uClinux feature support

Altera_Forum · ‎04-26-2005

Small question to NIOS2 uClinux users...

Has anyone used, nios2 uClinux with initrd support or RAMDisk ?

Is anyone using any alternative other than romfs.img support mounted as

readonly-root fs supplied as part of evalutaion kit ?

Any alternative suggestions to mount and use a r/w filesystem will be deeply appreciated.

Altera_Forum · ‎04-27-2005

Here is something you can try if you want to use a RAMdisk as the root fs:

Busybox comes with the chroot and pivot_root utilities, so you can use those to change the root fs to your RAMdisk once it is mounted.

What you have to do is create a ramdisk image that has all the files and directories needed for your root filesystem.

A ramdisk image can be created on a Linux host as shown:

dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
/sbin/mke2fs -s0 -vm0 /dev/ram0 2048
/sbin/tune2fs -i 0 /dev/ram0
mount /dev/ram0 /mnt
rmdir /mnt/lost+found
{ populate your filesystem here}
umount /mnt
dd if=/dev/ram0 of=ramfs2048 bs=1k count=2048
./holes ramfs2048 > ramfs2048.img

The holes utility compresses the ramdisk image, here is the source for that utility:

/* make a file with holes */
# include <unistd.h># include <stdlib.h># include <stdio.h># include <sys/types.h># include <sys/stat.h># include <fcntl.h>
char buf;
int
main(int argc, char *argv)
{
  int count;
  int pos;
  int zcount;
  int b;
  int len;
  int in = open(argv, O_RDONLY);
  
  len = lseek(in, 0, SEEK_END);
fprintf(stderr, "TOTAL LEN=%x", len);
  
  lseek(in, 0, SEEK_SET);
  
  len = htonl(len);
  
  write(1, &len, 4);
fprintf(stderr, "\n", len);
  pos = zcount = count = 0;
  while(read(in, &buf, 1) > 0) {
    pos++;
    if (count || buf) {
      count++;
      if (buf == 0) 
        zcount++;
      if ((zcount == 8) || (count == 2048)) {
          b = htonl(pos-count);
          write(1,&b,4);
fprintf(stderr, "POS=%x:", pos-count, b);
          b = htonl(count);
          write(1,&b,4);
fprintf(stderr, "LEN=%x", count, b);
        write(1,buf,count);
fprintf(stderr, "    -->   DATA=%x\n", buf);
    count = zcount = 0;
      }
    }
  }
  if (count) {
    b = htonl(pos-count);
    write(1,&b,4);
fprintf(stderr, "(%d)=%x\n", 4, b);
    b = htonl(count);
    write(1,&b,4);
    write(1,buf,count);
fprintf(stderr, "(%d)=%x    -->   (%d)=%x\n", 4, b, count, buf);
  }
}

Once you have your ramdisk image, you can include it in your Microtronix Nios II Filesystem Project. You can then edit your /etc/inittab and /etc/init.d/rcS files to use pivot_root and chroot on start up to switch the root filesystem to a RAMdisk during start up.

For example, your /etc/inittab file might look something like this:

::askfirst:-/bin/sh
::sysinit:/etc/init.d/rcS

And your /etc/init.d/rcS file could include something like this :

...#  for pivot root
/bin/expand /ramdisk.img /dev/ram0
/bin/mount -t ext2 /dev/ram0 /mnt/ide0 -n
cd /mnt/ide0
mkdir old-root
pivot_root . old-root
exec chroot . sh <dev/console >dev/console 2>&1
umount /old-root
...