Threading Considerations

Definitions

Thread

In the context of this document, a thread is any sequence of CPU instructions. In "bare-metal" implementations (i.e. no OS), threads include:

the main thread executing a while(1) loop that runs the system, and
interrupts.

When running under an OS, threads include:

each task (or process),
interrupts, and
advanced OSes can have multiple "execution threads" within a processes.

Atomic Operation

If operation X is atomic, that means that any thread observing the operation will see it either as not yet started, or as completed, and not in any state that is partially completed.

If other threads can see the operation in a partially performed state, or interfere with it, then operation X is not atomic.

If an atomic operation can fail, its implementation must return the the resource back to the state before the operation was started. To other threads it must appear as though the operation had not yet started.

Atomic Data

A datum (i.e. contents of a variable or data structure) is atomic if any thread observing it will always see it in a consistent state, as if operations on it have either not yet started, or have been successfully completed, and not in a state that is partially changed or otherwise inconsistent.

When reading or writing a value is started and completed with 1 CPU instruction, it is automatically atomic, since it can never been seen in an inconsistent (partially-changed) state, even from a CPU interrupt or exception. With such values, no special protection is required by programmers to ensure all threads see it in a consistent state.

LVGL and Threads

LVGL is not thread-safe.

That means it is the programmer's responsibility to see that no LVGL function is called while another LVGL call is in progress in another thread. This includes calls to lv_timer_handler().

Note

Assuming the above is the case, it is safe to call LVGL functions in

event callbacks, and in
timer callbacks

because the thread that drives both of these is the thread that calls lv_timer_handler().

Reason:

LVGL manages many complex data structures, and those structures are "system resources" that must be protected from being "seen" by other threads in an inconsistent state. A high percentage LVGL functions (functions that start with lv_) either read from or change those data structures. Those that change them place the data in an inconsistent state during execution (because such changes are multi-step sequences), but return them to a consistent state before those functions return. For this reason, execution of each LVGL function must be allowed to complete before any other LVGL function is started.

Exceptions to the Above:

These two LVGL functions may be called from any thread:

lv_tick_inc() (see Tick Interface for more information) and
lv_display_flush_ready() (see Flush Callback for more information)

The reason this is okay is that the LVGL data changed by them is atomic.

If an interrupt MUST convey information to part of your application that calls LVGL functions, set a flag or other atomic value that your LVGL-calling thread (or an LVGL Timer you create) can read from and take action.

If you are using an OS, there are a few other options. See below.

Tasks

Under an OS, it is common to have many threads of execution ("tasks" in some OSes) performing services for the application. In some cases, such threads can acquire data that should be shown (or otherwise reflected) in the user interface, and doing so requires making LVGL calls to get that data (or change) shown.

Yet it still remains the programmer's responsibility to see that no LVGL function is called while another LVGL call is in progress.

How do you do this?

Method 1: Use a Gateway Thread

A "Gateway Thread" (or "Gateway Task" in some OSes) is a thread (task) that the system designer designates to exclusively manage a system resource. An example is management of a remote chip, such as an EEPROM or other device that always needs to be brought into a consistent state before something new is started. Another example is management of multiple devices on an I2C bus (or any data bus). In this case the I2C bus is the "exclusively-managed resource", and having only one thread managing it guarantees that each action started is allowed to complete before another action with it is started.

LVGL's data structures are a system resource that requires such protection.

Using this method, creation, modification and deletion of all Widgets and other LVGL resources (i.e. all LVGL function calls excluding the exceptions mentioned above) are called by that thread. That means that thread is also the ONLY caller of lv_timer_handler(). (See Add LVGL to Your Project for more information.)

This ensures LVGL's data structures "appear" atomic (all threads using this data "see" it in a consistent state) by the fact that no other threads are "viewing" those data structures. This is enforced by programmer discipline that ensures the Gateway Thread is the only thread making LVGL calls (excluding the exceptions mentioned above).

If atomic data relevant to the user interface is updated in another thread (i.e. by another task or in an interrupt), the thread calling LVGL functions can read that data directly without worry that it is in an inconsistent state. (To avoid unnecessary CPU overhead, a mechanism can be provided [such as a flag raised by the updating thread] so that the user interface is only updated when it will result in a change visible to the end user.)

If non-atomic data relevant to the user interface is updated in another thread (i.e. by another task or in an interrupt), an alternate (and safe) way of convey that data to the thread calling LVGL functions is to pass a private copy of that data to that thread via a QUEUE or other OS mechanism that protects that data from being seen in an inconsistent state.

Use of a Gateway Thread avoids the CPU-overhead (and coding overhead) of using a MUTEX to protect LVGL data structures.

Method 2: Use a MUTEX

A MUTEX stands for "MUTually EXclusive" and is a synchronization primative that protects the state of a system resource from being modified or accessed by multiple threads of execution at once. In other words, it makes data so protected "appear" atomic (all threads using this data "see" it in a consistent state). Most OSes provide MUTEXes.

The system designer assigns a single MUTEX to product a single system resource. Once assigned, that MUTEX performs such protection by programmers:

acquiring the MUTEX (a.k.a. locking it) before accessing or modifying that resource, and
releasing the MUTEX (a.k.a. unlocking it) after that access or modification is complete.

If a thread attempts to acquire (lock) the MUTEX while another thread "owns" it, that thread waits on the other thread to release (unlock) it before it is allowed to continue execution.

To be clear: this must be done both by threads that READ from that resource, and threads that MODIFY that resource.

If a MUTEX is used to protect LVGL data structures, that means every LVGL function call (or group of function calls) must be preceeded by #1, and followed by #2, including calls to lv_timer_handler().

Note

If your OS is integrated with LVGL (the macro LV_USE_OS has a value other than LV_OS_NONE in lv_conf.h) you can use lv_lock() and lv_unlock() to perform #1 and #2.

When this is the case, lv_timer_handler() calls lv_lock() and lv_unlock() internally, so you do not have to bracket your calls to lv_timer_handler() with them.

If your OS is NOT integrated with LVGL, then these calls either return immediately with no effect, or are optimized away by the linker.

To enable lv_lock() and lv_unlock(), set LV_USE_OS to a value other than LV_OS_NONE.

This pseudocode illustrates the concept of using a MUTEX:

void lvgl_thread(void)
{
    while(1) {
        uint32_t time_till_next;
        time_till_next = lv_timer_handler(); /* lv_lock/lv_unlock is called internally */
        thread_sleep(time_till_next); /* sleep for a while */
    }
}

void other_thread(void)
{
    /* You must always hold (lock) the MUTEX while calling LVGL functions. */
    lv_lock();
    lv_obj_t *img = lv_image_create(lv_screen_active());
    lv_unlock();

    while(1) {
        lv_lock();
        /* Change to next image. */
        lv_image_set_src(img, next_image);
        lv_unlock();
        thread_sleep(2000);
    }
}

Sleep Management

The MCU can go to sleep when no user input has been received for a certain period. In this case, the main while(1) could look like this:

while(1) {
    /* Normal operation (no sleep) in < 1 sec inactivity */
    if(lv_display_get_inactive_time(NULL) < 1000) {
        lv_timer_handler();
    }
    /* Sleep after 1 sec inactivity */
    else {
        timer_stop();   /* Stop the timer where lv_tick_inc() is called */
        sleep();        /* Sleep the MCU */
    }
    my_delay_ms(5);
}

You should also add the following lines to your input device read function to signal a wake-up (press, touch, click, etc.) has happened:

lv_tick_inc(LV_DEF_REFR_PERIOD);  /* Force task execution on wake-up */
timer_start();                    /* Restart timer where lv_tick_inc() is called */
lv_timer_handler();               /* Call `lv_timer_handler()` manually to process the wake-up event */

In addition to lv_display_get_inactive_time() you can check lv_anim_count_running() to see if all animations have finished.