112 lines
3.1 KiB
C
112 lines
3.1 KiB
C
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/*
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* Copyright (C) 2012 Regents of the University of California
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation, version 2.
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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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#include <linux/delay.h>
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#include <linux/param.h>
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#include <linux/timex.h>
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#include <linux/export.h>
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/*
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* This is copies from arch/arm/include/asm/delay.h
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*
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* Loop (or tick) based delay:
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*
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* loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
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*
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* where:
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*
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* jiffies_per_sec = HZ
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* us_per_sec = 1000000
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*
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* Therefore the constant part is HZ / 1000000 which is a small
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* fractional number. To make this usable with integer math, we
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* scale up this constant by 2^31, perform the actual multiplication,
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* and scale the result back down by 2^31 with a simple shift:
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*
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* loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
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*
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* where:
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*
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* UDELAY_MULT = 2^31 * HZ / 1000000
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* = (2^31 / 1000000) * HZ
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* = 2147.483648 * HZ
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* = 2147 * HZ + 483648 * HZ / 1000000
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*
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* 31 is the biggest scale shift value that won't overflow 32 bits for
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* delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
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*/
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#define MAX_UDELAY_US 2000
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#define MAX_UDELAY_HZ 1000
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#define UDELAY_MULT (2147UL * HZ + 483648UL * HZ / 1000000UL)
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#define UDELAY_SHIFT 31
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#if HZ > MAX_UDELAY_HZ
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#error "HZ > MAX_UDELAY_HZ"
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#endif
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/*
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* RISC-V supports both UDELAY and NDELAY. This is largely the same as above,
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* but with different constants. I added 10 bits to the shift to get this, but
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* the result is that I need a 64-bit multiply, which is slow on 32-bit
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* platforms.
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*
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* NDELAY_MULT = 2^41 * HZ / 1000000000
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* = (2^41 / 1000000000) * HZ
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* = 2199.02325555 * HZ
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* = 2199 * HZ + 23255550 * HZ / 1000000000
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*
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* The maximum here is to avoid 64-bit overflow, but it isn't checked as it
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* won't happen.
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*/
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#define MAX_NDELAY_NS (1ULL << 42)
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#define MAX_NDELAY_HZ MAX_UDELAY_HZ
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#define NDELAY_MULT ((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL))
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#define NDELAY_SHIFT 41
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#if HZ > MAX_NDELAY_HZ
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#error "HZ > MAX_NDELAY_HZ"
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#endif
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void __delay(unsigned long cycles)
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{
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u64 t0 = get_cycles();
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while ((unsigned long)(get_cycles() - t0) < cycles)
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cpu_relax();
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}
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EXPORT_SYMBOL(__delay);
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void udelay(unsigned long usecs)
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{
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u64 ucycles = (u64)usecs * lpj_fine * UDELAY_MULT;
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if (unlikely(usecs > MAX_UDELAY_US)) {
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__delay((u64)usecs * riscv_timebase / 1000000ULL);
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return;
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}
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__delay(ucycles >> UDELAY_SHIFT);
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}
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EXPORT_SYMBOL(udelay);
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void ndelay(unsigned long nsecs)
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{
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/*
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* This doesn't bother checking for overflow, as it won't happen (it's
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* an hour) of delay.
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*/
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unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT;
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__delay(ncycles >> NDELAY_SHIFT);
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}
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EXPORT_SYMBOL(ndelay);
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