arch/common/boot/grub2/kern/i386/tsc.c

   1 /* kern/i386/tsc.c - x86 TSC time source implementation
   2  * Requires Pentium or better x86 CPU that supports the RDTSC instruction.
   3  * This module uses the RTC (via grub_get_rtc()) to calibrate the TSC to
   4  * real time.
   5  *
   6  *  GRUB  --  GRand Unified Bootloader
   7  *  Copyright (C) 2008  Free Software Foundation, Inc.
   8  *
   9  *  GRUB is free software: you can redistribute it and/or modify
  10  *  it under the terms of the GNU General Public License as published by
  11  *  the Free Software Foundation, either version 3 of the License, or
  12  *  (at your option) any later version.
  13  *
  14  *  GRUB is distributed in the hope that it will be useful,
  15  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  16  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  17  *  GNU General Public License for more details.
  18  *
  19  *  You should have received a copy of the GNU General Public License
  20  *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
  21  */
  22
  23 #include <grub/types.h>
  24 #include <grub/time.h>
  25 #include <grub/misc.h>
  26 #include <grub/i386/tsc.h>
  27 #include <grub/i386/pit.h>
  28
  29 /* This defines the value TSC had at the epoch (that is, when we calibrated it). */
  30 static grub_uint64_t tsc_boot_time;
  31
  32 /* Calibrated TSC rate.  (In TSC ticks per millisecond.) */
  33 static grub_uint64_t tsc_ticks_per_ms;
  34
  35
  36 grub_uint64_t
  37 grub_tsc_get_time_ms (void)
  38 {
  39   return tsc_boot_time + grub_divmod64 (grub_get_tsc (), tsc_ticks_per_ms, 0);
  40 }
  41
  42
  43 /* How many RTC ticks to use for calibration loop. (>= 1) */
  44 #define CALIBRATION_TICKS 2
  45
  46 /* Calibrate the TSC based on the RTC.  */
  47 static void
  48 calibrate_tsc (void)
  49 {
  50   /* First calibrate the TSC rate (relative, not absolute time). */
  51   grub_uint64_t start_tsc;
  52   grub_uint64_t end_tsc;
  53
  54   start_tsc = grub_get_tsc ();
  55   grub_pit_wait (0xffff);
  56   end_tsc = grub_get_tsc ();
  57
  58   tsc_ticks_per_ms = grub_divmod64 (end_tsc - start_tsc, 55, 0);
  59 }
  60
  61 void
  62 grub_tsc_init (void)
  63 {
  64   if (grub_cpu_is_tsc_supported ())
  65     {
  66       tsc_boot_time = grub_get_tsc ();
  67       calibrate_tsc ();
  68       grub_install_get_time_ms (grub_tsc_get_time_ms);
  69     }
  70   else
  71     {
  72       grub_install_get_time_ms (grub_rtc_get_time_ms);
  73     }
  74 }