Currently users have to choose a free snapshot id before
calling DEVLINK_CMD_REGION_NEW. This is potentially racy
and inconvenient.

Make the DEVLINK_ATTR_REGION_SNAPSHOT_ID optional and try
to allocate id automatically. Send a message back to the
caller with the snapshot info.

Example use:
$ devlink region new netdevsim/netdevsim1/dummy
netdevsim/netdevsim1/dummy: snapshot 1

$ id=$(devlink -j region new netdevsim/netdevsim1/dummy | \
       jq '.[][][][]')
$ devlink region dump netdevsim/netdevsim1/dummy snapshot $id
[...]
$ devlink region del netdevsim/netdevsim1/dummy snapshot $id

v3:
 - send the notification only once snapshot creation completed.
v2:
 - don't wrap the line containing extack;
 - add a few sentences to the docs.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 .../networking/devlink/devlink-region.rst     |  7 +-
 net/core/devlink.c                            | 69 +++++++++++++++----
 .../drivers/net/netdevsim/devlink.sh          | 13 ++++
 3 files changed, 74 insertions(+), 15 deletions(-)

diff --git a/Documentation/networking/devlink/devlink-region.rst b/Documentation/networking/devlink/devlink-region.rst
index 04e04d1ff627..daf35427fce1 100644
--- a/Documentation/networking/devlink/devlink-region.rst
+++ b/Documentation/networking/devlink/devlink-region.rst

@@ -23,7 +23,9 @@ states, but see also :doc:`devlink-health`
 Regions may optionally support capturing a snapshot on demand via the
 ``DEVLINK_CMD_REGION_NEW`` netlink message. A driver wishing to allow
 requested snapshots must implement the ``.snapshot`` callback for the region
-in its ``devlink_region_ops`` structure.
+in its ``devlink_region_ops`` structure. If snapshot id is not set in
+the ``DEVLINK_CMD_REGION_NEW`` request kernel will allocate one and send
+the snapshot information to user space.
 
 example usage
 -------------

@@ -45,7 +47,8 @@ example usage
     $ devlink region del pci/0000:00:05.0/cr-space snapshot 1
 
     # Request an immediate snapshot, if supported by the region
-    $ devlink region new pci/0000:00:05.0/cr-space snapshot 5
+    $ devlink region new pci/0000:00:05.0/cr-space
+    pci/0000:00:05.0/cr-space: snapshot 5
 
     # Dump a snapshot:
     $ devlink region dump pci/0000:00:05.0/fw-health snapshot 1

diff --git a/net/core/devlink.c b/net/core/devlink.c
index 92afb85bad89..4df947fb90d9 100644
--- a/net/core/devlink.c
+++ b/net/core/devlink.c

@@ -4082,10 +4082,41 @@ static int devlink_nl_cmd_region_del(struct sk_buff *skb,
 	return 0;
 }
 
+static int
+devlink_nl_snapshot_id_notify(struct devlink *devlink, struct genl_info *info,
+			      struct devlink_region *region, u32 snapshot_id)
+{
+	struct devlink_snapshot *snapshot;
+	struct sk_buff *msg;
+	int err;
+
+	snapshot = devlink_region_snapshot_get_by_id(region, snapshot_id);
+	if (WARN_ON(!snapshot))
+		return -EINVAL;
+
+	msg = devlink_nl_region_notify_build(region, snapshot,
+					     DEVLINK_CMD_REGION_NEW,
+					     info->snd_portid, info->snd_seq);
+	err = PTR_ERR_OR_ZERO(msg);
+	if (err)
+		goto err_notify;
+
+	err = genlmsg_reply(msg, info);
+	if (err)
+		goto err_notify;
+
+	return 0;
+
+err_notify:
+	devlink_region_snapshot_del(region, snapshot);
+	return err;
+}
+
 static int
 devlink_nl_cmd_region_new(struct sk_buff *skb, struct genl_info *info)
 {
 	struct devlink *devlink = info->user_ptr[0];
+	struct nlattr *snapshot_id_attr;
 	struct devlink_region *region;
 	const char *region_name;
 	u32 snapshot_id;

@@ -4097,11 +4128,6 @@ devlink_nl_cmd_region_new(struct sk_buff *skb, struct genl_info *info)
 		return -EINVAL;
 	}
 
-	if (!info->attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID]) {
-		NL_SET_ERR_MSG_MOD(info->extack, "No snapshot id provided");
-		return -EINVAL;
-	}
-
 	region_name = nla_data(info->attrs[DEVLINK_ATTR_REGION_NAME]);
 	region = devlink_region_get_by_name(devlink, region_name);
 	if (!region) {

@@ -4119,16 +4145,25 @@ devlink_nl_cmd_region_new(struct sk_buff *skb, struct genl_info *info)
 		return -ENOSPC;
 	}
 
-	snapshot_id = nla_get_u32(info->attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID]);
+	snapshot_id_attr = info->attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID];
+	if (snapshot_id_attr) {
+		snapshot_id = nla_get_u32(snapshot_id_attr);
 
-	if (devlink_region_snapshot_get_by_id(region, snapshot_id)) {
-		NL_SET_ERR_MSG_MOD(info->extack, "The requested snapshot id is already in use");
-		return -EEXIST;
-	}
+		if (devlink_region_snapshot_get_by_id(region, snapshot_id)) {
+			NL_SET_ERR_MSG_MOD(info->extack, "The requested snapshot id is already in use");
+			return -EEXIST;
+		}
 
-	err = __devlink_snapshot_id_insert(devlink, snapshot_id);
-	if (err)
-		return err;
+		err = __devlink_snapshot_id_insert(devlink, snapshot_id);
+		if (err)
+			return err;
+	} else {
+		err = __devlink_region_snapshot_id_get(devlink, &snapshot_id);
+		if (err) {
+			NL_SET_ERR_MSG_MOD(info->extack, "Failed to allocate a new snapshot id");
+			return err;
+		}
+	}
 
 	err = region->ops->snapshot(devlink, info->extack, &data);
 	if (err)

@@ -4138,6 +4173,14 @@ devlink_nl_cmd_region_new(struct sk_buff *skb, struct genl_info *info)
 	if (err)
 		goto err_snapshot_create;
 
+	if (!snapshot_id_attr) {
+		/* destroys the snapshot on failure */
+		err = devlink_nl_snapshot_id_notify(devlink, info,
+						    region, snapshot_id);
+		if (err)
+			return err;
+	}
+