output.txt

$ ./rmw 
set: from initial: 0xffffffff, setting mode: 6 => 0xffff6fff
set2: from initial: 0xffffffff, setting mode: 6 => 0xffff6fff
set: from initial: 0, setting mode: 6 => 0   <<<< failed to set properly.
set2: from initial: 0, setting mode: 6 => 0x6000

rmw.c

#include <stdint.h>
#include <stdio.h>

void set(uint32_t initial, uint32_t mode) {
	uint32_t reg = initial;
	uint32_t mask = 0xf << 12;
	reg &= ~mask | (mode << 12);
	printf("set: from initial: %#x, setting mode: %x => %#x\n", initial, mode, reg);
}

void set2(uint32_t initial, uint32_t mode) {
	uint32_t reg = initial;
	uint32_t mask = 0xf;
	uint32_t shift = 12;
	reg = (reg & ~(mask << shift)) | (mode << shift);
	printf("set2: from initial: %#x, setting mode: %x => %#x\n", initial, mode, reg);
}

void test_val(uint32_t initial, uint32_t mode) {
	set(initial, mode);
	set2(initial, mode);
}

int main() {
	test_val(0xffffffff, 6);
	test_val(0x0, 6);
}