[fiber] Fix PSPLIM triggering during context switch

examples/nucleo_h563zi/fiber/main.cpp

@@ -0,0 +1,99 @@
+/*
+ * Copyright (c) 2026, Niklas Hauser
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#include <modm/board.hpp>
+#include <modm/debug/logger.hpp>
+#include <modm/processing.hpp>
+
+using namespace Board;
+using namespace std::chrono_literals;
+
+namespace
+{
+
+constexpr uint32_t YieldGoal = 2'000'000;
+volatile uint32_t yieldCounter = 0;
+volatile uint32_t ringCounter = 0;
+
+void
+yield_worker()
+{
+	while (true)
+	{
+		++yieldCounter;
+		modm::this_fiber::yield();
+	}
+}
+
+extern modm::Fiber<512> fiberB;
+extern modm::Fiber<512> fiberC;
+
+modm_faststack modm::Fiber<768> fiberA([]{
+	++ringCounter;
+	if ((ringCounter & 0x3ff) == 0)
+	{
+		const auto ring = uint32_t(ringCounter);
+		const auto yields = uint32_t(yieldCounter);
+		MODM_LOG_INFO << "ring=" << ring
+				<< " yields=" << yields
+				<< " A=" << fiberA.stack_usage()
+				<< modm::endl;
+	}
+	modm::this_fiber::yield();
+	fiberB.start();
+});
+
+modm_faststack modm::Fiber<512> fiberB([]{
+	++ringCounter;
+	modm::this_fiber::yield();
+	fiberC.start();
+}, modm::fiber::Start::Later);
+
+modm_faststack modm::Fiber<512> fiberC([]{
+	++ringCounter;
+	modm::this_fiber::yield();
+	fiberA.start();
+}, modm::fiber::Start::Later);
+
+modm_faststack modm::Fiber<768> fiberYield(yield_worker);
+
+modm_faststack modm::Fiber<1024> fiberReporter([]{
+	while (true)
+	{
+		modm::this_fiber::sleep_for(250ms);
+		Leds::toggle();
+		const auto yields = uint32_t(yieldCounter);
+		const auto ring = uint32_t(ringCounter);
+		MODM_LOG_INFO << "report yields=" << yields
+				<< " ring=" << ring
+				<< " WY=" << fiberYield.stack_usage()
+				<< " WA=" << fiberA.stack_usage()
+				<< " WB=" << fiberB.stack_usage()
+				<< " WC=" << fiberC.stack_usage()
+				<< modm::endl;
+	}
+});
+
+}
+
+int
+main()
+{
+	Board::initialize();
+	Leds::setOutput();
+
+	MODM_LOG_INFO << "H563ZI fiber stress example" << modm::endl;
+	MODM_LOG_INFO << "Exercises repeated yield and restart handover on ARMv8-M" << modm::endl;
+	MODM_LOG_INFO.flush();
+
+	modm::fiber::Scheduler::run(modm::fiber::Scheduler::AutoWatermark);
+	return 0;
+}

examples/nucleo_h563zi/fiber/project.xml

@@ -0,0 +1,10 @@
+<library>
+  <extends>modm:nucleo-h563zi</extends>
+  <options>
+    <option name="modm:build:build.path">../../../build/nucleo_h563zi/fiber</option>
+  </options>
+  <modules>
+    <module>modm:build:scons</module>
+    <module>modm:processing:fiber</module>
+  </modules>
+</library>

src/modm/processing/fiber/context_arm_m.cpp.in

@@ -188,14 +188,17 @@ modm_context_start(modm_context_t*)
 		"orr r1, r1, #2		\n\t"	// Set SPSEL
 %% endif
 		"msr control, r1	\n\t"
+		"isb				\t\n\t"
 %#
 %% if is_cm0
 		"ldr r1, [r0]		\n\t"
 		"mov sp,  r1		\n\t"	// Set PSP to ctx->sp
 %% elif with_psplim
 		"ldm r0, {r1-r2}	\n\t"
-		"msr psplim, r2		\n\t"	// Set PSPLIM to ctx->bottom
+		"mov r3, #0			\n\t"
+		"msr psplim, r3		\n\t"	// Disable PSPLIM
 		"mov sp, r1			\n\t"	// Set PSP to ctx->sp
+		"msr psplim, r2		\n\t"	// Set PSPLIM to ctx->bottom
 %% else
 		"ldr sp, [r0]		\n\t"	// Set PSP to ctx->sp
 %% endif
@@ -240,8 +243,10 @@ modm_context_jump(modm_context_t*, modm_context_t*)
 		"mov sp,  r2			\n\t"	// Restore SP from to->sp
 %% elif with_psplim
 		"ldm r1, {r1-r2}		\n\t"
+		"mov r3, #0				\n\t"
+		"msr psplim, r3			\n\t"	// Disable PSPLIM
+		"mov sp, r1				\n\t"	// Restore SP from to->sp
 		"msr psplim, r2			\n\t"	// Set PSPLIM to ctx->bottom
-		"mov sp, r1				\n\t"	// Set PSP to ctx->sp
 %% else
 		"ldr sp, [r1]			\n\t"	// Restore SP from to->sp
 %% endif
@@ -260,6 +265,12 @@ modm_context_end(uintptr_t)
 {
 	asm volatile
 	(
+%% if with_psplim
+		"mov r3, #0			\n\t"
+		"msr psplim, r3		\n\t"	// Disable PSPLIM
+%#
+%% endif
+
 		"mrs r1, control	\n\t"
 %% if is_cm0
 		"mov r2, #2			\n\t"
@@ -268,6 +279,7 @@ modm_context_end(uintptr_t)
 		"bic r1, r1, #2		\n\t"	// Unset SPSEL
 %% endif
 		"msr control, r1	\n\t"
+		"isb				\n\t"
 
 		MODM_POP_CONTEXT()
 	);