using /dev/mem to talk to peripherals on VIIP system

scott scotto2hot-green at
Sat Jul 29 03:35:19 EST 2006

Hello again,
well i didn't get any responses at all to my last post so I thought i'd try this again.

I have kernel 2.4.26 installed on a Xilinx XUP board and everything works well except I can't seem to communicate with peripherals on the OPB or PLB bus through devfs.  devfs is on and mounted and everything, and I'm able open a file handle to it w/ the following code:

int main (int argc, char *argv[]) {
  int fd;
  int *ptr;

  fd = open("/dev/mem", O_RDWR);
  if(fd == -1) {
    printf("Err: cannot open /dev/mem\n");
    return -1;

  ptr = MAP_FAILED; // Initialize to bad value
  ptr = (int *) mmap(0, 256, PROT_READ|PROT_WRITE, MAP_SHARED, fd, USER_LOGIC_BASEADDR);

  if(ptr==MAP_FAILED) {
    printf("Err: cannot access address!\n");
    return -1;

  *ptr = 0xA0000000;

mmap also seems to work, and returns a virtual address that is supposed to point to the location in physical memory at USER_LOGIC_BASEADDR.  when I try to use ptr, however, I either get 
bus error


do_wp_page: bogus page at address 3001a000 (page 0xc2c4c004)
VM: killing process led_test.o

depending on whether I use MAP_SHARED or MAP_PRIVATE.

These errors occur if I try to access any device on either PLB or OPB - even the OPB UART (which i KNOW is working).  This technique DOES work, however, for address ranges in RAM.  This indicates to me that it's some sort of higher level memory management problem w/ linuxppc such that it doesn't realize these address ranges are valid.  I assume this is a function of the BSP that gets generated in EDK?  Anybody have any idea how I can tell linux to let me use these addresses?  Any other thoughts?

If I'm being too vague or something please let me know and I'll resolve it immediately...

Thanks much, --scott

Other details:
busybox 1.1.0
powerpc-405-linux-gnu-gcc 3.4.1

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