coldboot/lfsr.h

   1 #include <limits.h>
   2
   3 /*
   4  * Copyright (C) 2020, Anton Blanchard <anton@linux.ibm.com>, IBM
   5  *
   6  * Redistribution and use in source and binary forms, with or without modification,
   7  * are permitted provided that the following conditions are met:
   8  *
   9  * 1. Redistributions of source code must retain the above copyright notice, this
  10  *    list of conditions and the following disclaimer.
  11  * 2. Redistributions in binary form must reproduce the above copyright notice,
  12  *    this list of conditions and the following disclaimer in the documentation
  13  *    and/or other materials provided with the distribution.
  14
  15  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
  16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  17  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  18  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
  19  * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  20  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  21  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  22  * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  23  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  24  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  25  */
  26
  27 /*
  28  * Galois LFSR
  29  *
  30  * Polynomials verified with https://bitbucket.org/gallen/mlpolygen/
  31  */
  32 static inline unsigned long lfsr(unsigned long bits, unsigned long prev)
  33 {
  34        static const unsigned long lfsr_taps[] = {
  35                0x0,
  36                0x0,
  37                0x3,
  38                0x6,
  39                0xc,
  40                0x14,
  41                0x30,
  42                0x60,
  43                0xb8,
  44                0x110,
  45                0x240,
  46                0x500,
  47                0x829,
  48                0x100d,
  49                0x2015,
  50                0x6000,
  51                0xd008,
  52                0x12000,
  53                0x20400,
  54                0x40023,
  55                0x90000,
  56                0x140000,
  57                0x300000,
  58                0x420000,
  59                0xe10000,
  60                0x1200000,
  61                0x2000023,
  62                0x4000013,
  63                0x9000000,
  64                0x14000000,
  65                0x20000029,
  66                0x48000000,
  67                0x80200003,
  68 #ifdef __LP64__
  69                0x100080000,
  70                0x204000003,
  71                0x500000000,
  72                0x801000000,
  73                0x100000001f,
  74                0x2000000031,
  75                0x4400000000,
  76                0xa000140000,
  77                0x12000000000,
  78                0x300000c0000,
  79                0x63000000000,
  80                0xc0000030000,
  81                0x1b0000000000,
  82                0x300003000000,
  83                0x420000000000,
  84                0xc00000180000,
  85                0x1008000000000,
  86                0x3000000c00000,
  87                0x6000c00000000,
  88                0x9000000000000,
  89                0x18003000000000,
  90                0x30000000030000,
  91                0x40000040000000,
  92                0xc0000600000000,
  93                0x102000000000000,
  94                0x200004000000000,
  95                0x600003000000000,
  96                0xc00000000000000,
  97                0x1800300000000000,
  98                0x3000000000000030,
  99                0x6000000000000000,
 100                0x800000000000000d
 101 #endif
 102        };
 103        unsigned long lsb = prev & 1;
 104
 105        prev >>= 1;
 106        prev ^= (-lsb) & lfsr_taps[bits];
 107
 108        return prev;
 109 }