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"In computing, 'Not a typewriter' or ENOTTY is an error code defined in the errno.h found on many Unix systems. This code is now used to indicate that an invalid ioctl (input/output control) number was specified in an ioctl system call." Reference: https://en.wikipedia.org/wiki/Not_a_typewriter

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In ioctl() implementations in NuttX, -ENOTTY is always returned if the ioctl() command is not recognized.  You will often see driver ioctl() implementions implement ions with a general structure similar to the file followiningfollowing:

int driver_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
{
  int ret;

  switch (cmd)
    {
      ...

      default:
        ret = -ENOTTY;
        break;
      }
    }

  return ret;
}

Note that -ENOTTY is returned internally in NuttX. With This will subsequently be used to set the errno value to ENOTTY and to return -1 to indicate the error condition.

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Reference: https://lore.kernel.org/patchwork/patch/258361/

How is this useful?

Knowing that no error occurred but the ioctl() command was not recognized is a useful piece of information.  Suppose, for example, I have nfds open character drivers in an array fd[].  Then I could do something like this:

int do_command(FAR int *fd, int nfds, int cmd)
{
  int ret;
  int i;

  /* Try all file descriptors */

  for (i = 0; i < nfds; i++)
    {
      ret = ioctl(fd[i], cmd, 0ul);  /* No argument in this example */
      if (ret < 0)
        {
          int errcode = errno;

          /* Try the next file descriptor if this one return ENOTTY */

          if (errcode != ENOTTY)
            {
              /* Other errors, including EINVAL, are fatal */

              return -errcode
            }
        }
      else if (ret >= 0)
        {
          return OK;  /* Success! */
        }
    }

  return -ENOENT;
}