kernel: add k_can_yield helper function

Implements a function that application and driver code can use to check
whether it is valid to yield (or block) in the current context. This
check is required for functions that can feasibly be run from multiple
contexts. The primary intended use case is power management transition
functions, which can be run by application code explicitly or
automatically in the idle thread by system PM.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

Author: Jordan Yates

include/zephyr/kernel.h

@@ -464,6 +464,19 @@ __syscall int32_t k_usleep(int32_t us);
  */
 __syscall void k_busy_wait(uint32_t usec_to_wait);
 
+/**
+ * @brief Check whether it is possible to yield in the current context.
+ *
+ * This routine checks whether the kernel is in a state where it is possible to
+ * yield or call blocking API's. It should be used by code that needs to yield
+ * to perform correctly, but can feasibly be called from contexts where that
+ * is not possible. For example in the PRE_KERNEL initialization step, or when
+ * being run from the idle thread.
+ *
+ * @return True if it is possible to yield in the current context, false otherwise.
+ */
+bool k_can_yield(void);
+
 /**
  * @brief Yield the current thread.
  *

kernel/sched.c

@@ -1325,6 +1325,12 @@ static inline void z_vrfy_k_thread_deadline_set(k_tid_t tid, int deadline)
 #endif
 #endif
 
+bool k_can_yield(void)
+{
+	return !(k_is_pre_kernel() || k_is_in_isr() ||
+		 z_is_idle_thread_object(_current));
+}
+
 void z_impl_k_yield(void)
 {
 	__ASSERT(!arch_is_in_isr(), "");