modem: backend: tty: Support building with any C library

Ensure we call into the host C library open/close/read/write
independently of which embedded C library the code is built with.

We do this by:
a) Using the native simulator nsi_host* trampolines when we just want
   to call straight into the host libC.
b) Building in the native_simulator runner context (and therefore with
   the host C library) two functions which we call from the embedded
   side.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

subsys/modem/backends/CMakeLists.txt

@@ -3,7 +3,10 @@
 
 zephyr_library()
 
-zephyr_library_sources_ifdef(CONFIG_MODEM_BACKEND_TTY modem_backend_tty.c)
+if(CONFIG_MODEM_BACKEND_TTY)
+  zephyr_library_sources(modem_backend_tty.c)
+  target_sources(native_simulator INTERFACE modem_backend_tty_bottom.c)
+endif()
 zephyr_library_sources_ifdef(CONFIG_MODEM_BACKEND_UART modem_backend_uart.c)
 zephyr_library_sources_ifdef(CONFIG_MODEM_BACKEND_UART_ISR modem_backend_uart_isr.c)
 if(CONFIG_MODEM_BACKEND_UART_ASYNC)

subsys/modem/backends/modem_backend_tty.c

@@ -5,13 +5,12 @@
  */
 
 #include <zephyr/modem/backend/tty.h>
+#include "modem_backend_tty_bottom.h"
 
 #include <zephyr/logging/log.h>
 LOG_MODULE_REGISTER(modem_backend_tty, CONFIG_MODEM_MODULES_LOG_LEVEL);
 
-#include <fcntl.h>
-#include <unistd.h>
-#include <poll.h>
+#include <nsi_host_trampolines.h>
 #include <string.h>
 
 #define MODEM_BACKEND_TTY_THREAD_PRIO          (10)
@@ -23,25 +22,21 @@ LOG_MODULE_REGISTER(modem_backend_tty, CONFIG_MODEM_MODULES_LOG_LEVEL);
 static void modem_backend_tty_routine(void *p1, void *p2, void *p3)
 {
 	struct modem_backend_tty *backend = (struct modem_backend_tty *)p1;
-	struct pollfd pd;
 
 	ARG_UNUSED(p2);
 	ARG_UNUSED(p3);
 
-	pd.fd = backend->tty_fd;
-	pd.events = POLLIN;
-
 	/* Run until run flag is cleared. Check every MODEM_BACKEND_TTY_THREAD_RUN_PERIOD_MS */
 	while (atomic_test_bit(&backend->state, MODEM_BACKEND_TTY_STATE_RUN_BIT)) {
-		/* Clear events */
-		pd.revents = 0;
 
-		if (poll(&pd, 1, MODEM_BACKEND_TTY_THREAD_RUN_PERIOD_MS) < 0) {
+		int ret = modem_backend_tty_poll_bottom(backend->tty_fd);
+
+		if (ret < 0) {
 			LOG_ERR("Poll operation failed");
 			break;
 		}
 
-		if (pd.revents & POLLIN) {
+		if (ret) {
 			modem_pipe_notify_receive_ready(&backend->pipe);
 		}
 
@@ -57,7 +52,7 @@ static int modem_backend_tty_open(void *data)
 		return -EALREADY;
 	}
 
-	backend->tty_fd = open(backend->tty_path, (O_RDWR | O_NONBLOCK), 0644);
+	backend->tty_fd = modem_backend_tty_open_bottom(backend->tty_path);
 	if (backend->tty_fd < 0) {
 		return -EPERM;
 	}
@@ -75,7 +70,7 @@ static int modem_backend_tty_transmit(void *data, const uint8_t *buf, size_t siz
 	struct modem_backend_tty *backend = (struct modem_backend_tty *)data;
 	int ret;
 
-	ret = write(backend->tty_fd, buf, size);
+	ret = nsi_host_write(backend->tty_fd, buf, size);
 	modem_pipe_notify_transmit_idle(&backend->pipe);
 	return ret;
 }
@@ -85,7 +80,7 @@ static int modem_backend_tty_receive(void *data, uint8_t *buf, size_t size)
 	int ret;
 	struct modem_backend_tty *backend = (struct modem_backend_tty *)data;
 
-	ret = read(backend->tty_fd, buf, size);
+	ret = nsi_host_read(backend->tty_fd, buf, size);
 	return (ret < 0) ? 0 : ret;
 }
 
@@ -98,7 +93,7 @@ static int modem_backend_tty_close(void *data)
 	}
 
 	k_thread_join(&backend->thread, K_MSEC(MODEM_BACKEND_TTY_THREAD_RUN_PERIOD_MS * 2));
-	close(backend->tty_fd);
+	nsi_host_close(backend->tty_fd);
 	modem_pipe_notify_closed(&backend->pipe);
 	return 0;
 }

subsys/modem/backends/modem_backend_tty_bottom.c

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2022 Trackunit Corporation
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <fcntl.h>
+#include <poll.h>
+
+int modem_backend_tty_poll_bottom(int fd)
+{
+	struct pollfd pd = { .fd = fd, .events = POLLIN };
+	int ret;
+
+	ret = poll(&pd, 1, 0);
+
+	if ((ret > 0) && !(pd.revents & POLLIN)) {
+		/* we are only interested in POLLIN events */
+		ret = 0;
+	}
+
+	return ret;
+}
+
+int modem_backend_tty_open_bottom(const char *tty_path)
+{
+	return open(tty_path, (O_RDWR | O_NONBLOCK), 0644);
+}

subsys/modem/backends/modem_backend_tty_bottom.h

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_MODEM_BACKEND_UART_TTY_BOTTOM_
+#define ZEPHYR_MODEM_BACKEND_UART_TTY_BOTTOM_
+
+int modem_backend_tty_poll_bottom(int fd);
+int modem_backend_tty_open_bottom(const char *tty_path);
+
+#endif /* ZEPHYR_MODEM_BACKEND_UART_TTY_BOTTOM_ */