How to control hard drive motor speed with C on linux?

Question

I've got an old HDD with which I planned to fiddle around a little. First thing I'm trying to do is spinning the motor with different speeds.

Questions are:

1. Is there a general way to do this or does it depend on the HDD model?
2. Where do I find a list of commands, that I can send to the HDD controller to control the speed of the motor?

I actually found a function, that apparently spins down the motor, here it is:

/* spin-down a disk */
static void spindown_disk(const char *name)
{
  struct sg_io_hdr io_hdr;
  unsigned char sense_buf[255];
  char dev_name[100];
  int fd;

  dprintf("spindown: %s\n", name);

  /* fabricate SCSI IO request */
  memset(&io_hdr, 0x00, sizeof(io_hdr));
  io_hdr.interface_id = 'S';
  io_hdr.dxfer_direction = SG_DXFER_NONE;

  /* SCSI stop unit command */
  io_hdr.cmdp = (unsigned char *) "\x1b\x00\x00\x00\x00\x00";

  io_hdr.cmd_len = 6;
  io_hdr.sbp = sense_buf;
  io_hdr.mx_sb_len = (unsigned char) sizeof(sense_buf);

  /* open disk device (kernel 2.4 will probably need "sg" names here) */
  snprintf(dev_name, sizeof(dev_name), "/dev/%s", name);
  if ((fd = open(dev_name, O_RDONLY)) < 0) {
    perror(dev_name);
    return;
  }

  /* execute SCSI request */
  if (ioctl(fd, SG_IO, &io_hdr) < 0) {
    char buf[100];
    snprintf(buf, sizeof(buf), "ioctl on %s:", name);
    perror(buf);

  } else if (io_hdr.masked_status != 0) {
    fprintf(stderr, "error: SCSI command failed with status 0x%02x\n",
            io_hdr.masked_status);
    if (io_hdr.masked_status == CHECK_CONDITION) {
      phex(sense_buf, io_hdr.sb_len_wr, "sense buffer:\n");
    }
  }

  close(fd);
}

Though I don't really understand where the actual command is sent to the controller, nor do I know how to control the speed, I don't see any rpm specifications.

Answer 1

You cannot control a harddisk's rotational speed, and that is a good thing. If you could, you would inevitably destroy data.

The heads float in what is commonly called "air bearing". This is, in easy words, a spring mechanism pressing the head onto the disks's surface with a well-defined force and an air cussion from airflow due to the disk's rotation being in equilibrium at the disk's operational speed. When the disk is shut down, another spring mechanisms quickly pulls the heads out of the way into a kind of "parking position".

If you could run the drive at arbitrary speeds, the heads would scratch on the surface. Not good!

As to where the actual command is being sent in above snippet, it is the ioctl call in the line following /* execute SCSI request */.
If you are interested in playing with your old harddisk (be aware that you'll quite likely break it!), have a look at the hdparm tool and its source code. hdparm lets you tweak dozens of parameters such as power save modes, caching, or acustic management... pretty much everything that disk drives support.
In the tool's source code, you'll find a quite complete list of device commands, too.