From b9a76ea094c84d16e7548d95fcb9fdd7f99f6e18 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Tue, 9 Jan 2024 18:59:28 +0100
Subject: [PATCH 1/4] core/init: avoid context switch with !core_thread

---
 core/lib/init.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/core/lib/init.c b/core/lib/init.c
index cdf7358a4e..0dbbc6971c 100644
--- a/core/lib/init.c
+++ b/core/lib/init.c
@@ -106,8 +106,11 @@ void kernel_init(void)
                       main_trampoline, NULL, "main");
     }
     else {
+        /* RIOT without threads */
         irq_enable();
         main_trampoline(NULL);
+        while (1) {}
+        return;
     }
 
     cpu_switch_context_exit();

From 14d0cf5d87ad02ea002a223d8d751db45ae89cf2 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Tue, 9 Jan 2024 18:59:58 +0100
Subject: [PATCH 2/4] Makefile.dep: don't include idle thread with !core_thread

---
 makefiles/features_modules.inc.mk | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/makefiles/features_modules.inc.mk b/makefiles/features_modules.inc.mk
index 77e344a808..2f9792c68e 100644
--- a/makefiles/features_modules.inc.mk
+++ b/makefiles/features_modules.inc.mk
@@ -83,7 +83,9 @@ USEMODULE += $(filter cortexm_svc, $(FEATURES_USED))
 
 # select core_idle_thread if the feature no_idle_thread is *not* used
 ifeq (, $(filter no_idle_thread, $(FEATURES_USED)))
-  USEMODULE += core_idle_thread
+  ifneq (,$(filter core_thread, $(USEMODULE)))
+    USEMODULE += core_idle_thread
+  endif
 endif
 
 # use mpu_stack_guard if the feature is used

From 112e378fccf13b091a7f761395d390b2eaabebde Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Tue, 9 Jan 2024 19:00:22 +0100
Subject: [PATCH 3/4] cpu/avr8_common: fix build with !core_thread

---
 cpu/avr8_common/thread_arch.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/cpu/avr8_common/thread_arch.c b/cpu/avr8_common/thread_arch.c
index 1806db364b..f6529c9fb4 100644
--- a/cpu/avr8_common/thread_arch.c
+++ b/cpu/avr8_common/thread_arch.c
@@ -260,6 +260,10 @@ void NORETURN avr8_enter_thread_mode(void)
 
 void thread_yield_higher(void)
 {
+    if (!IS_USED(MODULE_CORE_THREAD)) {
+        return;
+    }
+
     if (irq_is_in() == 0) {
         avr8_context_save();
         sched_run();
@@ -278,7 +282,8 @@ void avr8_exit_isr(void)
     __asm__ volatile ("" : : : "memory");
 
     /* schedule should switch context when returning from a non nested interrupt */
-    if (sched_context_switch_request && avr8_state_irq_count == 0) {
+    if (sched_context_switch_request && avr8_state_irq_count == 0 &&
+        IS_USED(MODULE_CORE_THREAD)) {
         avr8_context_save();
         sched_run();
         avr8_context_restore();

From cb76cc17e9607e9d7c7aa90d01cac4db4ad5522b Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Tue, 9 Jan 2024 23:02:01 +0100
Subject: [PATCH 4/4] cpu/native: fix build with !core_thread

---
 cpu/native/native_cpu.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/cpu/native/native_cpu.c b/cpu/native/native_cpu.c
index 5e9e96875a..13fff4e2c4 100644
--- a/cpu/native/native_cpu.c
+++ b/cpu/native/native_cpu.c
@@ -156,7 +156,8 @@ void isr_cpu_switch_context_exit(void)
     ucontext_t *ctx;
 
     DEBUG("isr_cpu_switch_context_exit\n");
-    if ((sched_context_switch_request == 1) || (thread_get_active() == NULL)) {
+    if (((sched_context_switch_request == 1) || (thread_get_active() == NULL))
+        && IS_USED(MODULE_CORE_THREAD)) {
         sched_run();
     }
 
@@ -210,7 +211,11 @@ void isr_thread_yield(void)
         native_irq_handler();
     }
 
+    if (!IS_USED(MODULE_CORE_THREAD)) {
+        return;
+    }
     sched_run();
+
     /* Use intermediate cast to uintptr_t to silence -Wcast-align.
      * stacks are manually word aligned in thread_static_init() */
     ucontext_t *ctx = (ucontext_t *)(uintptr_t)(thread_get_active()->sp);