Fix shell threadless support

doc/kernel/services/threads/nothread.rst

@@ -119,3 +119,18 @@ peripheral drivers.
 
 *List/table of supported drivers to go here, including which API options
 are supported*
+
+Shell
+=====
+
+The :ref:`shell_api` along with a subset of backends are expected to work
+given the application calls :c:func:`shell_start` to start the shell
+backend, and intermittently calls :c:func:`shell_process` to check for and
+process new input.
+
+The following backends are supported:
+
+* Serial shell backend :kconfig:option:`CONFIG_SHELL_BACKEND_SERIAL`
+  configured to use either
+  :kconfig:option:`CONFIG_SHELL_BACKEND_SERIAL_API_INTERRUPT_DRIVEN` or
+  :kconfig:option:`CONFIG_SHELL_BACKEND_SERIAL_API_ASYNC`

include/zephyr/shell/shell.h

@@ -900,11 +900,10 @@ union shell_backend_ctx {
 };
 
 enum shell_signal {
-	SHELL_SIGNAL_RXRDY,
-	SHELL_SIGNAL_LOG_MSG,
-	SHELL_SIGNAL_KILL,
-	SHELL_SIGNAL_TXDONE, /* TXDONE must be last one before SHELL_SIGNALS */
-	SHELL_SIGNALS
+	SHELL_SIGNAL_RXRDY = BIT(0),
+	SHELL_SIGNAL_LOG_MSG = BIT(1),
+	SHELL_SIGNAL_KILL = BIT(2),
+	SHELL_SIGNAL_TXDONE = BIT(3),
 };
 
 /**
@@ -962,15 +961,11 @@ struct shell_ctx {
 	volatile union shell_backend_cfg cfg;
 	volatile union shell_backend_ctx ctx;
 
-	struct k_poll_signal signals[SHELL_SIGNALS];
-
-	/** Events that should be used only internally by shell thread.
-	 * Event for SHELL_SIGNAL_TXDONE is initialized but unused.
-	 */
-	struct k_poll_event events[SHELL_SIGNALS];
-
-	struct k_mutex wr_mtx;
+#if CONFIG_MULTITHREADING
+	struct k_event signal_event;
+	struct k_sem lock_sem;
 	k_tid_t tid;
+#endif
 	int ret_val;
 };
 
@@ -1006,13 +1001,29 @@ struct shell {
 	LOG_INSTANCE_PTR_DECLARE(log);
 
 	const char *name;
+
+#if CONFIG_MULTITHREADING
 	struct k_thread *thread;
 	k_thread_stack_t *stack;
+#endif
 };
 
 extern void z_shell_print_stream(const void *user_ctx, const char *data,
 				 size_t data_len);
 
+#if CONFIG_MULTITHREADING
+#define Z_SHELL_THREAD_DEFINE(_name)                                                               \
+	static K_KERNEL_STACK_DEFINE(_name##_stack, CONFIG_SHELL_STACK_SIZE);                      \
+	static struct k_thread _name##_thread
+
+#define Z_SHELL_THREAD_INIT(_name)                                                                 \
+	.thread = &_name##_thread,                                                                 \
+	.stack = _name##_stack,
+#else
+#define Z_SHELL_THREAD_DEFINE(_name)
+#define Z_SHELL_THREAD_INIT(_name)
+#endif
+
 /** @brief Internal macro for defining a shell instance.
  *
  * As it does not create the default shell logging backend it allows to use
@@ -1033,8 +1044,7 @@ extern void z_shell_print_stream(const void *user_ctx, const char *data,
 			       IS_ENABLED(CONFIG_SHELL_PRINTF_AUTOFLUSH), z_shell_print_stream);   \
 	LOG_INSTANCE_REGISTER(shell, _name, CONFIG_SHELL_LOG_LEVEL);                               \
 	Z_SHELL_STATS_DEFINE(_name);                                                               \
-	static K_KERNEL_STACK_DEFINE(_name##_stack, CONFIG_SHELL_STACK_SIZE);                      \
-	static struct k_thread _name##_thread;                                                     \
+	Z_SHELL_THREAD_DEFINE(_name);                                                              \
 	static const STRUCT_SECTION_ITERABLE(shell, _name) = {                                     \
 		.default_prompt = _prompt,                                                         \
 		.iface = _transport_iface,                                                         \
@@ -1044,8 +1054,9 @@ extern void z_shell_print_stream(const void *user_ctx, const char *data,
 		.fprintf_ctx = &_name##_fprintf,                                                   \
 		.stats = Z_SHELL_STATS_PTR(_name),                                                 \
 		.log_backend = _log_backend,                                                       \
-		LOG_INSTANCE_PTR_INIT(log, shell, _name).name =                                    \
-			STRINGIFY(_name), .thread = &_name##_thread, .stack = _name##_stack}
+		LOG_INSTANCE_PTR_INIT(log, shell, _name).name = STRINGIFY(_name),                  \
+		Z_SHELL_THREAD_INIT(_name)                                                         \
+	}
 
 /**
  * @brief Macro for defining a shell instance.

samples/subsys/shell/threadless/CMakeLists.txt

@@ -0,0 +1,9 @@
+# Copyright 2025 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(shell_threadless)
+
+target_sources(app PRIVATE src/main.c)

samples/subsys/shell/threadless/README.rst

@@ -0,0 +1,43 @@
+.. zephyr:code-sample:: threadless_shell
+   :name: Threadless shell
+
+   Run shell in single threaded build
+
+Overview
+********
+
+Demonstrate how to configure and use a shell in a single threaded build
+(:kconfig:option:`CONFIG_MULTITHREADING` disabled). The sample uses the
+UART shell backend using either the async
+(:kconfig:option:`CONFIG_UART_ASYNC_API`) or interrupt driven
+(:kconfig:option:`CONFIG_UART_INTERRUPT_DRIVEN`) APIs. The sample
+implements a single command, which simply echoes back the single
+argument passed to it, along with the built-in "help" command.
+
+.. code-block:: console
+
+   uart:~$ sample foobar
+   foobar
+   uart:~$
+
+Building and Running
+********************
+
+This application can be built and executed as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/shell/threadless
+   :host-os: unix
+   :board: nrf54l15dk/nrf54l15/cpuapp
+   :goals: run
+   :compact:
+
+To build for another board, change "nrf54l15dk/nrf54l15/cpuapp" above
+to that board's name.
+
+Sample Output
+*************
+
+.. code-block:: console
+
+   uart:~$

samples/subsys/shell/threadless/prj.conf

@@ -0,0 +1,3 @@
+CONFIG_MULTITHREADING=n
+CONFIG_SHELL=y
+CONFIG_SHELL_MINIMAL=y

samples/subsys/shell/threadless/sample.yaml

@@ -0,0 +1,18 @@
+sample:
+  description: Threadless shell using uart backend
+  name: Threadless shell
+tests:
+  sample.subsys.shell.threadless:
+    tags: threadless
+    min_ram: 2
+    min_flash: 32
+    filter: CONFIG_ARCH_HAS_SINGLE_THREAD_SUPPORT and
+            dt_chosen_enabled("zephyr,shell-uart")
+    harness: shell
+    harness_config:
+      shell_commands:
+        - command: "sample foobar"
+          expected: "foobar"
+    platform_exclude:
+      - qemu_cortex_a53
+      - qemu_cortex_a53/qemu_cortex_a53/smp

samples/subsys/shell/threadless/src/main.c

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/shell/shell.h>
+#include <zephyr/shell/shell_uart.h>
+
+static int sample_sh_echo(const struct shell *sh, size_t argc, char **argv)
+{
+	shell_print(sh, "%s", argv[1]);
+	return 0;
+}
+
+SHELL_CMD_ARG_REGISTER(sample, NULL, "Echo", sample_sh_echo, 2, 2);
+
+int main(void)
+{
+	/*
+	 * Start the shell. This prints the shell prompt and enables
+	 * input reception.
+	 */
+	shell_start(shell_backend_uart_get_ptr());
+
+	while (1) {
+		/* Check for new input and process it if present */
+		shell_process(shell_backend_uart_get_ptr());
+	}
+
+	return 0;
+}

subsys/shell/Kconfig

@@ -8,7 +8,7 @@
 menuconfig SHELL
 	bool "Shell"
 	imply LOG_RUNTIME_FILTERING
-	select POLL
+	select EVENTS if MULTITHREADING
 
 if SHELL