cortex-m0/task: Check if interrupts are enabled before switching task

Switching task with disabled interrupts leads to Hard Fault on
Cortex-M0 because:
- SVCall exception have configurable priority (full list can be found
  at 2.3.2 Exception types PM0215 p.23)
- We are using 'cpsid i' to disable interrupts. This instruction sets
  PRIMASK bit (3.7.2 CPSID CPSIE PM0215 p.62)
- When PRIMASK bit is set, all exceptions with configurable priority
  are disabled (PM0215 p.16), so SVCall is masked too

If Hard Fault is inevitable, it will be a good idea to catch this
earlier. It will save time spent debugging why Forced Hard Fault
happens. In functions responsible for enabling, disabling or making
task ready we postpone task switch when interrupts are disabled

BUG=b:190597666
BRANCH=none
TEST=Compile and flash EC on boards with Cortex-M0 and make
     sure that it works properly.

Signed-off-by: Patryk Duda <[email protected]>
Change-Id: Id3be74e977ae5d5eed79aad78ee378fa413ed4ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2953229
Commit-Queue: Marcin Wojtas <[email protected]>
Tested-by: Patryk Duda <[email protected]>
Reviewed-by: Aseda Aboagye <[email protected]>

Author: duda-patryk

core/cortex-m0/task.c

@@ -336,6 +336,12 @@ static uint32_t __wait_evt(int timeout_us, task_id_t resched)
 	uint32_t evt;
 	int ret __attribute__((unused));
 
+	/*
+	 * Scheduling task when interrupts are disabled will result in Forced
+	 * Hard Fault because disabling interrupt using 'cpsid i' also disables
+	 * SVCall handler (because it has configurable priority)
+	 */
+	ASSERT(is_interrupt_enabled());
 	ASSERT(!in_interrupt_context());
 
 	if (timeout_us > 0) {
@@ -373,7 +379,7 @@ uint32_t task_set_event(task_id_t tskid, uint32_t event)
 	atomic_or(&receiver->events, event);
 
 	/* Re-schedule if priorities have changed */
-	if (in_interrupt_context()) {
+	if (in_interrupt_context() || !is_interrupt_enabled()) {
 		/* The receiver might run again */
 		atomic_or(&tasks_ready, 1 << tskid);
 		if (start_called) {
@@ -436,7 +442,8 @@ void task_enable_all_tasks(void)
 	/* Mark all tasks as ready and able to run. */
 	tasks_ready = tasks_enabled = BIT(TASK_ID_COUNT) - 1;
 	/* Reschedule the highest priority task. */
-	__schedule(0, 0);
+	if (is_interrupt_enabled())
+		__schedule(0, 0);
 }
 
 void task_enable_task(task_id_t tskid)
@@ -448,7 +455,8 @@ void task_disable_task(task_id_t tskid)
 {
 	atomic_clear_bits(&tasks_enabled, BIT(tskid));
 
-	if (!in_interrupt_context() && tskid == task_get_current())
+	if (!in_interrupt_context() && is_interrupt_enabled() &&
+	    tskid == task_get_current())
 		__schedule(0, 0);
 }