State Machine Framework

Overview

The State Machine Framework (SMF) is an application agnostic framework that provides an easy way for developers to integrate state machines into their application. The framework can be added to any project by enabling the CONFIG_SMF option.

State Creation

A state is represented by three functions, where one function implements the Entry actions, another function implements the Run actions, and the last function implements the Exit actions. The prototype for these functions is as follows: void funct(void *obj), where the obj parameter is a user defined structure that has the state machine context, struct smf_ctx, as its first member. For example:

struct user_object {
   struct smf_ctx ctx;
   /* All User Defined Data Follows */
};

The struct smf_ctx member must be first because the state machine framework’s functions casts the user defined object to the struct smf_ctx type with the following macro: SMF_CTX(o)

For example instead of doing this (struct smf_ctx *)&user_obj, you could use SMF_CTX(&user_obj).

By default, a state can have no ancestor states, resulting in a flat state machine. But to enable the creation of a hierarchical state machine, the CONFIG_SMF_ANCESTOR_SUPPORT option must be enabled.

The following macro can be used for easy state creation:

  • SMF_CREATE_STATE Create a state

NOTE: The SMF_CREATE_STATE macro takes an additional parameter when CONFIG_SMF_ANCESTOR_SUPPORT is enabled.

State Machine Creation

A state machine is created by defining a table of states that’s indexed by an enum. For example, the following creates three flat states:

enum demo_state { S0, S1, S2 };

const struct smf_state demo_states[] = {
   [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit),
   [S1] = SMF_CREATE_STATE(s1_entry, s1_run, s1_exit),
   [S2] = SMF_CREATE_STATE(s2_entry, s2_run, s2_exit)
};

And this example creates three hierarchical states:

enum demo_state { S0, S1, S2 };

const struct smf_state demo_states[] = {
   [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit, parent_s0),
   [S1] = SMF_CREATE_STATE(s1_entry, s1_run, s1_exit, parent_s12),
   [S2] = SMF_CREATE_STATE(s2_entry, s2_run, s2_exit, parent_s12)
};

To set the initial state, the smf_set_initial function should be called. It has the following prototype: void smf_set_initial(smf_ctx *ctx, smf_state *state)

To transition from one state to another, the smf_set_state function is used and it has the following prototype: void smf_set_state(smf_ctx *ctx, smf_state *state)

NOTE: While the state machine is running, smf_set_state should only be called from the Entry and Run functions. Calling smf_set_state from the Exit functions doesn’t make sense and will generate a warning.

State Machine Execution

To run the state machine, the smf_run_state function should be called in some application dependent way. An application should cease calling smf_run_state if it returns a non-zero value. The function has the following prototype: int32_t smf_run_state(smf_ctx *ctx)

State Machine Termination

To terminate the state machine, the smf_terminate function should be called. It can be called from the entry, run, or exit action. The function takes a non-zero user defined value that’s returned by the smf_run_state function. The function has the following prototype: void smf_terminate(smf_ctx *ctx, int32_t val)

Flat State Machine Example

This example turns the following state diagram into code using the SMF, where the initial state is S0.

digraph smf_flat { node [style=rounded]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; STATE_S2 [shape = box]; init -> STATE_S0; STATE_S0 -> STATE_S1; STATE_S1 -> STATE_S2; STATE_S2 -> STATE_S0; }

Flat state machine diagram

Code:

#include <zephyr/smf.h>

/* Forward declaration of state table */
static const struct smf_state demo_states[];

/* List of demo states */
enum demo_state { S0, S1, S2 };

/* User defined object */
struct s_object {
        /* This must be first */
        struct smf_ctx ctx;

        /* Other state specific data add here */
} s_obj;

/* State S0 */
static void s0_entry(void *o)
{
        /* Do something */
}
static void s0_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S1]);
}
static void s0_exit(void *o)
{
        /* Do something */
}

/* State S1 */
static void s1_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S2]);
}
static void s1_exit(void *o)
{
        /* Do something */
}

/* State S2 */
static void s2_entry(void *o)
{
        /* Do something */
}
static void s2_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S0]);
}

/* Populate state table */
static const struct smf_state demo_states[] = {
        [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit),
        /* State S1 does not have an entry action */
        [S1] = SMF_CREATE_STATE(NULL, s1_run, s1_exit),
        /* State S2 does not have an exit action */
        [S2] = SMF_CREATE_STATE(s2_entry, s2_run, NULL),
};

int main(void)
{
        int32_t ret;

        /* Set initial state */
        smf_set_initial(SMF_CTX(&s_obj), &demo_states[S0]);

        /* Run the state machine */
        while(1) {
                /* State machine terminates if a non-zero value is returned */
                ret = smf_run_state(SMF_CTX(&s_obj));
                if (ret) {
                        /* handle return code and terminate state machine */
                        break;
                }
                k_msleep(1000);
        }
}

Hierarchical State Machine Example

This example turns the following state diagram into code using the SMF, where S0 and S1 share a parent state and S0 is the initial state.

digraph smf_hierarchical { node [style = rounded]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; STATE_S2 [shape = box]; subgraph cluster_0 { label = "PARENT"; style = rounded; STATE_S0 -> STATE_S1; } init -> STATE_S0; STATE_S1 -> STATE_S2; STATE_S2 -> STATE_S0; }

Hierarchical state machine diagram

Code:

#include <zephyr/smf.h>

/* Forward declaration of state table */
static const struct smf_state demo_states[];

/* List of demo states */
enum demo_state { PARENT, S0, S1, S2 };

/* User defined object */
struct s_object {
        /* This must be first */
        struct smf_ctx ctx;

        /* Other state specific data add here */
} s_obj;

/* Parent State */
static void parent_entry(void *o)
{
        /* Do something */
}
static void parent_exit(void *o)
{
        /* Do something */
}

/* State S0 */
static void s0_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S1]);
}

/* State S1 */
static void s1_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S2]);
}

/* State S2 */
static void s2_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S0]);
}

/* Populate state table */
static const struct smf_state demo_states[] = {
        /* Parent state does not have a run action */
        [PARENT] = SMF_CREATE_STATE(parent_entry, NULL, parent_exit, NULL),
        /* Child states do not have entry or exit actions */
        [S0] = SMF_CREATE_STATE(NULL, s0_run, NULL, &demo_states[PARENT]),
        [S1] = SMF_CREATE_STATE(NULL, s1_run, NULL, &demo_states[PARENT]),
        /* State S2 do ot have entry or exit actions and no parent */
        [S2] = SMF_CREATE_STATE(NULL, s2_run, NULL, NULL),
};

int main(void)
{
        int32_t ret;

        /* Set initial state */
        smf_set_initial(SMF_CTX(&s_obj), &demo_states[S0]);

        /* Run the state machine */
        while(1) {
                /* State machine terminates if a non-zero value is returned */
                ret = smf_run_state(SMF_CTX(&s_obj));
                if (ret) {
                        /* handle return code and terminate state machine */
                        break;
                }
                k_msleep(1000);
        }
}
When designing hierarchical state machines, the following should be considered:

  • Ancestor entry actions are executed before the sibling entry actions. For example, the parent_entry function is called before the s0_entry function.

  • Transitioning from one sibling to another with a shared ancestry does not re-execute the ancestor's entry action or execute the exit action. For example, the parent_entry function is not called when transitioning from S0 to S1, nor is the parent_exit function called.

  • Ancestor exit actions are executed after the sibling exit actions. For example, the s1_exit function is called before the parent_exit function is called.

  • The parent_run function only executes if the child_run function returns without transitioning to another state, ie. calling smf_set_state.

Event Driven State Machine Example

Events are not explicitly part of the State Machine Framework but an event driven state machine can be implemented using Zephyr Events.

digraph smf_flat { node [style=rounded]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; init -> STATE_S0; STATE_S0 -> STATE_S1 [label = "BTN EVENT"]; STATE_S1 -> STATE_S0 [label = "BTN EVENT"]; }

Event driven state machine diagram

Code:

#include <zephyr/kernel.h>
#include <zephyr/drivers/gpio.h>
#include <zephyr/smf.h>

#define SW0_NODE        DT_ALIAS(sw0)

/* List of events */
#define EVENT_BTN_PRESS BIT(0)

static const struct gpio_dt_spec button =
        GPIO_DT_SPEC_GET_OR(SW0_NODE, gpios, {0});

static struct gpio_callback button_cb_data;

/* Forward declaration of state table */
static const struct smf_state demo_states[];

/* List of demo states */
enum demo_state { S0, S1 };

/* User defined object */
struct s_object {
        /* This must be first */
        struct smf_ctx ctx;

        /* Events */
        struct k_event smf_event;
        int32_t events;

        /* Other state specific data add here */
} s_obj;

/* State S0 */
static void s0_entry(void *o)
{
        printk("STATE0\n");
}

static void s0_run(void *o)
{
        struct s_object *s = (struct s_object *)o;

        /* Change states on Button Press Event */
        if (s->events & EVENT_BTN_PRESS) {
                smf_set_state(SMF_CTX(&s_obj), &demo_states[S1]);
        }
}

/* State S1 */
static void s1_entry(void *o)
{
        printk("STATE1\n");
}

static void s1_run(void *o)
{
        struct s_object *s = (struct s_object *)o;

        /* Change states on Button Press Event */
        if (s->events & EVENT_BTN_PRESS) {
                smf_set_state(SMF_CTX(&s_obj), &demo_states[S0]);
        }
}

/* Populate state table */
static const struct smf_state demo_states[] = {
        [S0] = SMF_CREATE_STATE(s0_entry, s0_run, NULL),
        [S1] = SMF_CREATE_STATE(s1_entry, s1_run, NULL),
};

void button_pressed(const struct device *dev,
                struct gpio_callback *cb, uint32_t pins)
{
        /* Generate Button Press Event */
        k_event_post(&s_obj.smf_event, EVENT_BTN_PRESS);
}

int main(void)
{
        int ret;

        if (!gpio_is_ready_dt(&button)) {
                printk("Error: button device %s is not ready\n",
                        button.port->name);
                return;
        }

        ret = gpio_pin_configure_dt(&button, GPIO_INPUT);
        if (ret != 0) {
                printk("Error %d: failed to configure %s pin %d\n",
                        ret, button.port->name, button.pin);
                return;
        }

        ret = gpio_pin_interrupt_configure_dt(&button,
                GPIO_INT_EDGE_TO_ACTIVE);
        if (ret != 0) {
                printk("Error %d: failed to configure interrupt on %s pin %d\n",
                        ret, button.port->name, button.pin);
                return;
        }

        gpio_init_callback(&button_cb_data, button_pressed, BIT(button.pin));
        gpio_add_callback(button.port, &button_cb_data);

        /* Initialize the event */
        k_event_init(&s_obj.smf_event);

        /* Set initial state */
        smf_set_initial(SMF_CTX(&s_obj), &demo_states[S0]);

        /* Run the state machine */
        while(1) {
                /* Block until an event is detected */
                s_obj.events = k_event_wait(&s_obj.smf_event,
                                EVENT_BTN_PRESS, true, K_FOREVER);

                /* State machine terminates if a non-zero value is returned */
                ret = smf_run_state(SMF_CTX(&s_obj));
                if (ret) {
                        /* handle return code and terminate state machine */
                        break;
                }
        }
}