From e021f61946ca88cd762469762096710420bbe559 Mon Sep 17 00:00:00 2001
From: Maksim Salau <msalau@iotecha.com>
Date: Fri, 21 Mar 2025 17:16:46 +0100
Subject: [PATCH] modbus: Add locking API

Locking API allows atomic updates of modbus multi-register values in
server mode and pausing of an interface in client mode.

Signed-off-by: Maksim Salau <msalau@iotecha.com>
---
 include/zephyr/modbus/modbus.h  | 33 ++++++++++++++++++++++++++++++++
 subsys/modbus/Kconfig           | 14 ++++++++++++++
 subsys/modbus/modbus_core.c     | 34 +++++++++++++++++++++++++++++++++
 subsys/modbus/modbus_internal.h |  2 +-
 4 files changed, 82 insertions(+), 1 deletion(-)

diff --git a/include/zephyr/modbus/modbus.h b/include/zephyr/modbus/modbus.h
index 7043594b8ec6..1d9e2d28c27d 100644
--- a/include/zephyr/modbus/modbus.h
+++ b/include/zephyr/modbus/modbus.h
@@ -30,6 +30,7 @@
 #ifndef ZEPHYR_INCLUDE_MODBUS_H_
 #define ZEPHYR_INCLUDE_MODBUS_H_
 
+#include <zephyr/sys_clock.h>
 #include <zephyr/drivers/uart.h>
 #include <zephyr/sys/slist.h>
 #ifdef __cplusplus
@@ -561,6 +562,38 @@ int modbus_init_client(const int iface, struct modbus_iface_param param);
  */
 int modbus_disable(const uint8_t iface);
 
+#if IS_ENABLED(CONFIG_MODBUS_LOCKING)
+/**
+ * @brief Lock Modbus Interface
+ *
+ * The lock ensures that no new requests are handled while the inetraface is locked.asm
+ * May be used to ensure atomic update of register values in server role.
+ *
+ * @param iface   Modbus interface index
+ * @param timeout Waiting period to lock the interface,
+ *                or one of the special values K_NO_WAIT and
+ *                K_FOREVER.
+ *
+ * @retval 0 Interface locked.
+ * @retval -ENODEV Invalid interface index
+ * @retval -EBUSY Returned without waiting.
+ * @retval -EAGAIN Waiting period timed out.
+ */
+int modbus_lock(const int iface, k_timeout_t timeout);
+
+/**
+ * @brief Unlock Modbus Interface
+ *
+ * @param iface Modbus interface index
+ *
+ * @retval 0 Interface unlocked.
+ * @retval -ENODEV Invalid interface index
+ * @retval -EPERM The current thread does not own the lock
+ * @retval -EINVAL The interface is not locked
+ */
+int modbus_unlock(const int iface);
+#endif
+
 /**
  * @brief Submit raw ADU
  *
diff --git a/subsys/modbus/Kconfig b/subsys/modbus/Kconfig
index 4c49d6c09851..d5bad3da1eb9 100644
--- a/subsys/modbus/Kconfig
+++ b/subsys/modbus/Kconfig
@@ -79,6 +79,20 @@ config MODBUS_FC08_DIAGNOSTIC
 	help
 	  Enable function code 08 Diagnostic support
 
+config MODBUS_LOCKING
+	bool "Interface locking support"
+	default n
+	help
+	  Enable interface locking API
+
+config MODBUS_SERVER_LOCKING
+	bool "Server locking support"
+	depends on MODBUS_SERVER
+	select MODBUS_LOCKING
+	default n
+	help
+	  Enable server locking to allow for atomic request handling
+
 module = MODBUS
 module-str = Modbus Support
 module-help = Sets log level for Modbus support
diff --git a/subsys/modbus/modbus_core.c b/subsys/modbus/modbus_core.c
index 3e6f87c053b9..e197ebe3a1df 100644
--- a/subsys/modbus/modbus_core.c
+++ b/subsys/modbus/modbus_core.c
@@ -92,8 +92,16 @@ static void modbus_rx_handler(struct k_work *item)
 	if (ctx->client == true) {
 		k_sem_give(&ctx->client_wait_sem);
 	} else if (IS_ENABLED(CONFIG_MODBUS_SERVER)) {
+		if (IS_ENABLED(CONFIG_MODBUS_SERVER_LOCKING)) {
+			k_mutex_lock(&ctx->iface_lock, K_FOREVER);
+		}
+
 		bool respond = modbus_server_handler(ctx);
 
+		if (IS_ENABLED(CONFIG_MODBUS_SERVER_LOCKING)) {
+			k_mutex_unlock(&ctx->iface_lock);
+		}
+
 		if (respond) {
 			modbus_tx_adu(ctx);
 		} else {
@@ -407,3 +415,29 @@ int modbus_disable(const uint8_t iface)
 
 	return 0;
 }
+
+#if IS_ENABLED(CONFIG_MODBUS_LOCKING)
+
+int modbus_lock(const int iface, k_timeout_t timeout)
+{
+	struct modbus_context *ctx = modbus_get_context(iface);
+
+	if (ctx == NULL) {
+		return -ENODEV;
+	}
+
+	return k_mutex_lock(&ctx->iface_lock, timeout);
+}
+
+int modbus_unlock(const int iface)
+{
+	struct modbus_context *ctx = modbus_get_context(iface);
+
+	if (ctx == NULL) {
+		return -ENODEV;
+	}
+
+	return k_mutex_unlock(&ctx->iface_lock);
+}
+
+#endif
diff --git a/subsys/modbus/modbus_internal.h b/subsys/modbus/modbus_internal.h
index 080da58ea09e..5b3071f5a22b 100644
--- a/subsys/modbus/modbus_internal.h
+++ b/subsys/modbus/modbus_internal.h
@@ -109,7 +109,7 @@ struct modbus_context {
 	/* Interface state */
 	atomic_t state;
 
-	/* Client's mutually exclusive access */
+	/* Interface lock */
 	struct k_mutex iface_lock;
 	/* Wait for response semaphore */
 	struct k_sem client_wait_sem;