[RFC PATCH 06/10] uthread: add cooperative multi-tasking interface

[Jerome Forissier]

Add an new internal API called uthread (Kconfig symbol: UTHREAD) which
provides cooperative multi-tasking. The goal is to be able to improve
the performance of some parts of U-Boot by overlapping lengthy
operations. Each uthread has its own stack allocated on the heap. The
default stack size is defined by the UTHREAD_STACK_SIZE symbol and is
used when uthread_create() is passed zero for the stack_sz argument.

The implementation is based on context-switching via initjmp()/setjmp()/
longjmp() and is inspired from barebox threads [1].

[1] https://github.com/barebox/barebox/blob/master/common/bthread.c

Signed-off-by: Jerome Forissier <jerome.forissier at linaro.org>
---
 include/uthread.h |  31 ++++++++++++++
 lib/Kconfig       |  19 ++++++++
 lib/Makefile      |   2 +
 lib/uthread.c     | 107 ++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 159 insertions(+)
 create mode 100644 include/uthread.h
 create mode 100644 lib/uthread.c

diff --git a/include/uthread.h b/include/uthread.h
new file mode 100644
index 00000000000..ea1389e82c6
--- /dev/null
+++ b/include/uthread.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright 2025 Linaro Limited
+ */
+
+#include <linux/types.h>
+
+#ifndef _UTHREAD_H_
+#define _UTHREAD_H_
+
+#ifdef CONFIG_UTHREAD
+
+int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz);
+void uthread_free_all(void);
+/* Returns false when all threads are done */
+bool uthread_schedule(void);
+
+#else
+
+static inline int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz)
+{
+	fn(arg);
+	return 0;
+}
+
+static inline void uthread_free_all(void) { }
+
+static inline bool uthread_schedule(void) { return false; }
+
+#endif /* CONFIG_UTHREAD */
+#endif /* _UTHREAD_H_ */
diff --git a/lib/Kconfig b/lib/Kconfig
index 1a683dea670..c3416bbd0be 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -1255,6 +1255,25 @@ config PHANDLE_CHECK_SEQ
 	  enable this config option to distinguish them using
 	  phandles in fdtdec_get_alias_seq() function.
 
+config UTHREAD
+	bool "Enable thread support"
+	depends on HAVE_INITJMP
+	help
+	  Implement a simple form of cooperative multi-tasking based on
+	  context-switching via initjmp(), setjmp() and longjmp(). The
+	  uthread_ interface enables the main thread of execution to create
+	  one or more secondary threads and schedule them until they all have
+	  returned. At any point a thread may suspend its execution and
+	  schedule another thread, which allows for the efficient multiplexing
+	  of leghthy operations.
+
+config UTHREAD_STACK_SIZE
+	int "Default uthread stack size"
+	depends on UTHREAD
+	default 32168
+	help
+	  The default stask size for uthreads. Each uthread has its own stack.
+
 endmenu
 
 source "lib/fwu_updates/Kconfig"
diff --git a/lib/Makefile b/lib/Makefile
index a7bc2f3134a..3610694de7a 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -164,6 +164,8 @@ obj-$(CONFIG_LIB_ELF) += elf.o
 
 obj-$(CONFIG_$(PHASE_)SEMIHOSTING) += semihosting.o
 
+obj-$(CONFIG_UTHREAD) += uthread.o
+
 #
 # Build a fast OID lookup registry from include/linux/oid_registry.h
 #
diff --git a/lib/uthread.c b/lib/uthread.c
new file mode 100644
index 00000000000..bb132001fb6
--- /dev/null
+++ b/lib/uthread.c
@@ -0,0 +1,107 @@
+// SPDX-License-Identifier: GPL-2.0-only
+//
+// Copyright (C) 2021 Ahmad Fatoum, Pengutronix
+// Copyright (C) 2025 Linaro Limited
+//
+// An implementation of cooperative multi-tasking inspired from barebox threads
+// https://github.com/barebox/barebox/blob/master/common/bthread.c
+
+#include <compiler.h>
+#include <asm/setjmp.h>
+#include <linux/kernel.h>
+#include <linux/list.h>
+#include <malloc.h>
+#include <stdint.h>
+#include <uthread.h>
+
+static struct uthread {
+	void (*fn)(void *);
+	void *arg;
+	jmp_buf ctx;
+	void *stack;
+	bool done;
+	struct list_head list;
+} main_thread = {
+	.list = LIST_HEAD_INIT(main_thread.list),
+};
+
+static struct uthread *current = &main_thread;
+
+static void __noreturn uthread_trampoline(void)
+{
+	current->fn(current->arg);
+	current->done = true;
+	current = &main_thread;
+	longjmp(current->ctx, 1);
+	/* Not reached */
+	while (true)
+		;
+}
+
+static void uthread_free(struct uthread *uthread)
+{
+	if (!uthread)
+		return;
+	free(uthread->stack);
+	free(uthread);
+}
+
+int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz)
+{
+	struct uthread *uthread;
+
+	if (!stack_sz)
+		stack_sz = CONFIG_UTHREAD_STACK_SIZE;
+
+	uthread = calloc(1, sizeof(*uthread));
+	if (!uthread)
+		return -1;
+
+	uthread->stack = memalign(16, stack_sz);
+	if (!uthread->stack)
+		goto err;
+
+	uthread->fn = fn;
+	uthread->arg = arg;
+
+	list_add_tail(&uthread->list, &current->list);
+
+	initjmp(uthread->ctx, uthread_trampoline, uthread->stack + stack_sz);
+
+	return 0;
+err:
+	uthread_free(uthread);
+	return -1;
+}
+
+void uthread_free_all(void)
+{
+	struct uthread *next;
+	struct uthread *tmp;
+
+	list_for_each_entry_safe(next, tmp, &current->list, list) {
+		list_del(&next->list);
+		uthread_free(next);
+	}
+}
+
+static void uthread_resume(struct uthread *uthread)
+{
+	if (!setjmp(current->ctx)) {
+		current = uthread;
+		longjmp(uthread->ctx, 1);
+	}
+}
+
+bool uthread_schedule(void)
+{
+	struct uthread *next;
+
+	list_for_each_entry(next, &current->list, list) {
+		if (!next->done) {
+			uthread_resume(next);
+			return true;
+		}
+	}
+	return false;
+}
-- 
2.43.0