esp-idf/components/esp_driver_gptimer/README.md

GPTimer Driver Design

State Transition

State transition is achieved by using the primitives provided by <stdatomic.h>.

stateDiagram-v2
    [*] --> init: gptimer_new_timer
    init --> enable: gptimer_enable
    enable --> init: gptimer_disable
    enable --> run: gptimer_start*
    run --> enable: gptimer_stop*
    init --> [*]: gptimer_del_timer

Other functions won't change the driver state. The functions above labeled with * are allowed to be used in the interrupt context.

Concurrency

There might be race conditions when the user calls the APIs from a thread and interrupt at the same time. e.g. a Task is just running the gptimer_start, and suddenly an interrupt occurs, where the user calls gptimer_stop for the same timer handle. Which is possible to make a "stopped" timer continue to run if the interrupt is returned before the Task.

stateDiagram-v2
    state Race-Condition {
        Thread --> gptimer_start
        state gptimer_start {
            state is_enabled <<choice>>
            [*] --> is_enabled: Enabled?
            is_enabled --> run_wait: yes
            is_enabled --> [*] : no
            run_wait --> run: call HAL/LL functions to start timer
            }
        --
        Interrupt --> gptimer_stop
        state gptimer_stop {
            state is_running <<choice>>
            [*] --> is_running: Running?
            is_running --> enable_wait: yes
            is_running --> [*] : no
            enable_wait --> enable: call HAL/LL functions to stop timer
        }
    }

By introducing a "middle" state like run_wait and enable_wait, we make sure that the timer is in a safe state before we start/stop it. And if the state is invalid, it can return an error code to the user.