esp32: Reduce latency for handling of scheduled Python callbacks.

Prior to this patch there was a large latency for executing scheduled
callbacks when when Python code is sleeping: at the heart of the
implementation of sleep_ms() is a call to vTaskDelay(1), which always
sleeps for one 100Hz tick, before performing another call to
MICROPY_EVENT_POLL_HOOK.

This patch fixes this issue by using FreeRTOS Task Notifications to signal
the main thread that a new callback is pending.

ports/esp32/machine_pin.c

@@ -33,6 +33,7 @@
 
 #include "py/runtime.h"
 #include "py/mphal.h"
+#include "mphalport.h"
 #include "modmachine.h"
 #include "extmod/virtpin.h"
 #include "machine_rtc.h"
@@ -115,6 +116,7 @@ STATIC void IRAM_ATTR machine_pin_isr_handler(void *arg) {
     machine_pin_obj_t *self = arg;
     mp_obj_t handler = MP_STATE_PORT(machine_pin_irq_handler)[self->id];
     mp_sched_schedule(handler, MP_OBJ_FROM_PTR(self));
+    mp_hal_wake_main_task_from_isr();
 }
 
 gpio_num_t machine_pin_get_id(mp_obj_t pin_in) {

ports/esp32/machine_timer.c

@@ -34,6 +34,7 @@
 #include "py/obj.h"
 #include "py/runtime.h"
 #include "modmachine.h"
+#include "mphalport.h"
 
 #define TIMER_INTR_SEL TIMER_INTR_LEVEL
 #define TIMER_DIVIDER  40000
@@ -109,6 +110,7 @@ STATIC void machine_timer_isr(void *self_in) {
     device->hw_timer[self->index].config.alarm_en = self->repeat;
 
     mp_sched_schedule(self->callback, self);
+    mp_hal_wake_main_task_from_isr();
 }
 
 STATIC void machine_timer_enable(machine_timer_obj_t *self) {

ports/esp32/main.c

@@ -131,8 +131,8 @@ soft_reset:
 
 void app_main(void) {
     nvs_flash_init();
-    xTaskCreateStaticPinnedToCore(mp_task, "mp_task", MP_TASK_STACK_LEN, NULL, MP_TASK_PRIORITY,
-                                  &mp_task_stack[0], &mp_task_tcb, 0);
+    mp_main_task_handle = xTaskCreateStaticPinnedToCore(mp_task, "mp_task", MP_TASK_STACK_LEN, NULL, MP_TASK_PRIORITY,
+                                                        &mp_task_stack[0], &mp_task_tcb, 0);
 }
 
 void nlr_jump_fail(void *val) {

ports/esp32/mphalport.c

@@ -38,6 +38,9 @@
 #include "py/mphal.h"
 #include "extmod/misc.h"
 #include "lib/utils/pyexec.h"
+#include "mphalport.h"
+
+TaskHandle_t mp_main_task_handle;
 
 STATIC uint8_t stdin_ringbuf_array[256];
 ringbuf_t stdin_ringbuf = {stdin_ringbuf_array, sizeof(stdin_ringbuf_array)};
@@ -49,7 +52,7 @@ int mp_hal_stdin_rx_chr(void) {
             return c;
         }
         MICROPY_EVENT_POLL_HOOK
-        vTaskDelay(1);
+        ulTaskNotifyTake(pdFALSE, 1);
     }
 }
 
@@ -106,7 +109,7 @@ void mp_hal_delay_ms(uint32_t ms) {
             break;
         }
         MICROPY_EVENT_POLL_HOOK
-        vTaskDelay(1);
+        ulTaskNotifyTake(pdFALSE, 1);
     }
     if (dt < us) {
         // do the remaining delay accurately
@@ -154,3 +157,12 @@ int *__errno() {
     return &mp_stream_errno;
 }
 */
+
+// Wake up the main task if it is sleeping
+void mp_hal_wake_main_task_from_isr(void) {
+    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
+    vTaskNotifyGiveFromISR(mp_main_task_handle, &xHigherPriorityTaskWoken);
+    if (xHigherPriorityTaskWoken == pdTRUE) {
+        portYIELD_FROM_ISR();
+    }
+}

ports/esp32/mphalport.h

@@ -32,6 +32,11 @@
 #include "py/ringbuf.h"
 #include "lib/utils/interrupt_char.h"
 
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+
+extern TaskHandle_t mp_main_task_handle;
+
 extern ringbuf_t stdin_ringbuf;
 
 uint32_t mp_hal_ticks_us(void);
@@ -49,6 +54,9 @@ uint32_t mp_hal_get_cpu_freq(void);
 #define mp_hal_quiet_timing_enter() MICROPY_BEGIN_ATOMIC_SECTION()
 #define mp_hal_quiet_timing_exit(irq_state) MICROPY_END_ATOMIC_SECTION(irq_state)
 
+// Wake up the main task if it is sleeping
+void mp_hal_wake_main_task_from_isr(void);
+
 // C-level pin HAL
 #include "py/obj.h"
 #include "driver/gpio.h"