[PATCH 3/4] btrfs: sysfs: export supported checksums

[Johannes Thumshirn <jthumshirn@xxxxxxx>]

From: David Sterba <dsterba@xxxxxxxx>

Export supported checksum algorithms via sysfs.

Co-developed-by: David Sterba <dsterba@xxxxxxxx> 
Signed-off-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
---
 fs/btrfs/ctree.c |  5 +++++
 fs/btrfs/ctree.h |  2 ++
 fs/btrfs/sysfs.c | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index b66509ee62eb..5debd74dc61c 100644
--- a/fs/btrfs/ctree.c
+++ b/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 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);
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index d17e79a40930..0180554f6970 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2165,6 +2165,8 @@ 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 btrfs_get_num_csums(void);
+
 
 /*
  * The leaf data grows from end-to-front in the node.
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index f6d3c80f2e28..aeebbdfe1a98 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -246,6 +246,28 @@ static umode_t btrfs_feature_visible(struct kobject *kobj,
 	return mode;
 }
 
+static ssize_t btrfs_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_FEAT_ATTR_INCOMPAT(mixed_backref, MIXED_BACKREF);
 BTRFS_FEAT_ATTR_INCOMPAT(default_subvol, DEFAULT_SUBVOL);
 BTRFS_FEAT_ATTR_INCOMPAT(mixed_groups, MIXED_GROUPS);
@@ -259,6 +281,14 @@ BTRFS_FEAT_ATTR_INCOMPAT(no_holes, NO_HOLES);
 BTRFS_FEAT_ATTR_INCOMPAT(metadata_uuid, METADATA_UUID);
 BTRFS_FEAT_ATTR_COMPAT_RO(free_space_tree, FREE_SPACE_TREE);
 
+static struct btrfs_feature_attr btrfs_attr_features_checksums_name = {
+	.kobj_attr = __INIT_KOBJ_ATTR(supported_checksums, S_IRUGO,
+				      btrfs_supported_checksums_show,
+				      NULL),
+	.feature_set	= FEAT_INCOMPAT,
+	.feature_bit	= 0,
+};
+
 static struct attribute *btrfs_supported_feature_attrs[] = {
 	BTRFS_FEAT_ATTR_PTR(mixed_backref),
 	BTRFS_FEAT_ATTR_PTR(default_subvol),
@@ -272,6 +302,9 @@ static struct attribute *btrfs_supported_feature_attrs[] = {
 	BTRFS_FEAT_ATTR_PTR(no_holes),
 	BTRFS_FEAT_ATTR_PTR(metadata_uuid),
 	BTRFS_FEAT_ATTR_PTR(free_space_tree),
+
+	&btrfs_attr_features_checksums_name.kobj_attr.attr,
+
 	NULL
 };
 
-- 
2.16.4