samples/kobject: constify 'struct foo_attribute'

Showcase and test the new 'struct attribute' constification facilities.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20251029-sysfs-const-attr-prep-v5-6-ea7d745acff4@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: t-8ch

samples/kobject/kset-example.c

@@ -37,10 +37,11 @@ struct foo_obj {
 /* a custom attribute that works just for a struct foo_obj. */
 struct foo_attribute {
 	struct attribute attr;
-	ssize_t (*show)(struct foo_obj *foo, struct foo_attribute *attr, char *buf);
-	ssize_t (*store)(struct foo_obj *foo, struct foo_attribute *attr, const char *buf, size_t count);
+	ssize_t (*show)(struct foo_obj *foo, const struct foo_attribute *attr, char *buf);
+	ssize_t (*store)(struct foo_obj *foo, const struct foo_attribute *attr,
+			 const char *buf, size_t count);
 };
-#define to_foo_attr(x) container_of(x, struct foo_attribute, attr)
+#define to_foo_attr(x) container_of_const(x, struct foo_attribute, attr)
 
 /*
  * The default show function that must be passed to sysfs.  This will be
@@ -53,7 +54,7 @@ static ssize_t foo_attr_show(struct kobject *kobj,
 			     struct attribute *attr,
 			     char *buf)
 {
-	struct foo_attribute *attribute;
+	const struct foo_attribute *attribute;
 	struct foo_obj *foo;
 
 	attribute = to_foo_attr(attr);
@@ -73,7 +74,7 @@ static ssize_t foo_attr_store(struct kobject *kobj,
 			      struct attribute *attr,
 			      const char *buf, size_t len)
 {
-	struct foo_attribute *attribute;
+	const struct foo_attribute *attribute;
 	struct foo_obj *foo;
 
 	attribute = to_foo_attr(attr);
@@ -109,13 +110,13 @@ static void foo_release(struct kobject *kobj)
 /*
  * The "foo" file where the .foo variable is read from and written to.
  */
-static ssize_t foo_show(struct foo_obj *foo_obj, struct foo_attribute *attr,
+static ssize_t foo_show(struct foo_obj *foo_obj, const struct foo_attribute *attr,
 			char *buf)
 {
 	return sysfs_emit(buf, "%d\n", foo_obj->foo);
 }
 
-static ssize_t foo_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
+static ssize_t foo_store(struct foo_obj *foo_obj, const struct foo_attribute *attr,
 			 const char *buf, size_t count)
 {
 	int ret;
@@ -128,14 +129,14 @@ static ssize_t foo_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
 }
 
 /* Sysfs attributes cannot be world-writable. */
-static struct foo_attribute foo_attribute =
+static const struct foo_attribute foo_attribute =
 	__ATTR(foo, 0664, foo_show, foo_store);
 
 /*
  * More complex function where we determine which variable is being accessed by
  * looking at the attribute for the "baz" and "bar" files.
  */
-static ssize_t b_show(struct foo_obj *foo_obj, struct foo_attribute *attr,
+static ssize_t b_show(struct foo_obj *foo_obj, const struct foo_attribute *attr,
 		      char *buf)
 {
 	int var;
@@ -147,7 +148,7 @@ static ssize_t b_show(struct foo_obj *foo_obj, struct foo_attribute *attr,
 	return sysfs_emit(buf, "%d\n", var);
 }
 
-static ssize_t b_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
+static ssize_t b_store(struct foo_obj *foo_obj, const struct foo_attribute *attr,
 		       const char *buf, size_t count)
 {
 	int var, ret;
@@ -163,24 +164,24 @@ static ssize_t b_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
 	return count;
 }
 
-static struct foo_attribute baz_attribute =
+static const struct foo_attribute baz_attribute =
 	__ATTR(baz, 0664, b_show, b_store);
-static struct foo_attribute bar_attribute =
+static const struct foo_attribute bar_attribute =
 	__ATTR(bar, 0664, b_show, b_store);
 
 /*
  * Create a group of attributes so that we can create and destroy them all
  * at once.
  */
-static struct attribute *foo_default_attrs[] = {
+static const struct attribute *const foo_default_attrs[] = {
 	&foo_attribute.attr,
 	&baz_attribute.attr,
 	&bar_attribute.attr,
 	NULL,	/* need to NULL terminate the list of attributes */
 };
 
 static umode_t foo_default_attrs_is_visible(struct kobject *kobj,
-					    struct attribute *attr,
+					    const struct attribute *attr,
 					    int n)
 {
 	/* Hide attributes with the same name as the kobject. */
@@ -190,8 +191,8 @@ static umode_t foo_default_attrs_is_visible(struct kobject *kobj,
 }
 
 static const struct attribute_group foo_default_group = {
-	.attrs		= foo_default_attrs,
-	.is_visible	= foo_default_attrs_is_visible,
+	.attrs_const		= foo_default_attrs,
+	.is_visible_const	= foo_default_attrs_is_visible,
 };
 __ATTRIBUTE_GROUPS(foo_default);