LXR linux/arch/arc/include/asm/delay.h

   1/*
   2 * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
   3 *
   4 * This program is free software; you can redistribute it and/or modify
   5 * it under the terms of the GNU General Public License version 2 as
   6 * published by the Free Software Foundation.
   7 *
   8 * Delay routines using pre computed loops_per_jiffy value.
   9 *
  10 * vineetg: Feb 2012
  11 *  -Rewrote in "C" to avoid dealing with availability of H/w MPY
  12 *  -Also reduced the num of MPY operations from 3 to 2
  13 *
  14 * Amit Bhor: Codito Technologies 2004
  15 */
  16
  17#ifndef __ASM_ARC_UDELAY_H
  18#define __ASM_ARC_UDELAY_H
  19
  20#include <asm/param.h>          /* HZ */
  21
  22static inline void __delay(unsigned long loops)
  23{
  24        __asm__ __volatile__(
  25        "1:     sub.f %0, %0, 1 \n"
  26        "       jpnz 1b         \n"
  27        : "+r"(loops)
  28        :
  29        : "cc");
  30}
  31
  32extern void __bad_udelay(void);
  33
  34/*
  35 * Normal Math for computing loops in "N" usecs
  36 *  -we have precomputed @loops_per_jiffy
  37 *  -1 sec has HZ jiffies
  38 * loops per "N" usecs = ((loops_per_jiffy * HZ / 1000000) * N)
  39 *
  40 * Approximate Division by multiplication:
  41 *  -Mathematically if we multiply and divide a number by same value the
  42 *   result remains unchanged:  In this case, we use 2^32
  43 *  -> (loops_per_N_usec * 2^32 ) / 2^32
  44 *  -> (((loops_per_jiffy * HZ / 1000000) * N) * 2^32) / 2^32
  45 *  -> (loops_per_jiffy * HZ * N * 4295) / 2^32
  46 *
  47 *  -Divide by 2^32 is very simply right shift by 32
  48 *  -We simply need to ensure that the multiply per above eqn happens in
  49 *   64-bit precision (if CPU doesn't support it - gcc can emaulate it)
  50 */
  51
  52static inline void __udelay(unsigned long usecs)
  53{
  54        unsigned long loops;
  55
  56        /* (long long) cast ensures 64 bit MPY - real or emulated
  57         * HZ * 4295 is pre-evaluated by gcc - hence only 2 mpy ops
  58         */
  59        loops = ((long long)(usecs * 4295 * HZ) *
  60                 (long long)(loops_per_jiffy)) >> 32;
  61
  62        __delay(loops);
  63}
  64
  65#define udelay(n) (__builtin_constant_p(n) ? ((n) > 20000 ? __bad_udelay() \
  66                                : __udelay(n)) : __udelay(n))
  67
  68#endif /* __ASM_ARC_UDELAY_H */
  69