From b29f9d471bade1cf6d5e0994af7ccf722ac65ef0 Mon Sep 17 00:00:00 2001
From: Raul E Rangel <rrangel@chromium.org>
Date: Mon, 12 Jul 2021 13:49:59 -0600
Subject: [PATCH] lib/thread: Add mutex

We need a way to protect shared resources. Since we are using
cooperative multitasking the mutex implementation is pretty trivial.

BUG=b:179699789
TEST=Verify thread lock and unlock.

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ife1ac95ec064ebcdd00fcaacec37a06ac52885ff
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56230
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
---
 src/include/thread.h | 11 +++++++++++
 src/lib/thread.c     | 20 ++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/src/include/thread.h b/src/include/thread.h
index 293a666dbd..4bc04db00a 100644
--- a/src/include/thread.h
+++ b/src/include/thread.h
@@ -6,6 +6,10 @@
 #include <bootstate.h>
 #include <arch/cpu.h>
 
+struct thread_mutex {
+	bool locked;
+};
+
 #if ENV_RAMSTAGE && CONFIG(COOP_MULTITASKING)
 
 struct thread {
@@ -53,6 +57,9 @@ int thread_yield_microseconds(unsigned int microsecs);
 void thread_coop_enable(void);
 void thread_coop_disable(void);
 
+void thread_mutex_lock(struct thread_mutex *mutex);
+void thread_mutex_unlock(struct thread_mutex *mutex);
+
 static inline void thread_init_cpu_info_non_bsp(struct cpu_info *ci)
 {
 	ci->thread = NULL;
@@ -85,6 +92,10 @@ static inline void thread_coop_enable(void) {}
 static inline void thread_coop_disable(void) {}
 struct cpu_info;
 static inline void thread_init_cpu_info_non_bsp(struct cpu_info *ci) { }
+
+static inline void thread_mutex_lock(struct thread_mutex *mutex) {}
+
+static inline void thread_mutex_unlock(struct thread_mutex *mutex) {}
 #endif
 
 #endif /* THREAD_H_ */
diff --git a/src/lib/thread.c b/src/lib/thread.c
index 47a23acd96..e62c5d7239 100644
--- a/src/lib/thread.c
+++ b/src/lib/thread.c
@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -377,3 +378,22 @@ void thread_coop_disable(void)
 
 	current->can_yield--;
 }
+
+void thread_mutex_lock(struct thread_mutex *mutex)
+{
+	struct stopwatch sw;
+
+	stopwatch_init(&sw);
+
+	while (mutex->locked)
+		assert(thread_yield() == 0);
+	mutex->locked = true;
+
+	printk(BIOS_SPEW, "took %lu us to acquire mutex\n", stopwatch_duration_usecs(&sw));
+}
+
+void thread_mutex_unlock(struct thread_mutex *mutex)
+{
+	assert(mutex->locked);
+	mutex->locked = 0;
+}