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1================================================2Generic bitfield packing and unpacking functions3================================================4 5Problem statement6-----------------7 8When working with hardware, one has to choose between several approaches of9interfacing with it.10One can memory-map a pointer to a carefully crafted struct over the hardware11device's memory region, and access its fields as struct members (potentially12declared 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 register15definitions from the hardware documentation into bit field indices for the16structs. Also, some hardware (typically networking equipment) tends to group17its register fields in ways that violate any reasonable word boundaries18(sometimes even 64 bit ones). This creates the inconvenience of having to19define "high" and "low" portions of register fields within the struct.20A more robust alternative to struct field definitions would be to extract the21required fields by shifting the appropriate number of bits. But this would22still not protect from endianness mismatches, except if all memory accesses23were performed byte-by-byte. Also the code can easily get cluttered, and the24high-level idea might get lost among the many bit shifts required.25Many drivers take the bit-shifting approach and then attempt to reduce the26clutter with tailored macros, but more often than not these macros take27shortcuts that still prevent the code from being truly portable.28 29The solution30------------31 32This API deals with 2 basic operations:33 34 - Packing a CPU-usable number into a memory buffer (with hardware35 constraints/quirks)36 - Unpacking a memory buffer (which has hardware constraints/quirks)37 into a CPU-usable number.38 39The API offers an abstraction over said hardware constraints and quirks,40over CPU endianness and therefore between possible mismatches between41the two.42 43The basic unit of these API functions is the u64. From the CPU's44perspective, bit 63 always means bit offset 7 of byte 7, albeit only45logically. The question is: where do we lay this bit out in memory?46 47The following examples cover the memory layout of a packed u64 field.48The byte offsets in the packed buffer are always implicitly 0, 1, ... 7.49What the examples show is where the logical bytes and bits sit.50 511. Normally (no quirks), we would do it like this:52 53::54 55 63 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 3256 7 6 5 457 31 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 1 058 3 2 1 059 60That is, the MSByte (7) of the CPU-usable u64 sits at memory offset 0, and the61LSByte (0) of the u64 sits at memory offset 7.62This corresponds to what most folks would regard to as "big endian", where63bit i corresponds to the number 2^i. This is also referred to in the code64comments as "logical" notation.65 66 672. If QUIRK_MSB_ON_THE_RIGHT is set, we do it like this:68 69::70 71 56 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 3972 7 6 5 473 24 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 6 774 3 2 1 075 76That is, QUIRK_MSB_ON_THE_RIGHT does not affect byte positioning, but77inverts bit offsets inside a byte.78 79 803. If QUIRK_LITTLE_ENDIAN is set, we do it like this:81 82::83 84 39 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 5685 4 5 6 786 7 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 2487 0 1 2 388 89Therefore, QUIRK_LITTLE_ENDIAN means that inside the memory region, every90byte from each 4-byte word is placed at its mirrored position compared to91the boundary of that word.92 934. If QUIRK_MSB_ON_THE_RIGHT and QUIRK_LITTLE_ENDIAN are both set, we do it94 like this:95 96::97 98 32 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 6399 4 5 6 7100 0 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 31101 0 1 2 3102 103 1045. If just QUIRK_LSW32_IS_FIRST is set, we do it like this:105 106::107 108 31 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 0109 3 2 1 0110 63 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 32111 7 6 5 4112 113In this case the 8 byte memory region is interpreted as follows: first1144 bytes correspond to the least significant 4-byte word, next 4 bytes to115the more significant 4-byte word.116 117 1186. If QUIRK_LSW32_IS_FIRST and QUIRK_MSB_ON_THE_RIGHT are set, we do it like119 this:120 121::122 123 24 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 7124 3 2 1 0125 56 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 39126 7 6 5 4127 128 1297. If QUIRK_LSW32_IS_FIRST and QUIRK_LITTLE_ENDIAN are set, it looks like130 this:131 132::133 134 7 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 24135 0 1 2 3136 39 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 56137 4 5 6 7138 139 1408. If QUIRK_LSW32_IS_FIRST, QUIRK_LITTLE_ENDIAN and QUIRK_MSB_ON_THE_RIGHT141 are set, it looks like this:142 143::144 145 0 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 31146 0 1 2 3147 32 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 63148 4 5 6 7149 150 151We always think of our offsets as if there were no quirk, and we translate152them afterwards, before accessing the memory region.153 154Intended use155------------156 157Drivers that opt to use this API first need to identify which of the above 3158quirk combinations (for a total of 8) match what the hardware documentation159describes. Then they should wrap the packing() function, creating a new160xxx_packing() that calls it using the proper QUIRK_* one-hot bits set.161 162The packing() function returns an int-encoded error code, which protects the163programmer against incorrect API use. The errors are not expected to occur164during runtime, therefore it is reasonable for xxx_packing() to return void165and simply swallow those errors. Optionally it can dump stack or print the166error description.167