rp2/machine_timer: Support hard IRQ timer callbacks.

Unlike some boards like stm32, timer callbacks on the rp2 port are
unconditionally dispatched via mp_sched_schedule(), behaving like
soft IRQs with consequent GC jitter and delays.

Add a 'hard' keyword argument to the rp2 Timer constructor and init.
This defaults to False but if it is set True, the timer callback will
be dispatched in hard IRQ context rather than queued.

Signed-off-by: Chris Webb <chris@arachsys.com>

1 files changed, 28 insertions, 2 deletions

diff --git a/ports/rp2/machine_timer.c b/ports/rp2/machine_timer.c
index 6f8b04f10..ffb4c7024 100644
--- a/ports/rp2/machine_timer.c
+++ b/ports/rp2/machine_timer.c
@@ -27,6 +27,7 @@
 #include "py/runtime.h"
 #include "py/mperrno.h"
 #include "py/mphal.h"
+#include "py/gc.h"
 #include "pico/time.h"
 
 #define ALARM_ID_INVALID (-1)
@@ -40,13 +41,36 @@ typedef struct _machine_timer_obj_t {
     uint32_t mode;
     uint64_t delta_us; // for periodic mode
     mp_obj_t callback;
+    bool ishard;
 } machine_timer_obj_t;
 
 const mp_obj_type_t machine_timer_type;
 
 static int64_t alarm_callback(alarm_id_t id, void *user_data) {
     machine_timer_obj_t *self = user_data;
-    mp_sched_schedule(self->callback, MP_OBJ_FROM_PTR(self));
+
+    if (self->ishard) {
+        // When executing code within a handler we must lock the scheduler to
+        // prevent any scheduled callbacks from running, and lock the GC to
+        // prevent any memory allocations.
+        mp_sched_lock();
+        gc_lock();
+        nlr_buf_t nlr;
+        if (nlr_push(&nlr) == 0) {
+            mp_call_function_1(self->callback, MP_OBJ_FROM_PTR(self));
+            nlr_pop();
+        } else {
+            // Uncaught exception; disable the callback so it doesn't run again.
+            self->mode = TIMER_MODE_ONE_SHOT;
+            mp_printf(MICROPY_ERROR_PRINTER, "uncaught exception in timer callback\n");
+            mp_obj_print_exception(MICROPY_ERROR_PRINTER, MP_OBJ_FROM_PTR(nlr.ret_val));
+        }
+        gc_unlock();
+        mp_sched_unlock();
+    } else {
+        mp_sched_schedule(self->callback, MP_OBJ_FROM_PTR(self));
+    }
+
     if (self->mode == TIMER_MODE_ONE_SHOT) {
         return 0;
     } else {
@@ -66,13 +90,14 @@ static void machine_timer_print(const mp_print_t *print, mp_obj_t self_in, mp_pr
 }
 
 static mp_obj_t machine_timer_init_helper(machine_timer_obj_t *self, size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
-    enum { ARG_mode, ARG_callback, ARG_period, ARG_tick_hz, ARG_freq, };
+    enum { ARG_mode, ARG_callback, ARG_period, ARG_tick_hz, ARG_freq, ARG_hard, };
     static const mp_arg_t allowed_args[] = {
         { MP_QSTR_mode,         MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = TIMER_MODE_PERIODIC} },
         { MP_QSTR_callback,     MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_rom_obj = MP_ROM_NONE} },
         { MP_QSTR_period,       MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0xffffffff} },
         { MP_QSTR_tick_hz,      MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 1000} },
         { MP_QSTR_freq,         MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_rom_obj = MP_ROM_NONE} },
+        { MP_QSTR_hard,         MP_ARG_KW_ONLY | MP_ARG_BOOL, {.u_bool = false} },
     };
 
     // Parse args
@@ -96,6 +121,7 @@ static mp_obj_t machine_timer_init_helper(machine_timer_obj_t *self, size_t n_ar
     }
 
     self->callback = args[ARG_callback].u_obj;
+    self->ishard = args[ARG_hard].u_bool;
     self->alarm_id = alarm_pool_add_alarm_in_us(self->pool, self->delta_us, alarm_callback, self, true);
     if (self->alarm_id == -1) {
         mp_raise_OSError(MP_ENOMEM);