From ba58273b0435cabd68215685b0c3e4f50802f7c7 Mon Sep 17 00:00:00 2001
From: Koen Zandberg <koen@bergzand.net>
Date: Mon, 20 Jul 2020 10:54:17 +0200
Subject: [PATCH 1/2] cortexm_common: Enable using pendsv IRQ at lower priority

This modifies the cortex-m thread specifics to allow running the PendSV
interrupt continuously at lower priority and removes the priority
modifications during the interrupt itself. Interrupts are disabled
during the scheduling itself, but enabled briefly after the sleep to
ensure that they are handled if activated during the scheduling or the
sleep.
---
 cpu/cortexm_common/include/cpu_conf_common.h |  2 +-
 cpu/cortexm_common/thread_arch.c             | 20 ++++++--------------
 2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/cpu/cortexm_common/include/cpu_conf_common.h b/cpu/cortexm_common/include/cpu_conf_common.h
index 28e9643621..15c9bdef64 100644
--- a/cpu/cortexm_common/include/cpu_conf_common.h
+++ b/cpu/cortexm_common/include/cpu_conf_common.h
@@ -142,7 +142,7 @@ extern "C" {
  * If you want to set this, define it in your `cpu_conf.h`.
  */
 #ifndef CPU_CORTEXM_PENDSV_IRQ_PRIO
-#define CPU_CORTEXM_PENDSV_IRQ_PRIO (CPU_DEFAULT_IRQ_PRIO)
+#define CPU_CORTEXM_PENDSV_IRQ_PRIO (UINT8_MAX)
 #endif
 /** @} */
 
diff --git a/cpu/cortexm_common/thread_arch.c b/cpu/cortexm_common/thread_arch.c
index dfbefc0d48..145372019d 100644
--- a/cpu/cortexm_common/thread_arch.c
+++ b/cpu/cortexm_common/thread_arch.c
@@ -317,7 +317,9 @@ void __attribute__((naked)) __attribute__((used)) isr_pendsv(void) {
 #endif
     "push   {lr}                      \n" /* push exception return code */
 
+    "cpsid  i                         \n" /* Disable IRQs during sched_run */
     "bl     sched_run                 \n" /* perform scheduling */
+    "cpsie  i                         \n" /* Re-enable interrupts */
 
 #if CPU_CORE_CORTEXM_FULL_THUMB
     "cmp    r0, r12                   \n" /* if r0 == 0: (no switch required) */
@@ -495,24 +497,14 @@ void __attribute__((used)) isr_svc(void)
 
 void sched_arch_idle(void)
 {
-    /* by default, PendSV has the same priority as other ISRs.
-     * In this function, we temporarily lower the priority (set higher value),
-     * allowing other ISRs to interrupt.
-     *
-     * According to [this](http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0321a/BIHJICIE.html),
-     * dynamically changing the priority is not supported on CortexM0(+).
-     */
-    unsigned state = irq_disable();
-    NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO + 1);
-    __DSB();
-    __ISB();
 #ifdef MODULE_PM_LAYERED
     void pm_set_lowest(void);
     pm_set_lowest();
 #else
     __WFI();
 #endif
-    irq_restore(state);
-    NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO);
-    SCB->ICSR = SCB_ICSR_PENDSVCLR_Msk;
+    /* Briefly re-enable IRQs to allow pending interrupts to be serviced and
+     * have them update the runqueue */
+    __enable_irq();
+    __disable_irq();
 }

From eb73515e2cfdf5e42c84dfd53489c0e63dc6f00b Mon Sep 17 00:00:00 2001
From: Koen Zandberg <koen@bergzand.net>
Date: Mon, 20 Jul 2020 11:07:25 +0200
Subject: [PATCH 2/2] cortexm_common: Enable no_thread_idle for all
 architectures

---
 cpu/cortexm_common/Kconfig           | 1 +
 cpu/cortexm_common/Makefile.features | 5 +----
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/cpu/cortexm_common/Kconfig b/cpu/cortexm_common/Kconfig
index 13a67d2caa..8b9daa79c1 100644
--- a/cpu/cortexm_common/Kconfig
+++ b/cpu/cortexm_common/Kconfig
@@ -16,6 +16,7 @@ config CPU_ARCH_ARMV6M
     bool
     select HAS_ARCH_ARM
     select HAS_ARCH_32BIT
+    select HAS_NO_IDLE_THREAD
 
 config CPU_ARCH_ARMV7M
     bool
diff --git a/cpu/cortexm_common/Makefile.features b/cpu/cortexm_common/Makefile.features
index 7527c079d0..456647700f 100644
--- a/cpu/cortexm_common/Makefile.features
+++ b/cpu/cortexm_common/Makefile.features
@@ -33,9 +33,6 @@ else
   $(error Unkwnown cortexm core: $(CPU_CORE))
 endif
 
-# cortex-m3 and higher don't need the idle thread
-ifneq (,$(filter armv7m armv8m,$(CPU_ARCH)))
-  FEATURES_PROVIDED += no_idle_thread
-endif
+FEATURES_PROVIDED += no_idle_thread
 
 KCONFIG_ADD_CONFIG += $(RIOTCPU)/cortexm_common/cortexm_common.config