linux_kernel/arch/riscv/lib/delay.c

/*
 * Copyright (C) 2012 Regents of the University of California
 *
 *   This program 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, version 2.
 *
 *   This program 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.
 */

#include <linux/delay.h>
#include <linux/param.h>
#include <linux/timex.h>
#include <linux/export.h>

/*
 * This is copies from arch/arm/include/asm/delay.h
 *
 * Loop (or tick) based delay:
 *
 * loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
 *
 * where:
 *
 * jiffies_per_sec = HZ
 * us_per_sec = 1000000
 *
 * Therefore the constant part is HZ / 1000000 which is a small
 * fractional number. To make this usable with integer math, we
 * scale up this constant by 2^31, perform the actual multiplication,
 * and scale the result back down by 2^31 with a simple shift:
 *
 * loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
 *
 * where:
 *
 * UDELAY_MULT = 2^31 * HZ / 1000000
 *             = (2^31 / 1000000) * HZ
 *             = 2147.483648 * HZ
 *             = 2147 * HZ + 483648 * HZ / 1000000
 *
 * 31 is the biggest scale shift value that won't overflow 32 bits for
 * delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
 */
#define MAX_UDELAY_US	2000
#define MAX_UDELAY_HZ	1000
#define UDELAY_MULT	(2147UL * HZ + 483648UL * HZ / 1000000UL)
#define UDELAY_SHIFT	31

#if HZ > MAX_UDELAY_HZ
#error "HZ > MAX_UDELAY_HZ"
#endif

/*
 * RISC-V supports both UDELAY and NDELAY.  This is largely the same as above,
 * but with different constants.  I added 10 bits to the shift to get this, but
 * the result is that I need a 64-bit multiply, which is slow on 32-bit
 * platforms.
 *
 * NDELAY_MULT = 2^41 * HZ / 1000000000
 *             = (2^41 / 1000000000) * HZ
 *             = 2199.02325555 * HZ
 *             = 2199 * HZ + 23255550 * HZ / 1000000000
 *
 * The maximum here is to avoid 64-bit overflow, but it isn't checked as it
 * won't happen.
 */
#define MAX_NDELAY_NS   (1ULL << 42)
#define MAX_NDELAY_HZ	MAX_UDELAY_HZ
#define NDELAY_MULT	((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL))
#define NDELAY_SHIFT	41

#if HZ > MAX_NDELAY_HZ
#error "HZ > MAX_NDELAY_HZ"
#endif

void __delay(unsigned long cycles)
{
	u64 t0 = get_cycles();

	while ((unsigned long)(get_cycles() - t0) < cycles)
		cpu_relax();
}
EXPORT_SYMBOL(__delay);

void udelay(unsigned long usecs)
{
	unsigned long ucycles = usecs * lpj_fine * UDELAY_MULT;

	if (unlikely(usecs > MAX_UDELAY_US)) {
		__delay((u64)usecs * riscv_timebase / 1000000ULL);
		return;
	}

	__delay(ucycles >> UDELAY_SHIFT);
}
EXPORT_SYMBOL(udelay);

void ndelay(unsigned long nsecs)
{
	/*
	 * This doesn't bother checking for overflow, as it won't happen (it's
	 * an hour) of delay.
	 */
	unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT;
	__delay(ncycles >> NDELAY_SHIFT);
}
EXPORT_SYMBOL(ndelay);