From 8d07b7751ad2500cd03aacfe654184f16aeed380 Mon Sep 17 00:00:00 2001
From: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Date: Thu, 1 Apr 2021 13:46:57 +0200
Subject: [PATCH] smp: add a Kconfig option to delay booting secondary CPUs

Usually Zephyr boots all secondary CPUs as a part of system
boot. Some applications however need an ability to boot on
the main CPU only and enable secondary CPUs selectively at
run-time. Add a Kconfig option to support this behaviour.
When booting CPUs on demand applications also need helpers
to initialise a dummy thread and begin threaded execution
on those CPUs, add two such helpers.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
---
 include/kernel.h |  6 ++++++
 kernel/Kconfig   |  8 ++++++++
 kernel/smp.c     | 23 +++++++++++++++++++----
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/include/kernel.h b/include/kernel.h
index c5764b14730..35dc1459f2e 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -5553,6 +5553,12 @@ extern void z_init_static_threads(void);
  * @internal
  */
 extern bool z_is_thread_essential(void);
+
+#ifdef CONFIG_SMP
+void z_smp_thread_init(void *arg, struct k_thread *thread);
+void z_smp_thread_swap(void);
+#endif
+
 /**
  * @internal
  */
diff --git a/kernel/Kconfig b/kernel/Kconfig
index 6e3e0792202..5dbb91ccde9 100644
--- a/kernel/Kconfig
+++ b/kernel/Kconfig
@@ -785,6 +785,14 @@ config SMP
 	  When true, kernel will be built with SMP support, allowing
 	  more than one CPU to schedule Zephyr tasks at a time.
 
+config SMP_BOOT_DELAY
+	bool "Delay booting secondary cores"
+	depends on SMP
+	help
+	  By default Zephyr will boot all available CPUs during start up.
+	  Select this option to skip this and allow architecture code boot
+	  secondary CPUs at a later time.
+
 config MP_NUM_CPUS
 	int "Number of CPUs/cores"
 	default 1
diff --git a/kernel/smp.c b/kernel/smp.c
index 6e58a7b8964..193897bd36d 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -50,28 +50,43 @@ void z_smp_release_global_lock(struct k_thread *thread)
 }
 
 #if CONFIG_MP_NUM_CPUS > 1
-static FUNC_NORETURN void smp_init_top(void *arg)
+
+void z_smp_thread_init(void *arg, struct k_thread *thread)
 {
 	atomic_t *cpu_start_flag = arg;
-	struct k_thread dummy_thread;
 
 	/* Wait for the signal to begin scheduling */
 	while (!atomic_get(cpu_start_flag)) {
 	}
 
-	z_dummy_thread_init(&dummy_thread);
+	z_dummy_thread_init(thread);
+}
+
+void z_smp_thread_swap(void)
+{
+	z_swap_unlocked();
+}
+
+#ifndef CONFIG_SMP_BOOT_DELAY
+static FUNC_NORETURN void smp_init_top(void *arg)
+{
+	struct k_thread dummy_thread;
+
+	z_smp_thread_init(arg, &dummy_thread);
 	smp_timer_init();
+
 	z_swap_unlocked();
 
 	CODE_UNREACHABLE; /* LCOV_EXCL_LINE */
 }
 #endif
+#endif
 
 void z_smp_init(void)
 {
 	(void)atomic_clear(&start_flag);
 
-#if defined(CONFIG_SMP) && (CONFIG_MP_NUM_CPUS > 1)
+#if CONFIG_MP_NUM_CPUS > 1 && !defined(CONFIG_SMP_BOOT_DELAY)
 	for (int i = 1; i < CONFIG_MP_NUM_CPUS; i++) {
 		arch_start_cpu(i, z_interrupt_stacks[i], CONFIG_ISR_STACK_SIZE,
 			       smp_init_top, &start_flag);