/*
 * Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009
 *	The President and Fellows of Harvard College.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <types.h>
#include <kern/errno.h>
#include <kern/syscall.h>
#include <lib.h>
#include <mips/trapframe.h>
#include <thread.h>
#include <current.h>
#include <syscall.h>

/*
 * System call dispatcher.
 *
 * A pointer to the trapframe created during exception entry (in
 * exception-*.S) is passed in.
 *
 * The calling conventions for syscalls are as follows: Like ordinary
 * function calls, the first 4 32-bit arguments are passed in the 4
 * argument registers a0-a3. 64-bit arguments are passed in *aligned*
 * pairs of registers, that is, either a0/a1 or a2/a3. This means that
 * if the first argument is 32-bit and the second is 64-bit, a1 is
 * unused.
 *
 * This much is the same as the calling conventions for ordinary
 * function calls. In addition, the system call number is passed in
 * the v0 register.
 *
 * On successful return, the return value is passed back in the v0
 * register, or v0 and v1 if 64-bit. This is also like an ordinary
 * function call, and additionally the a3 register is also set to 0 to
 * indicate success.
 *
 * On an error return, the error code is passed back in the v0
 * register, and the a3 register is set to 1 to indicate failure.
 * (Userlevel code takes care of storing the error code in errno and
 * returning the value -1 from the actual userlevel syscall function.
 * See src/user/lib/libc/arch/mips/syscalls-mips.S and related files.)
 *
 * Upon syscall return the program counter stored in the trapframe
 * must be incremented by one instruction; otherwise the exception
 * return code will restart the "syscall" instruction and the system
 * call will repeat forever.
 *
 * If you run out of registers (which happens quickly with 64-bit
 * values) further arguments must be fetched from the user-level
 * stack, starting at sp+16 to skip over the slots for the
 * registerized values, with copyin().
 */
void syscall(struct trapframe *tf) {
  int callno;
  int32_t retval;
  int err;

  KASSERT(curthread != NULL);
  KASSERT(curthread->t_curspl == 0);
  KASSERT(curthread->t_iplhigh_count == 0);

  callno = tf->tf_v0;

  /*
   * Initialize retval to 0. Many of the system calls don't
   * really return a value, just 0 for success and -1 on
   * error. Since retval is the value returned on success,
   * initialize it to 0 by default; thus it's not necessary to
   * deal with it except for calls that return other values,
   * like write.
   */

  retval = 0;

  switch (callno) {
  case SYS_reboot:
    err = sys_reboot(tf->tf_a0);
    break;

  case SYS___time:
    err = sys___time((userptr_t)tf->tf_a0, (userptr_t)tf->tf_a1);
    break;

  case SYS__exit:
    err = 0;
    thread_exit();
    break;

  case SYS_printchar:
    err = sys_printchar((char)tf->tf_a0);
    break;

    /* Add stuff here */

  default:
    kprintf("Unknown syscall %d\n", callno);
    err = ENOSYS;
    break;
  }

  if (err) {
    /*
     * Return the error code. This gets converted at
     * userlevel to a return value of -1 and the error
     * code in errno.
     */
    tf->tf_v0 = err;
    tf->tf_a3 = 1; /* signal an error */
  } else {
    /* Success. */
    tf->tf_v0 = retval;
    tf->tf_a3 = 0; /* signal no error */
  }

  /*
   * Now, advance the program counter, to avoid restarting
   * the syscall over and over again.
   */

  tf->tf_epc += 4;

  /* Make sure the syscall code didn't forget to lower spl */
  KASSERT(curthread->t_curspl == 0);
  /* ...or leak any spinlocks */
  KASSERT(curthread->t_iplhigh_count == 0);
}

/*
 * Enter user mode for a newly forked process.
 *
 * This function is provided as a reminder. You need to write
 * both it and the code that calls it.
 *
 * Thus, you can trash it and do things another way if you prefer.
 */
void enter_forked_process(struct trapframe *tf) { (void)tf; }
