vty: Ensure radio-link-timeout is a multiple of 4

According to Table 10.5.2.3.1 in TS 144.018, radio-link-timeout
values are between 4 to 64 in steps of 4.

Change-Id: I733591d5f72f2e4f822761ca9eda85de7a4c6c81
---
M src/osmo-bsc/bts_vty.c
1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/osmo-bsc/bts_vty.c b/src/osmo-bsc/bts_vty.c
index ef8b8bd..54eb5cf 100644
--- a/src/osmo-bsc/bts_vty.c
+++ b/src/osmo-bsc/bts_vty.c
@@ -1481,8 +1481,15 @@
 	      "Radio link timeout value (lost SACCH block)\n")
 {
 	struct gsm_bts *bts = vty->index;
+	unsigned int radio_link_timeout = atoi(argv[0]);
 
-	gsm_bts_set_radio_link_timeout(bts, atoi(argv[0]));
+	/* According to Table 10.5.2.3.1 in TS 144.018 */
+	if (radio_link_timeout % 4 != 0) {
+		vty_out(vty, "%% Radio link timeout must be a multiple of 4%s", VTY_NEWLINE);
+		return CMD_WARNING;
+	}
+
+	gsm_bts_set_radio_link_timeout(bts, radio_link_timeout);
 
 	return CMD_SUCCESS;
 }

