grub/kern/i386/tsc.c

/* kern/i386/tsc.c - x86 TSC time source implementation
 * Requires Pentium or better x86 CPU that supports the RDTSC instruction.
 * This module uses the RTC (via grub_get_rtc()) to calibrate the TSC to 
 * real time.
 *
 *  GRUB  --  GRand Unified Bootloader
 *  Copyright (C) 2008  Free Software Foundation, Inc.
 *
 *  GRUB is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  GRUB is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <grub/types.h>
#include <grub/time.h>
#include <grub/misc.h>
#include <grub/i386/tsc.h>
#include <grub/i386/pit.h>

/* This defines the value TSC had at the epoch (that is, when we calibrated it). */
static grub_uint64_t tsc_boot_time;

/* Calibrated TSC rate.  (In TSC ticks per millisecond.) */
static grub_uint64_t tsc_ticks_per_ms;


grub_uint64_t
grub_tsc_get_time_ms (void)
{
  return tsc_boot_time + grub_divmod64 (grub_get_tsc (), tsc_ticks_per_ms, 0);
}


/* How many RTC ticks to use for calibration loop. (>= 1) */
#define CALIBRATION_TICKS 2

/* Calibrate the TSC based on the RTC.  */
static void
calibrate_tsc (void)
{
  /* First calibrate the TSC rate (relative, not absolute time). */
  grub_uint64_t start_tsc;
  grub_uint64_t end_tsc;

  start_tsc = grub_get_tsc ();
  grub_pit_wait (0xffff);
  end_tsc = grub_get_tsc ();

  tsc_ticks_per_ms = grub_divmod64 (end_tsc - start_tsc, 55, 0);
}

void
grub_tsc_init (void)
{
  if (grub_cpu_is_tsc_supported ())
    {
      tsc_boot_time = grub_get_tsc ();
      calibrate_tsc ();
      grub_install_get_time_ms (grub_tsc_get_time_ms);
    }
  else
    {
      grub_install_get_time_ms (grub_rtc_get_time_ms);
    }
}