How to access PCI / MCH bar in linux - c

Question

The background is based on Intel EDS document (example_doc)
see Section 5 ~ 5.2
I was trying to access PCI configuration space to get the CPU vendor ID,
and then accessing the MCH Bar to read out some information under OS base, not developing the driver.
I had search for few days, but most of information are driver related, or using linux kernel library such as pci.h, which is not I prefer.
My Process:
get mcfg base address from /sys/firmware/acpi/tables/MCFG
calculate actual address with bus0, dev0, func0, reg0 (ie. lspci 0000:00:00.0)
read data from the address
Code in C:
BOOLEAN IoConfigRead(uint32_t bus, uint32_t dev, uint32_t func, uint32_t reg, void* Buffer, unsigned long length){
static uint32_t pcieBar = 0;
int fd,fd1;
int sz;
uint32_t buffer[12];
uint32_t addr;
uint32_t *p;
uint32_t value;
if(pcieBar == 0){
fd = open("/sys/firmware/acpi/tables/MCFG",0);
if(fd == -1){
printf("Error opening /sys/firmware/acpi/tables/MCFG");
exit(EXIT_FAILURE);
}
sz = read(fd, buffer, 48);
if (sz != 48) {
printf("couldn't read 48 bytes from MCFG, sz=%d\n", sz);
exit(EXIT_FAILURE);
}
if (close(fd)) {
perror("Error closing /sys/firmware/acpi/tables/MCFG");
exit(EXIT_FAILURE);
}
if (buffer[0] != 0x4746434d) {
printf("MCFG signature not found\n");
exit(EXIT_FAILURE);
}
pcieBar = buffer[11];
}
addr = pcieBar;
addr |= (((DWORD)(func & 0x7)) << 12);
addr |= (((DWORD)(dev & 0x1f)) << 15);
addr |= ((bus & 0xff) << 20);
fd1 = open("/dev/mem", O_RDWR|O_DSYNC);
if (fd1 == -1) {
perror("Error opening /dev/mem");
return EXIT_FAILURE;
}
p = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd1, addr);
if(p == MAP_FAILED){
printf("Failed to mmap address,0x%x\n",addr);
}
else{
int i = 0;
while (i < length/4){
((uint32_t*)Buffer)[i] = p[(reg/4)+i];
i++;
}
munmap(p,4096);
close(fd1);
}
return 0;
}
Error Msg:
Failed to mmap address,0xe0000000
Base on the acpi table the base address is
at byte 0xc(12) which is 0xe0000000 but I'm not able to use mmap to allocate it correctly.
This code seems to be workable in the past, but I'm not able to make it work.
I'm not sure what I had missed. It's already run under sudo, maybe it need some special permission?
I had tried it on kernel: 5.3.18 / 4.12.14-23 neither of them worked.
gcc version : 7.5 / 7.3