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 22 static inline void __delay(unsigned long loops) 23 { 24 __asm__ __volatile__( 25 " lp 1f \n" 26 " nop \n" 27 "1: \n" 28 : "+l"(loops)); 29 } 30 31 extern void __bad_udelay(void); 32 33 /* 34 * Normal Math for computing loops in "N" usecs 35 * -we have precomputed @loops_per_jiffy 36 * -1 sec has HZ jiffies 37 * loops per "N" usecs = ((loops_per_jiffy * HZ / 1000000) * N) 38 * 39 * Approximate Division by multiplication: 40 * -Mathematically if we multiply and divide a number by same value the 41 * result remains unchanged: In this case, we use 2^32 42 * -> (loops_per_N_usec * 2^32 ) / 2^32 43 * -> (((loops_per_jiffy * HZ / 1000000) * N) * 2^32) / 2^32 44 * -> (loops_per_jiffy * HZ * N * 4295) / 2^32 45 * 46 * -Divide by 2^32 is very simply right shift by 32 47 * -We simply need to ensure that the multiply per above eqn happens in 48 * 64-bit precision (if CPU doesn't support it - gcc can emaulate it) 49 */ 50 51 static inline void __udelay(unsigned long usecs) 52 { 53 unsigned long loops; 54 55 /* (u64) cast ensures 64 bit MPY - real or emulated 56 * HZ * 4295 is pre-evaluated by gcc - hence only 2 mpy ops 57 */ 58 loops = ((u64) usecs * 4295 * HZ * loops_per_jiffy) >> 32; 59 60 __delay(loops); 61 } 62 63 #define udelay(n) (__builtin_constant_p(n) ? ((n) > 20000 ? __bad_udelay() \ 64 : __udelay(n)) : __udelay(n)) 65 66 #endif /* __ASM_ARC_UDELAY_H */ 67