btrfs: sysfs: export supported checksums

Export supported checksum algorithms via sysfs in the list of static
features:

  /sys/fs/btrfs/features/supported_checksums

Space spearated list of checksum algorithm names.
Co-developed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -53,6 +53,11 @@ const char *btrfs_super_csum_name(u16 csum_type)
 	return btrfs_csums[csum_type].name;
 }
 
+size_t __const btrfs_get_num_csums(void)
+{
+	return ARRAY_SIZE(btrfs_csums);
+}
+
 struct btrfs_path *btrfs_alloc_path(void)
 {
 	return kmem_cache_zalloc(btrfs_path_cachep, GFP_NOFS);

fs/btrfs/ctree.h

@@ -2163,6 +2163,8 @@ BTRFS_SETGET_STACK_FUNCS(super_uuid_tree_generation, struct btrfs_super_block,
 
 int btrfs_super_csum_size(const struct btrfs_super_block *s);
 const char *btrfs_super_csum_name(u16 csum_type);
+size_t __const btrfs_get_num_csums(void);
+
 
 /*
  * The leaf data grows from end-to-front in the node.

fs/btrfs/sysfs.c

@@ -295,8 +295,30 @@ static ssize_t rmdir_subvol_show(struct kobject *kobj,
 }
 BTRFS_ATTR(static_feature, rmdir_subvol, rmdir_subvol_show);
 
+static ssize_t supported_checksums_show(struct kobject *kobj,
+					struct kobj_attribute *a, char *buf)
+{
+	ssize_t ret = 0;
+	int i;
+
+	for (i = 0; i < btrfs_get_num_csums(); i++) {
+		/*
+		 * This "trick" only works as long as 'enum btrfs_csum_type' has
+		 * no holes in it
+		 */
+		ret += snprintf(buf + ret, PAGE_SIZE - ret, "%s%s",
+				(i == 0 ? "" : " "), btrfs_super_csum_name(i));
+
+	}
+
+	ret += snprintf(buf + ret, PAGE_SIZE - ret, "\n");
+	return ret;
+}
+BTRFS_ATTR(static_feature, supported_checksums, supported_checksums_show);
+
 static struct attribute *btrfs_supported_static_feature_attrs[] = {
 	BTRFS_ATTR_PTR(static_feature, rmdir_subvol),
+	BTRFS_ATTR_PTR(static_feature, supported_checksums),
 	NULL
 };