The shell sample, and a two-board rig for testing on hardware

samples/shell is `rb9704` on the shell with callbacks that print what
arrives, a CRC-32 and the start of each message from the network. Board
overlays for native_sim (the modem a host serial port, a pty or a real
9704 on USB), NUCLEO-U575ZI-Q, XIAO ESP32S3 and ESP32-S3-DevKitC.

tests/rig/bridge copies bytes between a board's USB serial port and a
UART, so two boards wired together stand in for a USB-serial adapter: one
runs the sample, the other bridges its modem UART to the host, where the
fake modem answers. Overlays for a Nucleo and a XIAO, either way round.

tests/rig/bridge/CMakeLists.txt

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: MIT
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(rb9704_bridge)
+target_sources(app PRIVATE src/main.c)

tests/rig/bridge/boards/nucleo_u575zi_q.overlay

@@ -0,0 +1,18 @@
+/* host: USART1, the ST-LINK's virtual COM port. target: USART2 on PD5/PD6,
+ * the CN9 UART block, wired to the other board's modem UART. Both at the
+ * modem's 230400. */
+/ {
+	aliases {
+		bridge-host = &usart1;
+		bridge-target = &usart2;
+	};
+};
+
+&usart1 {
+	current-speed = <230400>;
+};
+
+&usart2 {
+	current-speed = <230400>;
+	fifo-enable;
+};

tests/rig/bridge/boards/xiao_esp32s3_procpu.overlay

@@ -0,0 +1,33 @@
+/* host: the native USB serial port. target: uart1 on D8 (GPIO7, TX) and D9
+ * (GPIO8, RX), wired to the other board's modem UART. 230400, the modem's
+ * rate; the USB side has none. */
+/ {
+	aliases {
+		bridge-host = &usb_serial;
+		bridge-target = &uart1;
+	};
+};
+
+&spi2 {
+	status = "disabled";	/* the board puts it on GPIO7 to 9 */
+};
+
+&pinctrl {
+	uart1_bridge: uart1_bridge {
+		group1 {
+			pinmux = <UART1_TX_GPIO7>;
+			output-high;
+		};
+		group2 {
+			pinmux = <UART1_RX_GPIO8>;
+			bias-pull-up;
+		};
+	};
+};
+
+&uart1 {
+	pinctrl-0 = <&uart1_bridge>;
+	pinctrl-names = "default";
+	current-speed = <230400>;
+	status = "okay";
+};

tests/rig/bridge/prj.conf

@@ -0,0 +1,9 @@
+CONFIG_SERIAL=y
+CONFIG_UART_INTERRUPT_DRIVEN=y
+CONFIG_RING_BUFFER=y
+# Both UARTs carry the bridged bytes; nothing else may print on them.
+CONFIG_CONSOLE=n
+CONFIG_UART_CONSOLE=n
+CONFIG_PRINTK=n
+CONFIG_LOG=n
+CONFIG_BOOT_BANNER=n

tests/rig/bridge/src/main.c

@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * A two-way UART bridge, for the test rig: every byte in on `host` goes out
+ * on `target`, and the other way round. With `host` a board's USB serial
+ * port and `target` a UART wired to another board's modem pins, the fake
+ * modem on the host answers that board as a modem would. The aliases are
+ * the board overlay's. */
+#include <zephyr/kernel.h>
+#include <zephyr/device.h>
+#include <zephyr/drivers/uart.h>
+#include <zephyr/sys/ring_buffer.h>
+
+static const struct device *const host = DEVICE_DT_GET(DT_ALIAS(bridge_host));
+static const struct device *const target = DEVICE_DT_GET(DT_ALIAS(bridge_target));
+
+RING_BUF_DECLARE(to_target, 4096);
+RING_BUF_DECLARE(to_host, 4096);
+
+static void isr(const struct device *dev, void *user)
+{
+	struct ring_buf *out = user;	/* bytes read here go into this ring */
+	const struct device *other = dev == host ? target : host;
+	struct ring_buf *back = out == &to_target ? &to_host : &to_target;
+	uint8_t b[64];
+
+	while (uart_irq_update(dev) && uart_irq_is_pending(dev)) {
+		if (uart_irq_rx_ready(dev)) {
+			int n = uart_fifo_read(dev, b, sizeof(b));
+
+			if (n > 0) {
+				ring_buf_put(out, b, n);
+				uart_irq_tx_enable(other);
+			}
+		}
+		if (uart_irq_tx_ready(dev)) {
+			uint8_t *p;
+			uint32_t n = ring_buf_get_claim(back, &p, 64);
+
+			if (n == 0) {
+				uart_irq_tx_disable(dev);
+			} else {
+				int w = uart_fifo_fill(dev, p, n);
+
+				ring_buf_get_finish(back, w > 0 ? w : 0);
+			}
+		}
+	}
+}
+
+int main(void)
+{
+	uart_irq_callback_user_data_set(host, isr, &to_target);
+	uart_irq_callback_user_data_set(target, isr, &to_host);
+	uart_irq_rx_enable(host);
+	uart_irq_rx_enable(target);
+	return 0;
+}

tests/rig/nucleo_u575zi_q_rig.overlay

@@ -0,0 +1,13 @@
+/* The shell sample on a NUCLEO-U575ZI-Q in the test rig: the modem UART on
+ * USART2, PD5 (TX) and PD6 (RX) on the CN9 UART block, wired to the bridge
+ * board's target UART (bridge/). The shell stays on the ST-LINK's port.
+ *   west build -b nucleo_u575zi_q samples/shell -- \
+ *     -DDTC_OVERLAY_FILE=$PWD/tests/rig/nucleo_u575zi_q_rig.overlay */
+&usart2 {
+	current-speed = <230400>;
+	fifo-enable;
+
+	rockblock {
+		compatible = "rock7,rockblock-9704";
+	};
+};

tests/rig/xiao_esp32s3_rig.overlay

@@ -0,0 +1,32 @@
+/* The shell sample on a XIAO ESP32S3 in the test rig: the modem UART on
+ * D8 (GPIO7, TX) and D9 (GPIO8, RX), wired to the bridge board's target
+ * UART (bridge/). Use it in place of the sample's own XIAO overlay:
+ *   west build -b xiao_esp32s3/esp32s3/procpu samples/shell -- \
+ *     -DDTC_OVERLAY_FILE=$PWD/tests/rig/xiao_esp32s3_rig.overlay */
+&spi2 {
+	status = "disabled";	/* the board puts it on GPIO7 to 9 */
+};
+
+&pinctrl {
+	uart1_rig: uart1_rig {
+		group1 {
+			pinmux = <UART1_TX_GPIO7>;
+			output-high;
+		};
+		group2 {
+			pinmux = <UART1_RX_GPIO8>;
+			bias-pull-up;
+		};
+	};
+};
+
+&uart1 {
+	pinctrl-0 = <&uart1_rig>;
+	pinctrl-names = "default";
+	current-speed = <230400>;
+	status = "okay";
+
+	rockblock {
+		compatible = "rock7,rockblock-9704";
+	};
+};