drivers: modem: Add nRF91x SLM Socket Offloaded driver

[wilkinsw]

This is a socket offloaded modem driver for the nRF91 Serial LTE Modem. The driver structure is based on Bjarki's modem_cellular driver and uses the modem chat interface.

For clarity the driver is broken into multiple files. These can be merged in to one file during final rebase/squash if desired.

1. nrf91_slm.c contains the state machine and events. It is a lightly modified version of Bjark's cellular_modem driver to disable PPP (if enabled).
2. nrf91_slm_dns.c contains the socket_dns_offload functions.
3. nrf91_slm_socket.c contains the socket_op_vtable functions.

Right now the driver only supports a single modem and TCP sockets (XSEND and XRECV). I have tested it mostly using MQTT.

Areas for future development:

• Support more than one driver instance.
• Support more than one socket per driver instance.
• Support UDP (XRECVFROM and XSENDTO).
• Support GNSS

Testing details for the mqtt_publisher sample are here: #91601 (comment)

[wilkinsw]

@MarkusLassila you asked to be tagged for this draft PR

[wilkinsw]

My testing was done with samples/net/mqtt_publisher on an NXP LPC5516.

In config.h I set SERVER_ADDR to the address of my server running mosquitto -v

I applied the following patch to wait for registration. (because wait_for_network requires NET_NATIVE)

diff --git a/samples/net/mqtt_publisher/src/main.c b/samples/net/mqtt_publisher/src/main.c
index 03050685b41..3d14ffb8644 100644
--- a/samples/net/mqtt_publisher/src/main.c
+++ b/samples/net/mqtt_publisher/src/main.c
@@ -563,9 +563,45 @@ K_THREAD_DEFINE(app_thread, STACK_SIZE,
 static K_HEAP_DEFINE(app_mem_pool, 1024 * 2);
 #endif
 
+#include <zephyr/device.h>
+#include <zephyr/pm/device.h>
+#include <zephyr/pm/device_runtime.h>
+#include <zephyr/drivers/cellular.h>
+
+static bool modem_registered(const struct device *modem)
+{
+    enum cellular_registration_status status;
+
+    int ret = cellular_get_registration_status(
+            modem, CELLULAR_ACCESS_TECHNOLOGY_LTE, &status);
+
+    if (ret < 0) {
+        return false;
+    }
+
+    return (status == CELLULAR_REGISTRATION_REGISTERED_HOME
+        || status == CELLULAR_REGISTRATION_REGISTERED_ROAMING);
+}
+
 int main(void)
 {
-	wait_for_network();
+	const struct device *modem = DEVICE_DT_GET(DT_ALIAS(modem));
+    struct net_if *iface = net_if_get_first_by_type(&NET_L2_GET_NAME(OFFLOADED_NETDEV));
+
+    LOG_INF("powering on modem");
+    pm_device_action_run(modem, PM_DEVICE_ACTION_RESUME);
+
+    LOG_INF("bring up network interface");
+    if (net_if_up(iface) < 0) {
+        LOG_ERR("failed to bring up network interface");
+    }
+
+	while (!modem_registered(modem)) {
+		k_sleep(K_SECONDS(1));
+	}
+	k_sleep(K_SECONDS(1));
 
 #if defined(CONFIG_MQTT_LIB_TLS)
 	int rc;

I used this configuration overlay to disable NET_NATIVE and enable socket offloading

# UART console
CONFIG_SERIAL=y
CONFIG_CONSOLE=y
CONFIG_LOG=y
CONFIG_UART_INTERRUPT_DRIVEN=y

CONFIG_UART_CONSOLE=y
CONFIG_LOG_BACKEND_UART=y
#CONFIG_MODEM_LOG_LEVEL_DBG=y
#CONFIG_MODEM_MODULES_LOG_LEVEL_DBG=y
#CONFIG_MQTT_LOG_LEVEL_DBG=y

# Enable basic networking and sockets.
CONFIG_NETWORKING=y
CONFIG_NET_SOCKETS=y

# Disable NET_NATIVE
CONFIG_NET_NATIVE=n
CONFIG_NET_L2_PPP=n
CONFIG_NET_IPV4=n
CONFIG_NET_IPV6=n
CONFIG_NET_TCP=n
CONFIG_NET_UDP=n

CONFIG_NET_IPV6_RA_RDNSS=n
CONFIG_NET_CONFIG_SETTINGS=n
CONFIG_NET_SHELL=n

# Enable the socket offloaded driver.
CONFIG_NET_SOCKETS_OFFLOAD=y
CONFIG_MODEM=y
CONFIG_MODEM_NRF91_SLM=y
CONFIG_PM_DEVICE=y

And I used this device tree overlay for my LPC5516.

#include <nxp/nxp_lpc55S16_ns.dtsi>

/ {
	aliases {
		modem = &modem;
	};
};

&pinctrl {
	pinmux_flexcomm6_usart: pinmux_flexcomm6_usart {
		group0 {
			pinmux =
				<FC6_TXD_SCL_MISO_WS_PIO1_16>, 
				<FC6_RXD_SDA_MOSI_DATA_PIO1_13>; 
			slew-rate = "standard";
		};
	};
};

&flexcomm6 {
	status = "okay";
	compatible = "nxp,lpc-usart";
	current-speed = <115200>;
	pinctrl-0 = <&pinmux_flexcomm6_usart>;
	pinctrl-names = "default";

	modem: modem {
		compatible = "nordic,nrf91-slm";
		status = "okay";
		mdm-power-gpios = <&gpio1 20 GPIO_ACTIVE_LOW>;
	};
};

Sample size with socket offloading:

Memory region         Used Size  Region Size  %age Used
           FLASH:       59348 B       246 KB     23.56%
             RAM:       25224 B        64 KB     38.49%
           SRAMX:          0 GB        16 KB      0.00%
           SRAM0:          0 GB        64 KB      0.00%
        USB_SRAM:          0 GB        16 KB      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
2.4% Duplication on New Code

See analysis details on SonarQube Cloud