driver core: Add device_show_string() helper for sysfs attributes

For drivers wishing to expose an unsigned long, int or bool at a static
memory location in sysfs, the driver core provides ready-made helpers
such as device_show_ulong() to be used as ->show() callback.

Some drivers need to expose a string and so far they all provide their
own ->show() implementation. arch/powerpc/perf/hv-24x7.c went so far
as to create a device_show_string() helper but kept it private.

Make it public for reuse by other drivers. The pattern seems to be
sufficiently frequent to merit a public helper.

Add a DEVICE_STRING_ATTR_RO() macro in line with the existing
DEVICE_ULONG_ATTR() and similar macros to ease declaration of string
attributes.

Signed-off-by: Lukas Wunner <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

arch/powerpc/perf/hv-24x7.c

···
 	return kasprintf(gfp, "%.*s", max_len, maybe_str);
 }
 
-static ssize_t device_show_string(struct device *dev,
-		struct device_attribute *attr, char *buf)
-{
-	struct dev_ext_attribute *d;
-
-	d = container_of(attr, struct dev_ext_attribute, attr);
-
-	return sprintf(buf, "%s\n", (char *)d->var);
-}
-
 static ssize_t cpumask_show(struct device *dev,
 			    struct device_attribute *attr, char *buf)
 {

drivers/base/core.c

···
 }
 EXPORT_SYMBOL_GPL(device_show_bool);
 
+ssize_t device_show_string(struct device *dev,
+			   struct device_attribute *attr, char *buf)
+{
+	struct dev_ext_attribute *ea = to_ext_attr(attr);
+
+	return sysfs_emit(buf, "%s\n", (char *)ea->var);
+}
+EXPORT_SYMBOL_GPL(device_show_string);
+
 /**
  * device_release - free device structure.
  * @kobj: device's kobject.

include/linux/device.h

···
 			char *buf);
 ssize_t device_store_bool(struct device *dev, struct device_attribute *attr,
 			 const char *buf, size_t count);
+ssize_t device_show_string(struct device *dev, struct device_attribute *attr,
+			   char *buf);
 
 /**
  * DEVICE_ATTR - Define a device attribute.
···
 #define DEVICE_BOOL_ATTR(_name, _mode, _var) \
 	struct dev_ext_attribute dev_attr_##_name = \
 		{ __ATTR(_name, _mode, device_show_bool, device_store_bool), &(_var) }
+
+/**
+ * DEVICE_STRING_ATTR_RO - Define a device attribute backed by a r/o string.
+ * @_name: Attribute name.
+ * @_mode: File mode.
+ * @_var: Identifier of string.
+ *
+ * Like DEVICE_ULONG_ATTR(), but @_var is a string. Because the length of the
+ * string allocation is unknown, the attribute must be read-only.
+ */
+#define DEVICE_STRING_ATTR_RO(_name, _mode, _var) \
+	struct dev_ext_attribute dev_attr_##_name = \
+		{ __ATTR(_name, (_mode) & ~0222, device_show_string, NULL), (_var) }
 
 #define DEVICE_ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store) \
 	struct device_attribute dev_attr_##_name =		\