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

[Heinrich Schuchardt]

On 14.02.25 15:00, Jerome Forissier wrote:
> 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);
> +	}
> +}
> +

For every function, please, create a Sphinx style documentation.

For the whole uthread library, please, create a document in 
/doc/develop/ explaining how to use the API functions.

Best regards

Heinrich

> +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;
> +}