[PATCH iproute2] ip link: add sub-command to view and change DSA master

[Vladimir Oltean <vladimir.oltean@nxp.com>]

Support the "dsa" kind of rtnl_link_ops exported by the kernel, and
export reads/writes to IFLA_DSA_MASTER.

Examples:

$ ip link set swp0 type dsa master eth1

$ ip -d link show dev swp0
    (...)
    dsa master eth0

$ ip -d -j link show swp0
[
	{
		"link": "eth1",
		"linkinfo": {
			"info_kind": "dsa",
			"info_data": {
				"master": "eth1"
			}
		},
	}
]

Note that by construction and as shown in the example, the IFLA_LINK
reported by a DSA user port is identical to what is reported through
IFLA_DSA_MASTER. However IFLA_LINK is not writable, and overloading its
meaning to make it writable would clash with other users of IFLA_LINK
(vlan etc) for which writing this property does not make sense.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
This is semi-RFC, as in: pls don't apply yet, because the kernel patches
weren't merged yet (according to feedback so far, it's likely that the
proposed UAPI won't change, but still).

 include/uapi/linux/if_link.h | 10 ++++++
 ip/Makefile                  |  2 +-
 ip/iplink_dsa.c              | 67 ++++++++++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+), 1 deletion(-)
 create mode 100644 ip/iplink_dsa.c

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index e0fbbfeeb3a1..6f4d7b1ff9fb 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -1372,4 +1372,14 @@ enum {
 
 #define IFLA_MCTP_MAX (__IFLA_MCTP_MAX - 1)
 
+/* DSA section */
+
+enum {
+	IFLA_DSA_UNSPEC,
+	IFLA_DSA_MASTER,
+	__IFLA_DSA_MAX,
+};
+
+#define IFLA_DSA_MAX	(__IFLA_DSA_MAX - 1)
+
 #endif /* _LINUX_IF_LINK_H */
diff --git a/ip/Makefile b/ip/Makefile
index 6c2e072049a2..8fd9e295f344 100644
--- a/ip/Makefile
+++ b/ip/Makefile
@@ -8,7 +8,7 @@ IPOBJ=ip.o ipaddress.o ipaddrlabel.o iproute.o iprule.o ipnetns.o \
     iplink_macvlan.o ipl2tp.o link_vti.o link_vti6.o link_xfrm.o \
     iplink_vxlan.o tcp_metrics.o iplink_ipoib.o ipnetconf.o link_ip6tnl.o \
     link_iptnl.o link_gre6.o iplink_bond.o iplink_bond_slave.o iplink_hsr.o \
-    iplink_bridge.o iplink_bridge_slave.o ipfou.o iplink_ipvlan.o \
+    iplink_bridge.o iplink_bridge_slave.o iplink_dsa.o ipfou.o iplink_ipvlan.o \
     iplink_geneve.o iplink_vrf.o iproute_lwtunnel.o ipmacsec.o ipila.o \
     ipvrf.o iplink_xstats.o ipseg6.o iplink_netdevsim.o iplink_rmnet.o \
     ipnexthop.o ipmptcp.o iplink_bareudp.o iplink_wwan.o ipioam6.o \
diff --git a/ip/iplink_dsa.c b/ip/iplink_dsa.c
new file mode 100644
index 000000000000..d984e8b3b534
--- /dev/null
+++ b/ip/iplink_dsa.c
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * iplink_dsa.c		DSA switch support
+ */
+
+#include "utils.h"
+#include "ip_common.h"
+
+static void print_usage(FILE *f)
+{
+	fprintf(f, "Usage: ... dsa [ master DEVICE ]\n");
+}
+
+static int dsa_parse_opt(struct link_util *lu, int argc, char **argv,
+			 struct nlmsghdr *n)
+{
+	while (argc > 0) {
+		if (strcmp(*argv, "master") == 0) {
+			__u32 ifindex;
+
+			NEXT_ARG();
+			ifindex = ll_name_to_index(*argv);
+			if (!ifindex)
+				invarg("Device does not exist\n", *argv);
+			addattr_l(n, 1024, IFLA_DSA_MASTER, &ifindex, 4);
+		} else if (strcmp(*argv, "help") == 0) {
+			print_usage(stderr);
+			return -1;
+		} else {
+			fprintf(stderr, "dsa: unknown command \"%s\"?\n", *argv);
+			print_usage(stderr);
+			return -1;
+		}
+		argc--;
+		argv++;
+	}
+
+	return 0;
+}
+
+static void dsa_print_opt(struct link_util *lu, FILE *f, struct rtattr *tb[])
+{
+	if (!tb)
+		return;
+
+	if (tb[IFLA_DSA_MASTER]) {
+		__u32 master = rta_getattr_u32(tb[IFLA_DSA_MASTER]);
+
+		print_string(PRINT_ANY,
+			     "master", "master %s ",
+			     ll_index_to_name(master));
+	}
+}
+
+static void dsa_print_help(struct link_util *lu, int argc, char **argv,
+			   FILE *f)
+{
+	print_usage(f);
+}
+
+struct link_util dsa_link_util = {
+	.id		= "dsa",
+	.maxattr	= IFLA_DSA_MAX,
+	.parse_opt	= dsa_parse_opt,
+	.print_opt	= dsa_print_opt,
+	.print_help     = dsa_print_help,
+};
-- 
2.34.1