Re: read PCI memory and config spyce through /dev/mem

May 7, 2013


      On Tue, May 07, 2013 at 06:35:43AM +0000, Warlich, Christof wrote:
[...]
...
Furthermore, read / write access through a simple program using /dev/mem
_does_ work as expected:
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <assert.h>
#include <sys/mman.h>
int main( int argc, char *argv[]) {
  char *mem;
  int fd;
  fd = open ("/dev/mem", O_RDWR);
  assert(fd >= 0);
  mem = mmap(NULL, getpagesize(), PROT_READ|PROT_WRITE, MAP_SHARED, fd, (off_t) 0x81423000);
  assert(mem != MAP_FAILED);
  printf("Memory pointer: %p\n", mem);
  printf("The PCI memory is : %#x\n", *mem);
  *mem = *argv[1];
  printf("The PCI memory is : %#x\n", *mem);
  munmap(mem, getpagesize());
  close(fd);
  return 0;
}
Calling it, yields:
$ led 4
Memory pointer: 0xb7678000
The PCI memory is : 0x33
The PCI memory is : 0x34
And the LED matrix display being behind that address switches from 3 to
4 as expected.
Thus, I still cannot see why the access through /dev/mem using dd doesn't
work.
I suspect dd uses read instead of mmap.


Thanks,
Jonathan Neuschäfer