[nrf noup] modem: backend: uart: SLM optimized modem UART backend

This noup does the following:
- Adds the  modem_backend_uart_slm modem backend which is optimized
  for UART communication to SLM.

The noup is intended to live until upstream zephyr has proper
consistent RTS/CTS flow control support across multiple drivers,
and DTR, RTS and CTS support has been added to the upstream modem
UART backends, and lastly, SLM has been updated to use DTR, so
everything is in sync.

Ask Bjarki Arge Andreasen, Markus Lassila or Seppo Takalo when this
noup is to be altered as part of an upmerge for example.

Signed-off-by: Bjarki Arge Andreasen <[email protected]>
Signed-off-by: Markus Lassila <[email protected]>
(cherry picked from commit 964736d)

Author: bjarki-andreasen

include/zephyr/modem/backend/uart_slm.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/drivers/uart.h>
+#include <zephyr/modem/pipe.h>
+#include <zephyr/modem/stats.h>
+
+#ifndef ZEPHYR_MODEM_BACKEND_UART_SLM_
+#define ZEPHYR_MODEM_BACKEND_UART_SLM_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct slm_rx_queue_event {
+	uint8_t *buf;
+	size_t len;
+};
+
+struct modem_backend_uart_slm {
+	const struct device *uart;
+	struct modem_pipe pipe;
+	struct k_work_delayable receive_ready_work;
+	struct k_work transmit_idle_work;
+
+#ifdef CONFIG_MODEM_STATS
+	struct modem_stats_buffer receive_buf_stats;
+	struct modem_stats_buffer transmit_buf_stats;
+#endif
+	struct ring_buf transmit_rb;
+	struct k_work rx_disabled_work;
+	atomic_t state;
+
+	struct k_mem_slab rx_slab;
+	struct k_msgq rx_queue;
+	struct slm_rx_queue_event rx_event;
+	struct slm_rx_queue_event rx_queue_buf[CONFIG_MODEM_BACKEND_UART_SLM_BUFFER_COUNT];
+	uint32_t rx_buf_size;
+	uint8_t rx_buf_count;
+};
+
+struct modem_backend_uart_slm_config {
+	const struct device *uart;
+	uint8_t *receive_buf; /* Address must be word-aligned. */
+	uint32_t receive_buf_size;
+	uint8_t *transmit_buf;
+	uint32_t transmit_buf_size;
+};
+
+struct modem_pipe *modem_backend_uart_slm_init(struct modem_backend_uart_slm *backend,
+					       const struct modem_backend_uart_slm_config *config);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_MODEM_BACKEND_UART_SLM_ */

subsys/modem/backends/CMakeLists.txt

@@ -10,3 +10,4 @@ if(CONFIG_MODEM_BACKEND_UART_ASYNC)
   zephyr_library_sources_ifdef(CONFIG_MODEM_BACKEND_UART_ASYNC_HWFC modem_backend_uart_async_hwfc.c)
   zephyr_library_sources_ifndef(CONFIG_MODEM_BACKEND_UART_ASYNC_HWFC modem_backend_uart_async.c)
 endif()
+zephyr_library_sources_ifdef(CONFIG_MODEM_BACKEND_UART_SLM modem_backend_uart_slm.c)

subsys/modem/backends/Kconfig

@@ -64,3 +64,26 @@ endif # MODEM_BACKEND_UART_ASYNC_HWFC
 endif # MODEM_BACKEND_UART_ASYNC
 
 endif # MODEM_BACKEND_UART
+
+config MODEM_BACKEND_UART_SLM
+	bool "SLM-optimized modem UART backend"
+	select MODEM_PIPE
+	select RING_BUFFER
+	depends on UART_ASYNC_API
+
+if MODEM_BACKEND_UART_SLM
+
+config MODEM_BACKEND_UART_SLM_BUFFER_COUNT
+	int "SLM modem UART backend buffer count"
+	range 2 4
+	default 3
+
+config MODEM_BACKEND_UART_SLM_TRANSMIT_TIMEOUT_MS
+	int "SLM modem UART transmit timeout in milliseconds"
+	default 1000
+
+config MODEM_BACKEND_UART_SLM_RECEIVE_IDLE_TIMEOUT_MS
+	int "SLM modem UART receive idle timeout in milliseconds"
+	default 30
+
+endif # MODEM_BACKEND_UART_SLM