103 lines
3 KiB
C
103 lines
3 KiB
C
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/* kern/i386/tsc.c - x86 TSC time source implementation
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* Requires Pentium or better x86 CPU that supports the RDTSC instruction.
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* This module uses the RTC (via grub_get_rtc()) to calibrate the TSC to
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* real time.
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*
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* GRUB -- GRand Unified Bootloader
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* Copyright (C) 2008 Free Software Foundation, Inc.
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*
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* GRUB is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* GRUB is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with GRUB. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <grub/types.h>
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#include <grub/time.h>
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#include <grub/misc.h>
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#include <grub/i386/tsc.h>
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/* Calibrated reference for TSC=0. This defines the time since the epoch in
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milliseconds that TSC=0 refers to. */
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static grub_uint64_t tsc_boot_time;
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/* Calibrated TSC rate. (In TSC ticks per millisecond.) */
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static grub_uint64_t tsc_ticks_per_ms;
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grub_uint64_t
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grub_tsc_get_time_ms (void)
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{
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return tsc_boot_time + grub_divmod64 (grub_get_tsc (), tsc_ticks_per_ms, 0);
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}
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/* How many RTC ticks to use for calibration loop. (>= 1) */
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#define CALIBRATION_TICKS 2
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/* Calibrate the TSC based on the RTC. */
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static void
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calibrate_tsc (void)
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{
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/* First calbrate the TSC rate (relative, not absolute time). */
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grub_uint64_t start_tsc;
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grub_uint64_t end_tsc;
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grub_uint32_t initial_tick;
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grub_uint32_t start_tick;
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grub_uint32_t end_tick;
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/* Wait for the start of the next tick;
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we'll base out timing off this edge. */
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initial_tick = grub_get_rtc ();
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do
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{
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start_tick = grub_get_rtc ();
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}
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while (start_tick == initial_tick);
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start_tsc = grub_get_tsc ();
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/* Wait for the start of the next tick. This will
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be the end of the 1-tick period. */
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do
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{
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end_tick = grub_get_rtc ();
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}
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while (end_tick - start_tick < CALIBRATION_TICKS);
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end_tsc = grub_get_tsc ();
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tsc_ticks_per_ms =
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grub_divmod64 (grub_divmod64
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(end_tsc - start_tsc, end_tick - start_tick, 0)
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* GRUB_TICKS_PER_SECOND, 1000, 0);
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/* Reference the TSC zero (boot time) to the epoch to
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get an absolute real time reference. */
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grub_uint64_t ms_since_boot = grub_divmod64 (end_tsc, tsc_ticks_per_ms, 0);
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grub_uint64_t mstime_now = grub_divmod64 ((grub_uint64_t) 1000 * end_tick,
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GRUB_TICKS_PER_SECOND,
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0);
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tsc_boot_time = mstime_now - ms_since_boot;
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}
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void
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grub_tsc_init (void)
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{
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if (grub_cpu_is_tsc_supported ())
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{
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calibrate_tsc ();
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grub_install_get_time_ms (grub_tsc_get_time_ms);
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}
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else
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{
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grub_install_get_time_ms (grub_rtc_get_time_ms);
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}
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}
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