Porting the Modem library to your OS

The Modem library from Nordic Semiconductor is an OS-agnostic C library. OS-specific parts are moved out of the library. To use the library with your OS or scheduler, you must first port it by creating a custom nrf_modem_os.c file, which serves as an OS abstraction layer. The library exposes a nrf_modem_os.h header file that defines the functions that must be implemented in nrf_modem_os.c. The header file also exposes the functions provided by the library that the OS integration module must interface with.

The following diagram presents the Modem library OS abstraction layer. Arrows indicate that the elements can communicate with each other directly.

Modem library OS abstraction overview

Modem library OS abstraction overview

Creating the OS abstraction layer

To create an OS abstraction layer for the Modem library, you must implement the functions in the nrf_modem_os.h file.

nrf_modem_os_init

This function is called by the Modem library when the application has issued nrf_modem_init(). It is responsible for preparing IRQ for low priority Modem library scheduling and trace scheduling.

Note

When working with an application based on Zephyr, set the IRQs to a low priority (6 or 7) and enable them before exiting the function.

The function must also initialize the timers and threads (if there is a context that needs a timeout). If Nordic Proprietary trace is enabled, the library generates trace data and forwards it to a medium that can be initialized or configured by using the nrf_modem_os_init() function. The forwarded trace data is handled in the nrf_modem_os_trace_put() function. See nrf_modem_os_trace_put for more information.

Required actions:

  • Initialize timers/threads.

  • Configure low priority Modem library scheduling IRQ (SoftIRQ).

  • Configure low priority trace scheduling IRQ (SoftIRQ).

  • Configure medium for trace (for example, UART or SPI).

nrf_modem_os_timedwait

This function is called by the Modem library when a timed context or sleep is required. A blind return value of 0 will make all the Modem library operations always blocking.

Note

To ensure all waiting threads are awake on the nrf_modem_event_notify() function, the waiting threads must not be cleared on nrf_modem_os_init() or nrf_modem_os_shutdown() functions.

Required actions to be taken that do not block the operations:

  • Start counting the time (this can be based on a Timer or Thread for instance).

  • In the case of a call to nrf_modem_os_event_notify() function, all waiting threads must wake up, reporting back the remaining time of the timer through the timeout variable.

The following points decide the Function return value:

  • If the modem is not initialized, that is, if nrf_modem_is_initialized() returns false, function return value will be -NRF_ESHUTDOWN.

  • If there is a time out, function return value will be -NRF_EAGAIN.

  • In all other cases, function return value will be 0.

nrf_modem_os_event_notify

This function is called by the Modem library when an event occurs and all threads waiting in nrf_modem_os_timedwait() function wake up.

Required action:

Wake up all sleeping threads in nrf_modem_os_timedwait() function. For details, see nrf_modem_os_timedwait().

nrf_modem_os_alloc

This function is called by the library to allocate memory dynamically, and it is like a malloc call. There are no specific requirements related to the location where this memory must be allocated in RAM.

nrf_modem_os_free

This function must free the memory allocated by nrf_modem_os_alloc().

nrf_modem_os_shm_tx_alloc

This function is called by the library to dynamically allocate the memory that must be shared with the modem core. This function allocates memory on the TX memory region that is passed to the nrf_modem_init() function during the initialization.

nrf_modem_os_shm_tx_free

This function releases the memory allocated by nrf_modem_os_shm_tx_alloc().

nrf_modem_os_errno_set

This function translates errnos from the Modem library to the OS-defined ones.

Required action:

  • Implement a translation for each errno set by the Modem library. If it overlaps with your OS errno, the translation is not needed.

nrf_modem_os_application_irq_clear

This function is called by the Modem library when the library wants to clear IRQ on the low priority Modem library scheduling IRQ.

Required action:

  • Clear the low priority Modem library scheduling IRQ using OS primitives or NVIC functions.

nrf_modem_os_application_irq_set

This function is called by the Modem library when the library wants to set a pending IRQ on the low priority Modem library scheduling IRQ.

Required action:

  • Set a pending IRQ on the low priority Modem library scheduling IRQ using OS primitives or NVIC functions.

nrf_modem_os_trace_irq_clear

This function is called by the Modem library when the library wants to clear IRQ on the low priority trace scheduling IRQ.

Required action:

  • Clear the low priority trace scheduling IRQ using OS primitives or NVIC functions.

nrf_modem_os_trace_irq_set

This function is called by the Modem library when the library wants to set a pending IRQ on the low priority trace scheduling IRQ.

Required action:

  • Set a pending IRQ on the low priority trace scheduling IRQ using OS primitives or NVIC functions.

nrf_modem_os_trace_put

This function puts the trace string to the desired medium, typically UART. However, the medium used to forward and store the traces is up to the implementation and must be initialized correctly before using. Once the traces are processed or stored, the nrf_modem_trace_processed_callback() must be called. Even if you do not want the traces further, you need to ensure that nrf_modem_trace_processed_callback() is called for each received trace. Until the nrf_modem_trace_processed_callback() is called, the Modem library do not free up the memory allocated for that trace in the trace memory area. Since the modem uses this trace memory area to send traces, not calling the nrf_modem_trace_processed_callback(), leads to losing modem traces.

nrf_modem_application_irq_handler

This function is implemented in the Modem library and must be called upon the low priority Modem library IRQ handler, triggered by the nrf_modem_os_application_irq_set() function.

nrf_modem_trace_irq_handler

This function is implemented in the Modem library and must be called upon the low priority trace IRQ handler, triggered by the nrf_modem_os_trace_irq_set() function.

nrf_modem_os_log

This function is called by the library to output logs. This function can be called in an interrupt context.

nrf_modem_os_logdump

This function is called by the library to dump binary data. This function can be called in an interrupt context.

nrf_modem_os_log_strdup

The Modem library calls this function for each logged string that does not reside in read-only memory. The returned value will be a pointer to a string that can be logged correctly by the logging functions. This function might be necessary for some implementations of the logging functions, which might, for example, defer the logging at a later point in time.

Other scenarios to handle in nrf_modem_os.c

  1. In case the OS has its own IRQ handler scheme, which is not directly forwarding the IPC_IRQHandler to the Modem library, this must be routed by the OS. The OS must call IPC_IRQHandler() upon all IRQs with IRQ number IPC_IRQn.

  2. In nrf_modem_os.c, you can configure a desired medium for forwarding the trace data upon nrf_modem_os_trace_put() calls.

Memory

The Modem library needs a region of RAM within the first lower 128KB to share with the modem. To be accessible by both the modem and the application, this region of RAM must be configured as non-secure by the secure application.

The following RAM overview diagram shows the placement of Modem library in the sequential RAM, and it also indicates the configurable memory position values.

Modem library memory overview

Modem library memory overview

Peripheral requirements

As the Modem library has been compiled to operate on peripherals in the non-secure domain, the following two peripherals must be configured to be non-secure:

  • NRF_IPC

  • NRF_POWER

If you are using the hard-float variant of the Modem library, the FPU must be activated in both the secure domain and the non-secure domain, and must be configured to allow the non-secure application to run FPU instructions.

The nrfx/mdk/system_nrf9160.c file provides a template on how to configure the FPU in both cases. The system file also provides several Errata workarounds specific to the chip variant used, which are needed for any secure domain application.

Message sequence diagrams

The following message sequence diagrams show the interactions between the application, Modem library, and the OS.

  1. Sequence of the initialization of the Modem library. Configuration of the high and low priority IRQs:

    Initialization (main thread)

    Initialization (main thread)

  2. Handling an event sent from the Modem library to a lower priority to be able to receive new events:

    Event handling, lowering priority

    Event handling, lowering priority

  3. Handling traces:

    Trace handling, lowering priority

    Trace handling, lowering priority

  4. Handling a timeout or sleep:

    Timers

    Timers

Reference template for the nrf_modem_os.c file

The following code snippet shows a simple implementation of the Modem library OS abstraction layer. You can use it as a template and customize it for your OS or scheduler.

#include <nrf_modem_os.h>
#include <nrf_errno.h>
#include <nrf_modem_platform.h>
#include <nrf_modem_limits.h>

#include <nrf.h>
#include "errno.h"

#define TRACE_IRQ EGU2_IRQn
#define TRACE_IRQ_PRIORITY 6
#define TRACE_IRQ_HANDLER EGU2_IRQHandler

void read_task_create(void)
{
        // The read task is achieved using SW interrupt.
        NVIC_SetPriority(NRF_MODEM_APPLICATION_IRQ, NRF_MODEM_APPLICATION_IRQ_PRIORITY);
        NVIC_ClearPendingIRQ(NRF_MODEM_APPLICATION_IRQ);
        NVIC_EnableIRQ(NRF_MODEM_APPLICATION_IRQ);
}

void trace_task_create(void) {
        NVIC_SetPriority(TRACE_IRQ, TRACE_IRQ_PRIORITY);
        NVIC_ClearPendingIRQ(TRACE_IRQ);
        NVIC_EnableIRQ(TRACE_IRQ);
}

void nrf_modem_os_init(void) {
        read_task_create();
        trace_task_create();
        // Initialize timers / sleeping threads used in the nrf_modem_os_timedwait function.
        // Initialize trace medium used in the nrf_modem_os_trace_put function.
}

int32_t nrf_modem_os_timedwait(uint32_t context, int32_t * timeout)
{
        // Return remaining time by reference in timeout parameter,
        // if not yet timed out.
        // Else return NRF_ETIMEDOUT if timeout has triggered.
        // A blind return value of 0 will make all Modem library operations
        // always block.
        return 0;
}

void nrf_modem_os_event_notify()
{
        // Wake threads in nrf_modem_os_timedwait()
}

void nrf_modem_os_errno_set(int errno_val) {
        // Translate nrf_errno.h errno to the OS specific value.
}

void nrf_modem_os_application_irq_set(void) {
        NVIC_SetPendingIRQ(NRF_MODEM_APPLICATION_IRQ);
}

void nrf_modem_os_application_irq_clear(void) {
        NVIC_ClearPendingIRQ(NRF_MODEM_APPLICATION_IRQ);
}

void NRF_MODEM_APPLICATION_IRQ_HANDLER(void) {
        nrf_modem_application_irq_handler();
}

void nrf_modem_os_trace_irq_set(void) {
        NVIC_SetPendingIRQ(TRACE_IRQ);
}

void nrf_modem_os_trace_irq_clear(void)
{
        NVIC_ClearPendingIRQ(TRACE_IRQ);
}

void TRACE_IRQ_HANDLER(void) {
        nrf_modem_trace_irq_handler();
}

int32_t nrf_modem_os_trace_put(const uint8_t * const p_buffer, uint32_t buf_len) {
        // Store buffer to chosen medium.
        // Traces can be dropped if not needed.
        // Either call nrf_modem_trace_processed_callback() here or at a later point (for example, in a
        // thread or a work queue handler function).
        int err = nrf_modem_trace_processed_callback(p_buffer, buf_len);
        return 0;
}