add osmo_tdef API, originally adopted from osmo-bsc T_def

Move T_def from osmo-bsc to libosmocore as osmo_tdef. Adjust naming to be more
consistent. Upgrade to first class API:
- add timer grouping
- add generic vty support
- add mising API doc
- add C test
- add VTY transcript tests, also as examples for using the API

From osmo_fsm_inst_state_chg() API doc, cross reference to osmo_tdef API.

The root reason for moving to libosmocore is that I want to use the
mgw_endpoint_fsm in osmo-msc for inter-MSC handover, and hence want to move the
FSM to libosmo-mgcp-client. This FSM uses the T_def from osmo-bsc. Though the
mgw_endpoint_fsm's use of T_def is minimal, I intend to use the osmo_tdef API
in osmo-msc (and probably elsewhere) as well. libosmocore is the most sensible
place for this.

osmo_tdef provides:

- a list of Tnnnn (GSM) timers with description, unit and default value.
- vty UI to allow users to configure non-default timeouts.
- API to tie T timers to osmo_fsm states and set them on state transitions.

- a few standard units (minute, second, millisecond) as well as a custom unit
  (which relies on the timer's human readable description to indicate the
  meaning of the value).
- conversion for standard units: for example, some GSM timers are defined in
  minutes, while our FSM definitions need timeouts in seconds. Conversion is
  for convenience only and can be easily avoided via the custom unit.

By keeping separate osmo_tdef arrays, several groups of timers can be kept
separately. The VTY tests in tests/tdef/ showcase different schemes:

- tests/vty/tdef_vty_test_config_root.c:
  Keep several timer definitions in separately named groups: showcase the
  osmo_tdef_vty_groups*() API. Each timer group exists exactly once.

- tests/vty/tdef_vty_test_config_subnode.c:
  Keep a single list of timers without separate grouping.
  Put this list on a specific subnode below the CONFIG_NODE.
  There could be several separate subnodes with timers like this, i.e.
  continuing from this example, sets timers could be separated by placing
  timers in specific config subnodes instead of using the global group name.

- tests/vty/tdef_vty_test_dynamic.c:
  Dynamically allocate timer definitions per each new created object.
  Thus there can be an arbitrary number of independent timer definitions, one
  per allocated object.

T_def was introduced during the recent osmo-bsc refactoring for inter-BSC
handover, and has proven useful:

- without osmo_tdef, each invocation of osmo_fsm_inst_state_chg() needs to be
  programmed with the right timeout value, for all code paths that invoke this
  state change. It is a likely source of errors to get one of them wrong.  By
  defining a T timer exactly for an FSM state, the caller can merely invoke the
  state change and trust on the original state definition to apply the correct
  timeout.

- it is helpful to have a standardized config file UI to provide user
  configurable timeouts, instead of inventing new VTY commands for each
  separate application of T timer numbers.

Change-Id: Ibd6b1ed7f1bd6e1f2e0fde53352055a4468f23e5

18 files changed, 2998 insertions, 2 deletions

diff --git a/include/Makefile.am b/include/Makefile.am
index 25a6d75f..17f7d1ce 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -47,6 +47,7 @@ nobase_include_HEADERS = \
                        osmocom/core/statistics.h \
                        osmocom/core/strrb.h \
                        osmocom/core/talloc.h \
+                       osmocom/core/tdef.h \
                        osmocom/core/timer.h \
                        osmocom/core/timer_compat.h \
                        osmocom/core/utils.h \
@@ -154,6 +155,7 @@ nobase_include_HEADERS += \
                           osmocom/vty/vector.h \
                           osmocom/vty/vty.h \
                           osmocom/vty/ports.h \
+                          osmocom/vty/tdef_vty.h \
                           osmocom/ctrl/control_vty.h
 endif
 
diff --git a/include/osmocom/core/tdef.h b/include/osmocom/core/tdef.h
new file mode 100644
index 00000000..92b71597
--- /dev/null
+++ b/include/osmocom/core/tdef.h
@@ -0,0 +1,172 @@
+/*! \file tdef.h
+ * API to define Tnnn timers globally and use for FSM state changes.
+ */
+/*
+ * (C) 2018-2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <stdint.h>
+#include <osmocom/core/utils.h>
+
+struct osmo_fsm_inst;
+
+/*! \defgroup Tdef  Tnnn timer configuration
+ * @{
+ * \file tdef.h
+ */
+
+enum osmo_tdef_unit {
+	OSMO_TDEF_S = 0,	/*!< most T are in seconds, keep 0 as default. */
+	OSMO_TDEF_MS,		/*!< milliseconds */
+	OSMO_TDEF_M,		/*!< minutes */
+	OSMO_TDEF_CUSTOM,	/*!< unspecified unit, explained in osmo_tdef.desc. */
+};
+
+extern const struct value_string osmo_tdef_unit_names[];
+/*! \return enum osmo_tdef_unit value as human readable unit letter, or "custom-unit". */
+static inline const char *osmo_tdef_unit_name(enum osmo_tdef_unit val)
+{ return get_value_string(osmo_tdef_unit_names, val); }
+
+/*! Define a GSM timer of the form Tnnn, with unit, default value and doc string.
+ * Typically used as an array with the last entry being left zero-initialized, e.g.:
+ *
+ *         struct osmo_tdef tdefs[] = {
+ *                 { .T=10, .default_val=6, .desc="RR Assignment" },
+ *                 { .T=101, .default_val=10, .desc="inter-BSC Handover MT, HO Request to HO Accept" },
+ *                 { .T=3101, .default_val=3, .desc="RR Immediate Assignment" },
+ *                 {}
+ *         };
+ *
+ * Program initialization should call osmo_tdefs_reset() so that all timers return the default_val, until e.g. the VTY
+ * configuration sets user-defined values (see osmo_tdef_vty_init()).
+ */
+struct osmo_tdef {
+	/*! T1234 number; type corresponds to struct osmo_fsm_inst.T. Negative and zero T numbers are actually possible,
+	 * but be aware that osmo_tdef_fsm_inst_state_chg() interprets T == 0 as "no timer". */
+	const int T;
+	/*! Timeout duration (according to unit), default value; type corresponds to osmo_fsm_inst_state_chg()'s
+	 * timeout_secs argument. Note that osmo_fsm_inst_state_chg() clamps the range. */
+	const unsigned long default_val;
+	const enum osmo_tdef_unit unit;
+	/*! Human readable description. For unit == OSMO_TDEF_CUSTOM, this should include an explanation of the value's
+	 * unit. Best keep this a short one-liner (e.g. for VTY output). */
+	const char *desc;
+	/*! Currently active timeout value, e.g. set by user config. This is the only mutable member: a user may
+	 * configure the timeout value, but neither unit nor any other field. */
+	unsigned long val;
+};
+
+/*! Iterate an array of struct osmo_tdef, the last item should be fully zero, i.e. "{}".
+ * Example:
+ *
+ *     struct osmo_tdef *t;
+ *     osmo_tdef_for_each(t, tdefs) {
+ *             printf("%lu %s %s\n", t->val, osmo_tdef_unit_name(t->unit), t->desc);
+ *     }
+ *
+ * \param[inout] t  A struct osmo_tdef *t used for iteration, will point at the current entry inside the loop scope.
+ * \param[in] tdefs  Array of struct osmo_tdef to iterate, zero-terminated.
+ */
+#define osmo_tdef_for_each(t, tdefs) \
+	for (t = tdefs; t && (t->T || t->default_val || t->desc); t++)
+
+void osmo_tdefs_reset(struct osmo_tdef *tdefs);
+unsigned long osmo_tdef_get(const struct osmo_tdef *tdefs, int T, enum osmo_tdef_unit as_unit,
+			    unsigned long val_if_not_present);
+struct osmo_tdef *osmo_tdef_get_entry(struct osmo_tdef *tdefs, int T);
+
+/*! Using osmo_tdef for osmo_fsm_inst: array entry for a mapping of state numbers to timeout definitions.
+ * For a usage example, see osmo_tdef_get_state_timeout() and test_tdef_state_timeout() in tdef_test.c. */
+struct osmo_tdef_state_timeout {
+	/*! Timer number to match struct osmo_tdef.T, and to pass to osmo_fsm_inst_state_chg(). */
+	int T;
+	/*! If true, call osmo_fsm_inst_state_chg_keep_timer().
+	 * If T == 0, keep previous T number, otherwise also set fi->T. */
+	bool keep_timer;
+};
+
+const struct osmo_tdef_state_timeout *osmo_tdef_get_state_timeout(uint32_t state,
+								  const struct osmo_tdef_state_timeout *timeouts_array);
+
+/*! Call osmo_fsm_inst_state_chg() or osmo_fsm_inst_state_chg_keep_timer(), depending on the timeouts_array, tdefs and
+ * default_timeout.
+ *
+ * A T timer configured in sub-second precision is rounded up to the next full second. A timer in unit =
+ * OSMO_TDEF_CUSTOM is applied as if the unit is in seconds (i.e. this macro does not make sense for custom units!).
+ *
+ * See osmo_tdef_get_state_timeout() and osmo_tdef_get().
+ *
+ * If no T timer is defined for the given state (T == 0), invoke the state change without a timeout.
+ *
+ * Should a T number be defined in timeouts_array that is not defined in tdefs, use default_timeout (in seconds). If
+ * default_timeout is negative, a missing T definition in tdefs instead causes a program abort.
+ *
+ * This is best used by wrapping this function call in a macro suitable for a specific FSM implementation, which can
+ * become as short as: my_fsm_state_chg(fi, NEXT_STATE):
+ *
+ *     #define my_fsm_state_chg(fi, NEXT_STATE) \
+ *             osmo_tdef_fsm_inst_state_chg(fi, NEXT_STATE, my_fsm_timeouts, global_T_defs, 5)
+ *
+ *     my_fsm_state_chg(fi, MY_FSM_STATE_1);
+ *     // -> No timeout configured, will enter state without timeout.
+ *
+ *     my_fsm_state_chg(fi, MY_FSM_STATE_3);
+ *     // T423 configured for this state, will look up T423 in tdefs, or use 5 seconds if unset.
+ *
+ *     my_fsm_state_chg(fi, MY_FSM_STATE_8);
+ *     // keep_timer == true for this state, will invoke osmo_fsm_inst_state_chg_keep_timer().
+ *
+ * \param[inout] fi  osmo_fsm_inst to transition to another state.
+ * \param[in] state  State number to transition to.
+ * \param[in] timeouts_array  Array of struct osmo_tdef_state_timeout[32] to look up state in.
+ * \param[in] tdefs  Array of struct osmo_tdef (last entry zero initialized) to look up T in.
+ * \param[in] default_timeout  If a T is set in timeouts_array, but no timeout value is configured for T, then use this
+ *                             default timeout value as fallback, or pass -1 to abort the program.
+ * \return Return value from osmo_fsm_inst_state_chg() or osmo_fsm_inst_state_chg_keep_timer().
+ */
+#define osmo_tdef_fsm_inst_state_chg(fi, state, timeouts_array, tdefs, default_timeout) \
+	_osmo_tdef_fsm_inst_state_chg(fi, state, timeouts_array, tdefs, default_timeout, \
+				      __FILE__, __LINE__)
+int _osmo_tdef_fsm_inst_state_chg(struct osmo_fsm_inst *fi, uint32_t state,
+				  const struct osmo_tdef_state_timeout *timeouts_array,
+				  const struct osmo_tdef *tdefs, unsigned long default_timeout,
+				  const char *file, int line);
+
+/*! Manage timer definitions in named groups.
+ * This should be defined as an array with the final element kept fully zero-initialized,
+ * to be compatible with osmo_tdef_vty* API. There must not be any tdefs == NULL entries except on the final
+ * zero-initialized entry. */
+struct osmo_tdef_group {
+	const char *name;
+	const char *desc;
+	struct osmo_tdef *tdefs;
+};
+
+/*! Iterate an array of struct osmo_tdef_group, the last item should be fully zero, i.e. "{}".
+ * \param[inout] g  A struct osmo_tdef_group *g used for iteration, will point at the current entry inside the loop scope.
+ * \param[in] tdefs  Array of struct osmo_tdef_group to iterate, zero-terminated.
+ */
+#define osmo_tdef_groups_for_each(g, tdef_groups) \
+	for (g = tdef_groups; g && g->tdefs; g++)
+
+/*! @} */
diff --git a/include/osmocom/vty/tdef_vty.h b/include/osmocom/vty/tdef_vty.h
new file mode 100644
index 00000000..f55239ad
--- /dev/null
+++ b/include/osmocom/vty/tdef_vty.h
@@ -0,0 +1,67 @@
+/*! \file tdef_vty.h
+ * API to configure osmo_tdef Tnnn timers from VTY configuration.
+ */
+/* (C) 2018-2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <stdint.h>
+
+struct vty;
+
+/*! \defgroup Tdef_VTY  Tnnn timer VTY configuration
+ * @{
+ * \file tdef_vty.h
+ */
+
+struct osmo_tdef;
+struct osmo_tdef_group;
+
+#define OSMO_TDEF_VTY_ARG_T "TNNNN"
+#define OSMO_TDEF_VTY_DOC_T "T-number, optionally preceded by 't' or 'T'.\n"
+#define OSMO_TDEF_VTY_ARG_T_OPTIONAL "[" OSMO_TDEF_VTY_ARG_T "]"
+
+#define OSMO_TDEF_VTY_ARG_VAL "(<0-2147483647>|default)"
+#define OSMO_TDEF_VTY_DOC_VAL "New timer value\n" "Set to default timer value\n"
+#define OSMO_TDEF_VTY_ARG_VAL_OPTIONAL "[" OSMO_TDEF_VTY_ARG_VAL "]"
+
+#define OSMO_TDEF_VTY_ARG_SET	OSMO_TDEF_VTY_ARG_T " " OSMO_TDEF_VTY_ARG_VAL
+#define OSMO_TDEF_VTY_DOC_SET	OSMO_TDEF_VTY_DOC_T OSMO_TDEF_VTY_DOC_VAL
+#define OSMO_TDEF_VTY_ARG_SET_OPTIONAL	OSMO_TDEF_VTY_ARG_T_OPTIONAL " " OSMO_TDEF_VTY_ARG_VAL_OPTIONAL
+
+int osmo_tdef_vty_set_cmd(struct vty *vty, struct osmo_tdef *tdefs, const char **args);
+int osmo_tdef_vty_show_cmd(struct vty *vty, struct osmo_tdef *tdefs, const char *T_arg,
+			   const char *prefix_fmt, ...);
+void osmo_tdef_vty_write(struct vty *vty, struct osmo_tdef *tdefs,
+			 const char *prefix_fmt, ...);
+
+void osmo_tdef_vty_out_one(struct vty *vty, struct osmo_tdef *t, const char *prefix_fmt, ...);
+void osmo_tdef_vty_out_all(struct vty *vty, struct osmo_tdef *tdefs, const char *prefix_fmt, ...);
+
+void osmo_tdef_vty_out_one_va(struct vty *vty, struct osmo_tdef *t, const char *prefix_fmt, va_list va);
+void osmo_tdef_vty_out_all_va(struct vty *vty, struct osmo_tdef *tdefs, const char *prefix_fmt, va_list va);
+
+struct osmo_tdef *osmo_tdef_vty_parse_T_arg(struct vty *vty, struct osmo_tdef *tdefs, const char *osmo_tdef_str);
+unsigned long osmo_tdef_vty_parse_val_arg(const char *val_arg, unsigned long default_val);
+
+void osmo_tdef_vty_groups_init(enum node_type parent_node, struct osmo_tdef_group *groups);
+void osmo_tdef_vty_groups_write(struct vty *vty, const char *indent);
+
+/*! @} */
diff --git a/src/Makefile.am b/src/Makefile.am
index 6840f798..27ab7026 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -23,7 +23,8 @@ libosmocore_la_SOURCES = timer.c timer_gettimeofday.c timer_clockgettime.c \
 			 loggingrb.c crc8gen.c crc16gen.c crc32gen.c crc64gen.c \
 			 macaddr.c stat_item.c stats.c stats_statsd.c prim.c \
 			 conv_acc.c conv_acc_generic.c sercomm.c prbs.c \
-			 isdnhdlc.c
+			 isdnhdlc.c \
+			 tdef.c
 
 if HAVE_SSSE3
 libosmocore_la_SOURCES += conv_acc_sse.c
diff --git a/src/fsm.c b/src/fsm.c
index 0d31f872..6e15ab70 100644
--- a/src/fsm.c
+++ b/src/fsm.c
@@ -498,6 +498,10 @@ static int state_chg(struct osmo_fsm_inst *fi, uint32_t new_state,
  *  timer_cb. If passing timeout_secs == 0, it is recommended to also pass T ==
  *  0, so that fi->T is reset to 0 when no timeout is invoked.
  *
+ *  See also osmo_tdef_fsm_inst_state_chg() from the osmo_tdef API, which
+ *  provides a unified way to configure and apply GSM style Tnnnn timers to FSM
+ *  state transitions.
+ *
  *  Range: since time_t's maximum value is not well defined in a cross platform
  *  way, clamp timeout_secs to the maximum of the signed 32bit range, or roughly
  *  68 years (float(0x7fffffff) / (60. * 60 * 24 * 365.25) = 68.0497). Thus
diff --git a/src/tdef.c b/src/tdef.c
new file mode 100644
index 00000000..7e79d680
--- /dev/null
+++ b/src/tdef.c
@@ -0,0 +1,282 @@
+/*! \file tdef.c
+ * Implementation to define Tnnn timers globally and use for FSM state changes.
+ */
+/*
+ * (C) 2018-2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <limits.h>
+
+#include <osmocom/core/fsm.h>
+#include <osmocom/core/tdef.h>
+
+/*! \addtogroup Tdef
+ *
+ * Implementation to define Tnnn timers globally and use for FSM state changes.
+ *
+ * See also \ref Tdef_VTY
+ *
+ * osmo_tdef provides:
+ *
+ * - a list of Tnnnn (GSM) timers with description, unit and default value.
+ * - vty UI to allow users to configure non-default timeouts.
+ * - API to tie T timers to osmo_fsm states and set them on state transitions.
+ *
+ * - a few standard units (minute, second, millisecond) as well as a custom unit
+ *   (which relies on the timer's human readable description to indicate the
+ *   meaning of the value).
+ * - conversion for standard units: for example, some GSM timers are defined in
+ *   minutes, while our FSM definitions need timeouts in seconds. Conversion is
+ *   for convenience only and can be easily avoided via the custom unit.
+ *
+ * By keeping separate osmo_tdef arrays, several groups of timers can be kept
+ * separately. The VTY tests in tests/tdef/ showcase different schemes:
+ *
+ * - \ref tests/vty/tdef_vty_test_config_root.c:
+ *   Keep several timer definitions in separately named groups: showcase the
+ *   osmo_tdef_vty_groups*() API. Each timer group exists exactly once.
+ *
+ * - \ref tests/vty/tdef_vty_test_config_subnode.c:
+ *   Keep a single list of timers without separate grouping.
+ *   Put this list on a specific subnode below the CONFIG_NODE.
+ *   There could be several separate subnodes with timers like this, i.e.
+ *   continuing from this example, sets of timers could be separated by placing
+ *   timers in specific config subnodes instead of using the global group name.
+ *
+ * - \ref tests/vty/tdef_vty_test_dynamic.c:
+ *   Dynamically allocate timer definitions per each new created object.
+ *   Thus there can be an arbitrary number of independent timer definitions, one
+ *   per allocated object.
+ *
+ * osmo_tdef was introduced because:
+ *
+ * - without osmo_tdef, each invocation of osmo_fsm_inst_state_chg() needs to be
+ *   programmed with the right timeout value, for all code paths that invoke this
+ *   state change. It is a likely source of errors to get one of them wrong.  By
+ *   defining a T timer exactly for an FSM state, the caller can merely invoke the
+ *   state change and trust on the original state definition to apply the correct
+ *   timeout.
+ *
+ * - it is helpful to have a standardized config file UI to provide user
+ *   configurable timeouts, instead of inventing new VTY commands for each
+ *   separate application of T timer numbers. See \ref tdef_vty.h.
+ *
+ * @{
+ * \file tdef.c
+ */
+
+/*! a = return_val * b. \return 0 if factor is below 1. */
+static unsigned long osmo_tdef_factor(enum osmo_tdef_unit a, enum osmo_tdef_unit b)
+{
+	if (b == a
+	    || b == OSMO_TDEF_CUSTOM || a == OSMO_TDEF_CUSTOM)
+		return 1;
+
+	switch (b) {
+	case OSMO_TDEF_MS:
+		switch (a) {
+		case OSMO_TDEF_S:
+			return 1000;
+		case OSMO_TDEF_M:
+			return 60*1000;
+		default:
+			return 0;
+		}
+	case OSMO_TDEF_S:
+		switch (a) {
+		case OSMO_TDEF_M:
+			return 60;
+		default:
+			return 0;
+		}
+	default:
+		return 0;
+	}
+}
+
+/*! \return val in unit to_unit, rounded up to the next integer value and clamped to ULONG_MAX, or 0 if val == 0. */
+static unsigned long osmo_tdef_round(unsigned long val, enum osmo_tdef_unit from_unit, enum osmo_tdef_unit to_unit)
+{
+	unsigned long f;
+	if (!val)
+		return 0;
+
+	f = osmo_tdef_factor(from_unit, to_unit);
+	if (f == 1)
+		return val;
+	if (f < 1) {
+		f = osmo_tdef_factor(to_unit, from_unit);
+		return (val / f) + (val % f? 1 : 0);
+	}
+	/* range checking */
+	if (f > (ULONG_MAX / val))
+		return ULONG_MAX;
+	return val * f;
+}
+
+/*! Set all osmo_tdef values to the default_val.
+ * It is convenient to define a tdefs array by setting only the default_val, and calling osmo_tdefs_reset() once for
+ * program startup. (See also osmo_tdef_vty_init())
+ * \param[in] tdefs  Array of timer definitions, last entry being fully zero.
+ */
+void osmo_tdefs_reset(struct osmo_tdef *tdefs)
+{
+	struct osmo_tdef *t;
+	osmo_tdef_for_each(t, tdefs)
+		t->val = t->default_val;
+}
+
+/*! Return the value of a T timer from a list of osmo_tdef, in the given unit.
+ * If no such timer is defined, return the default value passed, or abort the program if default < 0.
+ *
+ * Round up any value match as_unit: 1100 ms as OSMO_TDEF_S becomes 2 seconds, as OSMO_TDEF_M becomes one minute.
+ * However, always return a value of zero as zero (0 ms as OSMO_TDEF_M still is 0 m).
+ *
+ * Range: even though the value range is unsigned long here, in practice, using ULONG_MAX as value for a timeout in
+ * seconds may actually wrap to negative or low timeout values (e.g. in struct timeval). It is recommended to stay below
+ * INT_MAX seconds. See also osmo_fsm_inst_state_chg().
+ *
+ * Usage example:
+ *
+ * 	struct osmo_tdef global_T_defs[] = {
+ * 		{ .T=7, .default_val=50, .desc="Water Boiling Timeout" },  // default is .unit=OSMO_TDEF_S == 0
+ * 		{ .T=8, .default_val=300, .desc="Tea brewing" },
+ * 		{ .T=9, .default_val=5, .unit=OSMO_TDEF_M, .desc="Let tea cool down before drinking" },
+ * 		{ .T=10, .default_val=20, .unit=OSMO_TDEF_M, .desc="Forgot to drink tea while it's warm" },
+ * 		{}  //  <-- important! last entry shall be zero
+ * 	};
+ * 	osmo_tdefs_reset(global_T_defs); // make all values the default
+ * 	osmo_tdef_vty_init(global_T_defs, CONFIG_NODE);
+ *
+ * 	val = osmo_tdef_get(global_T_defs, 7, OSMO_TDEF_S, -1); // -> 50
+ * 	sleep(val);
+ *
+ * 	val = osmo_tdef_get(global_T_defs, 7, OSMO_TDEF_M, -1); // 50 seconds becomes 1 minute -> 1
+ * 	sleep_minutes(val);
+ *
+ * 	val = osmo_tdef_get(global_T_defs, 99, OSMO_TDEF_S, 3); // not defined, returns 3
+ *
+ * 	val = osmo_tdef_get(global_T_defs, 99, OSMO_TDEF_S, -1); // not defined, program aborts!
+ *
+ * \param[in] tdefs  Array of timer definitions, last entry must be fully zero initialized.
+ * \param[in] T  Timer number to get the value for.
+ * \param[in] as_unit  Return timeout value in this unit.
+ * \param[in] val_if_not_present  Fallback value to return if no timeout is defined.
+ * \return Timeout value in the unit given by as_unit, rounded up if necessary, or val_if_not_present.
+ */
+unsigned long osmo_tdef_get(const struct osmo_tdef *tdefs, int T, enum osmo_tdef_unit as_unit, unsigned long val_if_not_present)
+{
+	const struct osmo_tdef *t = osmo_tdef_get_entry((struct osmo_tdef*)tdefs, T);
+	if (!t) {
+		OSMO_ASSERT(val_if_not_present >= 0);
+		return val_if_not_present;
+	}
+	return osmo_tdef_round(t->val, t->unit, as_unit);
+}
+
+/*! Find tdef entry matching T.
+ * This is useful for manipulation, which is usually limited to the VTY configuration. To retrieve a timeout value,
+ * most callers probably should use osmo_tdef_get() instead.
+ * \param[in] tdefs  Array of timer definitions, last entry being fully zero.
+ * \param[in] T  Timer number to get the entry for.
+ * \return osmo_tdef entry matching T in given array, or NULL if no match is found.
+ */
+struct osmo_tdef *osmo_tdef_get_entry(struct osmo_tdef *tdefs, int T)
+{
+	struct osmo_tdef *t;
+	osmo_tdef_for_each(t, tdefs) {
+		if (t->T == T)
+			return t;
+	}
+	return NULL;
+}
+
+/*! Using osmo_tdef for osmo_fsm_inst: find a given state's osmo_tdef_state_timeout entry.
+ *
+ * The timeouts_array shall contain exactly 32 elements, regardless whether only some of them are actually populated
+ * with nonzero values. 32 corresponds to the number of states allowed by the osmo_fsm_* API. Lookup is by array index.
+ * Not populated entries imply a state change invocation without timeout.
+ *
+ * For example:
+ *
+ * 	struct osmo_tdef_state_timeout my_fsm_timeouts[32] = {
+ * 		[MY_FSM_STATE_3] = { .T = 423 }, // look up timeout configured for T423
+ * 		[MY_FSM_STATE_7] = { .T = 235 },
+ * 		[MY_FSM_STATE_8] = { .keep_timer = true }, // keep previous state's T number, continue timeout.
+ * 		// any state that is omitted will remain zero == no timeout
+ *	};
+ *	osmo_tdef_get_state_timeout(MY_FSM_STATE_0, &my_fsm_timeouts) -> NULL,
+ *	osmo_tdef_get_state_timeout(MY_FSM_STATE_7, &my_fsm_timeouts) -> { .T = 235 }
+ *
+ * The intention is then to obtain the timer like osmo_tdef_get(global_T_defs, T=235); see also
+ * fsm_inst_state_chg_T() below.
+ *
+ * \param[in] state  State constant to look up.
+ * \param[in] timeouts_array  Array[32] of struct osmo_tdef_state_timeout defining which timer number to use per state.
+ * \return A struct osmo_tdef_state_timeout entry, or NULL if that entry is zero initialized.
+ */
+const struct osmo_tdef_state_timeout *osmo_tdef_get_state_timeout(uint32_t state, const struct osmo_tdef_state_timeout *timeouts_array)
+{
+	const struct osmo_tdef_state_timeout *t;
+	OSMO_ASSERT(state < 32);
+	t = &timeouts_array[state];
+	if (!t->keep_timer && !t->T)
+		return NULL;
+	return t;
+}
+
+/*! See invocation macro osmo_tdef_fsm_inst_state_chg() instead.
+ * \param[in] file  Source file name, like __FILE__.
+ * \param[in] line  Source file line number, like __LINE__.
+ */
+int _osmo_tdef_fsm_inst_state_chg(struct osmo_fsm_inst *fi, uint32_t state,
+				  const struct osmo_tdef_state_timeout *timeouts_array,
+				  const struct osmo_tdef *tdefs, unsigned long default_timeout,
+				  const char *file, int line)
+{
+	const struct osmo_tdef_state_timeout *t = osmo_tdef_get_state_timeout(state, timeouts_array);
+	unsigned long val;
+
+	/* No timeout defined for this state? */
+	if (!t)
+		return _osmo_fsm_inst_state_chg(fi, state, 0, 0, file, line);
+
+	if (t->keep_timer) {
+		int rc = _osmo_fsm_inst_state_chg_keep_timer(fi, state, file, line);
+		if (t->T && !rc)
+			fi->T = t->T;
+		return rc;
+	}
+
+	val = osmo_tdef_get(tdefs, t->T, OSMO_TDEF_S, default_timeout);
+	return _osmo_fsm_inst_state_chg(fi, state, val, t->T, file, line);
+}
+
+const struct value_string osmo_tdef_unit_names[] = {
+	{ OSMO_TDEF_S, "s" },
+	{ OSMO_TDEF_MS, "ms" },
+	{ OSMO_TDEF_M, "m" },
+	{ OSMO_TDEF_CUSTOM, "custom-unit" },
+	{}
+};
+
+/*! @} */
diff --git a/src/vty/Makefile.am b/src/vty/Makefile.am
index 2e494982..cdde0fa5 100644
--- a/src/vty/Makefile.am
+++ b/src/vty/Makefile.am
@@ -11,7 +11,8 @@ lib_LTLIBRARIES = libosmovty.la
 
 libosmovty_la_SOURCES = buffer.c command.c vty.c vector.c utils.c \
 			telnet_interface.c logging_vty.c stats_vty.c \
-			fsm_vty.c talloc_ctx_vty.c
+			fsm_vty.c talloc_ctx_vty.c \
+			tdef_vty.c
 libosmovty_la_LDFLAGS = -version-info $(LIBVERSION) -no-undefined
 libosmovty_la_LIBADD = $(top_builddir)/src/libosmocore.la $(TALLOC_LIBS)
 endif
diff --git a/src/vty/tdef_vty.c b/src/vty/tdef_vty.c
new file mode 100644
index 00000000..1c6af70a
--- /dev/null
+++ b/src/vty/tdef_vty.c
@@ -0,0 +1,372 @@
+/*! \file tdef_vty.c
+ * Implementation to configure osmo_tdef Tnnn timers from VTY configuration.
+ */
+/* (C) 2018-2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <string.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <limits.h>
+
+#include <osmocom/vty/vty.h>
+#include <osmocom/vty/command.h>
+#include <osmocom/vty/tdef_vty.h>
+#include <osmocom/core/tdef.h>
+
+/*! \addtogroup Tdef_VTY
+ *
+ * VTY API for \ref Tdef.
+ *
+ * @{
+ * \file tdef_vty.c
+ */
+
+/*! Parse an argument like "T1234", "t1234" or "1234", as from OSMO_TDEF_VTY_ARG_T.
+ * \param[in] vty  VTY context for vty_out() of error messages.
+ * \param[in] tdefs  Array of timer definitions to look up T timer.
+ * \param[in] T_str  Argument string. It is not validated, expected to be checked by VTY input.
+ * \return the corresponding osmo_tdef entry from the tdefs array, or NULL if no such entry exists.
+ */
+struct osmo_tdef *osmo_tdef_vty_parse_T_arg(struct vty *vty, struct osmo_tdef *tdefs, const char *T_str)
+{
+	long l;
+	int T;
+	struct osmo_tdef *t;
+	char *endptr;
+	const char *T_nr_str;
+
+	if (!tdefs) {
+		vty_out(vty, "%% Error: no timers found%s", VTY_NEWLINE);
+		return NULL;
+	}
+
+	T_nr_str = T_str;
+	if (T_nr_str[0] == 't' || T_nr_str[0] == 'T')
+		T_nr_str++;
+
+	errno = 0;
+	l = strtol(T_nr_str, &endptr, 10);
+	if (errno || *endptr || l > INT_MAX) {
+		vty_out(vty, "%% No such timer: '%s'%s", T_str, VTY_NEWLINE);
+		return NULL;
+	}
+	T = l;
+
+	t = osmo_tdef_get_entry(tdefs, T);
+	if (!t)
+		vty_out(vty, "%% No such timer: T%d%s", T, VTY_NEWLINE);
+	return t;
+}
+
+/*! Parse an argument of the form "(0-2147483647|default)", as from OSMO_TDEF_VTY_ARG_VAL.
+ * \param[in] val_arg  Argument string (not format checked).
+ * \param[in] default_val  Value to return in case of val_arg being "default".
+ * \return Parsed value or default_val.
+ */
+unsigned long osmo_tdef_vty_parse_val_arg(const char *val_arg, unsigned long default_val)
+{
+        if (!strcmp(val_arg, "default"))
+                return default_val;
+	return atoll(val_arg);
+}
+
+/*! Apply a timer configuration from VTY argument strings.
+ * Employ both osmo_tdef_vty_parse_T_arg() and osmo_tdef_vty_parse_val_arg() to configure a T timer in an array of
+ * tdefs. Evaluate two arguments, a "T1234" argument and a "(0-2147483647|default)" argument, as from
+ * OSMO_TDEF_VTY_ARGS.  If the T timer given in the first argument is found in tdefs, set it to the value given in the
+ * second argument.
+ * \param[in] vty  VTY context for vty_out() of error messages.
+ * \param[in] tdefs  Array of timer definitions to look up T timer.
+ * \param[in] args  Array of string arguments like { "T1234", "23" }.
+ * \return CMD_SUCCESS, or CMD_WARNING if no such timer is found in tdefs.
+ */
+int osmo_tdef_vty_set_cmd(struct vty *vty, struct osmo_tdef *tdefs, const char **args)
+{
+	const char *T_arg = args[0];
+	const char *val_arg = args[1];
+	struct osmo_tdef *t = osmo_tdef_vty_parse_T_arg(vty, tdefs, T_arg);
+	if (!t)
+		return CMD_WARNING;
+	t->val = osmo_tdef_vty_parse_val_arg(val_arg, t->default_val);
+	return CMD_SUCCESS;
+}
+
+/*! Output one or all timers to the VTY, as for a VTY command like 'show timer [TNNNN]'.
+ * If T_arg is NULL, print all timers in tdefs to the VTY.
+ * If T_arg is not NULL, employ osmo_tdef_vty_parse_T_arg() to select one timer from tdefs and print only that to the
+ * VTY.
+ * \param[in] vty  VTY context for vty_out() of error messages.
+ * \param[in] tdefs  Array of timer definitions.
+ * \param[in] T_arg  Argument string like "T1234", or NULL.
+ * \param[in] prefix_fmt  Arbitrary string to start each line with, with variable printf like arguments.
+ * \return CMD_SUCCESS, or CMD_WARNING if no such timer is found in tdefs.
+ */
+int osmo_tdef_vty_show_cmd(struct vty *vty, struct osmo_tdef *tdefs, const char *T_arg,
+			   const char *prefix_fmt, ...)
+{
+	va_list va;
+	if (T_arg) {
+		struct osmo_tdef *t = osmo_tdef_vty_parse_T_arg(vty, tdefs, T_arg);
+		if (!t)
+			return CMD_WARNING;
+		va_start(va, prefix_fmt);
+		osmo_tdef_vty_out_one_va(vty, t, prefix_fmt, va);
+		va_end(va);
+	} else {
+		va_start(va, prefix_fmt);
+		osmo_tdef_vty_out_all_va(vty, tdefs, prefix_fmt, va);
+		va_end(va);
+	}
+	return CMD_SUCCESS;
+}
+
+/*! Write to VTY the current status of one timer.
+ * \param[in] vty  VTY context for vty_out().
+ * \param[in] t  The timer to print.
+ * \param[in] prefix_fmt  Arbitrary string to start each line with, with variable vprintf like arguments.
+ * \param[in] va  va_list instance. As always, call va_start() before, and va_end() after this call.
+ */
+void osmo_tdef_vty_out_one_va(struct vty *vty, struct osmo_tdef *t, const char *prefix_fmt, va_list va)
+{
+	if (!t) {
+		vty_out(vty, "%% Error: no such timer%s", VTY_NEWLINE);
+		return;
+	}
+	if (prefix_fmt)
+		vty_out_va(vty, prefix_fmt, va);
+	vty_out(vty, "T%d = %lu%s%s\t%s (default: %lu%s%s)%s",
+		t->T, t->val,
+		t->unit == OSMO_TDEF_CUSTOM ? "" : " ", t->unit == OSMO_TDEF_CUSTOM ? "" : osmo_tdef_unit_name(t->unit),
+		t->desc, t->default_