57ca654bef
We have recently had an example of someone wanting to use a 90kHz timer for the software delay loop. udelay() needs to have at least microsecond resolution to allow drivers access to a delay mechanism with a reasonable chance of delaying the period they requested within at least a 50% marging of error, especially for small delays. Discussion about the udelay() accuracy can be found at: https://lkml.org/lkml/2011/1/9/37 Reject timers which are unable to supply this level of resolution. Acked-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
117 lines
3 KiB
C
117 lines
3 KiB
C
/*
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* Delay loops based on the OpenRISC implementation.
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*
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* Copyright (C) 2012 ARM Limited
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*
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* This program 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 this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*
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* Author: Will Deacon <will.deacon@arm.com>
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*/
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#include <linux/clocksource.h>
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#include <linux/delay.h>
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#include <linux/init.h>
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/timex.h>
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/*
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* Default to the loop-based delay implementation.
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*/
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struct arm_delay_ops arm_delay_ops = {
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.delay = __loop_delay,
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.const_udelay = __loop_const_udelay,
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.udelay = __loop_udelay,
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};
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static const struct delay_timer *delay_timer;
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static bool delay_calibrated;
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static u64 delay_res;
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int read_current_timer(unsigned long *timer_val)
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{
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if (!delay_timer)
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return -ENXIO;
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*timer_val = delay_timer->read_current_timer();
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return 0;
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}
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EXPORT_SYMBOL_GPL(read_current_timer);
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static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
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{
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return (cyc * mult) >> shift;
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}
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static void __timer_delay(unsigned long cycles)
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{
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cycles_t start = get_cycles();
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while ((get_cycles() - start) < cycles)
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cpu_relax();
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}
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static void __timer_const_udelay(unsigned long xloops)
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{
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unsigned long long loops = xloops;
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loops *= arm_delay_ops.ticks_per_jiffy;
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__timer_delay(loops >> UDELAY_SHIFT);
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}
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static void __timer_udelay(unsigned long usecs)
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{
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__timer_const_udelay(usecs * UDELAY_MULT);
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}
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void __init register_current_timer_delay(const struct delay_timer *timer)
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{
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u32 new_mult, new_shift;
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u64 res;
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clocks_calc_mult_shift(&new_mult, &new_shift, timer->freq,
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NSEC_PER_SEC, 3600);
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res = cyc_to_ns(1ULL, new_mult, new_shift);
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if (res > 1000) {
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pr_err("Ignoring delay timer %ps, which has insufficient resolution of %lluns\n",
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timer, res);
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return;
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}
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if (!delay_calibrated && (!delay_res || (res < delay_res))) {
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pr_info("Switching to timer-based delay loop, resolution %lluns\n", res);
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delay_timer = timer;
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lpj_fine = timer->freq / HZ;
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delay_res = res;
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/* cpufreq may scale loops_per_jiffy, so keep a private copy */
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arm_delay_ops.ticks_per_jiffy = lpj_fine;
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arm_delay_ops.delay = __timer_delay;
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arm_delay_ops.const_udelay = __timer_const_udelay;
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arm_delay_ops.udelay = __timer_udelay;
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} else {
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pr_info("Ignoring duplicate/late registration of read_current_timer delay\n");
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}
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}
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unsigned long calibrate_delay_is_known(void)
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{
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delay_calibrated = true;
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return lpj_fine;
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}
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void calibration_delay_done(void)
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{
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delay_calibrated = true;
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}
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