SW4STM32 and SW4Linux fully supports the STM32MP1 asymmetric multicore Cortex/A7+M4 MPUs

   With System Workbench for Linux, Embedded Linux on the STM32MP1 family of MPUs from ST was never as simple to build and maintain, even for newcomers in the Linux world. And, if you install System Workbench for Linux in System Workbench for STM32 you can seamlessly develop and debug asymmetric applications running partly on Linux, partly on the Cortex-M4.
You can get more information from the ac6-tools website and download (registration required) various documents highlighting:

System Workbench for STM32

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printf through uart

In fact there is a simpler way to do that; all these functions are already implemented in the syscalls.c file provided in projects created by Ac6 System Workbench. The only think you need to provide are the blocking USART read/write routins that should be defined as:
int __io_putchar(int ch) {
    // Code to write character 'ch' on the UART

int __io_getchar(void) {
    // Code to read a character from the UART

The only thing that was not (yet) done by the provided code s to allow to open several “files” on various devices and write or read from them. For this to work you should just redefine _open, _read and _write, probably by modifying the syscalls.c file, and this will work seamlessly.

Furthermore, if you use some RTOS (like FreeRTOS) you can write your low-level I/O routines to sleep while waiting characters and send output using a buffer and interrupts; probably this would need modifying the _read and _write routines in a quite natural way.