-
-	return kobj;
+		return sysfs_dev_char_kobj;
 }
 
 static int device_create_sys_dev_entry(struct device *dev)

drivers/base/devtmpfs.c

@@ -94,15 +94,6 @@ static struct file_system_type dev_fs_type = {
 	.mount = public_dev_mount,
 };
 
-#ifdef CONFIG_BLOCK
-static inline int is_blockdev(struct device *dev)
-{
-	return dev->class == &block_class;
-}
-#else
-static inline int is_blockdev(struct device *dev) { return 0; }
-#endif
-
 static int devtmpfs_submit_req(struct req *req, const char *tmp)
 {
 	init_completion(&req->done);