linux/Documentation/packing.txt
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   1================================================
   2Generic bitfield packing and unpacking functions
   3================================================
   4
   5Problem statement
   6-----------------
   7
   8When working with hardware, one has to choose between several approaches of
   9interfacing with it.
  10One can memory-map a pointer to a carefully crafted struct over the hardware
  11device's memory region, and access its fields as struct members (potentially
  12declared as bitfields). But writing code this way would make it less portable,
  13due to potential endianness mismatches between the CPU and the hardware device.
  14Additionally, one has to pay close attention when translating register
  15definitions from the hardware documentation into bit field indices for the
  16structs. Also, some hardware (typically networking equipment) tends to group
  17its register fields in ways that violate any reasonable word boundaries
  18(sometimes even 64 bit ones). This creates the inconvenience of having to
  19define "high" and "low" portions of register fields within the struct.
  20A more robust alternative to struct field definitions would be to extract the
  21required fields by shifting the appropriate number of bits. But this would
  22still not protect from endianness mismatches, except if all memory accesses
  23were performed byte-by-byte. Also the code can easily get cluttered, and the
  24high-level idea might get lost among the many bit shifts required.
  25Many drivers take the bit-shifting approach and then attempt to reduce the
  26clutter with tailored macros, but more often than not these macros take
  27shortcuts that still prevent the code from being truly portable.
  28
  29The solution
  30------------
  31
  32This API deals with 2 basic operations:
  33  - Packing a CPU-usable number into a memory buffer (with hardware
  34    constraints/quirks)
  35  - Unpacking a memory buffer (which has hardware constraints/quirks)
  36    into a CPU-usable number.
  37
  38The API offers an abstraction over said hardware constraints and quirks,
  39over CPU endianness and therefore between possible mismatches between
  40the two.
  41
  42The basic unit of these API functions is the u64. From the CPU's
  43perspective, bit 63 always means bit offset 7 of byte 7, albeit only
  44logically. The question is: where do we lay this bit out in memory?
  45
  46The following examples cover the memory layout of a packed u64 field.
  47The byte offsets in the packed buffer are always implicitly 0, 1, ... 7.
  48What the examples show is where the logical bytes and bits sit.
  49
  501. Normally (no quirks), we would do it like this:
  51
  5263 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32
  537                       6                       5                        4
  5431 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
  553                       2                       1                        0
  56
  57That is, the MSByte (7) of the CPU-usable u64 sits at memory offset 0, and the
  58LSByte (0) of the u64 sits at memory offset 7.
  59This corresponds to what most folks would regard to as "big endian", where
  60bit i corresponds to the number 2^i. This is also referred to in the code
  61comments as "logical" notation.
  62
  63
  642. If QUIRK_MSB_ON_THE_RIGHT is set, we do it like this:
  65
  6656 57 58 59 60 61 62 63 48 49 50 51 52 53 54 55 40 41 42 43 44 45 46 47 32 33 34 35 36 37 38 39
  677                       6                        5                       4
  6824 25 26 27 28 29 30 31 16 17 18 19 20 21 22 23  8  9 10 11 12 13 14 15  0  1  2  3  4  5  6  7
  693                       2                        1                       0
  70
  71That is, QUIRK_MSB_ON_THE_RIGHT does not affect byte positioning, but
  72inverts bit offsets inside a byte.
  73
  74
  753. If QUIRK_LITTLE_ENDIAN is set, we do it like this:
  76
  7739 38 37 36 35 34 33 32 47 46 45 44 43 42 41 40 55 54 53 52 51 50 49 48 63 62 61 60 59 58 57 56
  784                       5                       6                       7
  797  6  5  4  3  2  1  0  15 14 13 12 11 10  9  8 23 22 21 20 19 18 17 16 31 30 29 28 27 26 25 24
  800                       1                       2                       3
  81
  82Therefore, QUIRK_LITTLE_ENDIAN means that inside the memory region, every
  83byte from each 4-byte word is placed at its mirrored position compared to
  84the boundary of that word.
  85
  864. If QUIRK_MSB_ON_THE_RIGHT and QUIRK_LITTLE_ENDIAN are both set, we do it
  87   like this:
  88
  8932 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
  904                       5                       6                       7
  910  1  2  3  4  5  6  7  8   9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
  920                       1                       2                       3
  93
  94
  955. If just QUIRK_LSW32_IS_FIRST is set, we do it like this:
  96
  9731 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
  983                       2                       1                        0
  9963 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32
 1007                       6                       5                        4
 101
 102In this case the 8 byte memory region is interpreted as follows: first
 1034 bytes correspond to the least significant 4-byte word, next 4 bytes to
 104the more significant 4-byte word.
 105
 106
 1076. If QUIRK_LSW32_IS_FIRST and QUIRK_MSB_ON_THE_RIGHT are set, we do it like
 108   this:
 109
 11024 25 26 27 28 29 30 31 16 17 18 19 20 21 22 23  8  9 10 11 12 13 14 15  0  1  2  3  4  5  6  7
 1113                       2                        1                       0
 11256 57 58 59 60 61 62 63 48 49 50 51 52 53 54 55 40 41 42 43 44 45 46 47 32 33 34 35 36 37 38 39
 1137                       6                        5                       4
 114
 115
 1167. If QUIRK_LSW32_IS_FIRST and QUIRK_LITTLE_ENDIAN are set, it looks like
 117   this:
 118
 1197  6  5  4  3  2  1  0  15 14 13 12 11 10  9  8 23 22 21 20 19 18 17 16 31 30 29 28 27 26 25 24
 1200                       1                       2                       3
 12139 38 37 36 35 34 33 32 47 46 45 44 43 42 41 40 55 54 53 52 51 50 49 48 63 62 61 60 59 58 57 56
 1224                       5                       6                       7
 123
 124
 1258. If QUIRK_LSW32_IS_FIRST, QUIRK_LITTLE_ENDIAN and QUIRK_MSB_ON_THE_RIGHT
 126   are set, it looks like this:
 127
 1280  1  2  3  4  5  6  7  8   9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
 1290                       1                       2                       3
 13032 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
 1314                       5                       6                       7
 132
 133
 134We always think of our offsets as if there were no quirk, and we translate
 135them afterwards, before accessing the memory region.
 136
 137Intended use
 138------------
 139
 140Drivers that opt to use this API first need to identify which of the above 3
 141quirk combinations (for a total of 8) match what the hardware documentation
 142describes. Then they should wrap the packing() function, creating a new
 143xxx_packing() that calls it using the proper QUIRK_* one-hot bits set.
 144
 145The packing() function returns an int-encoded error code, which protects the
 146programmer against incorrect API use.  The errors are not expected to occur
 147durring runtime, therefore it is reasonable for xxx_packing() to return void
 148and simply swallow those errors. Optionally it can dump stack or print the
 149error description.
 150