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diff --git a/Documentation/core-api/packing.rst b/Documentation/core-api/packing.rst new file mode 100644 index 000000000..d8c341fe3 --- /dev/null +++ b/Documentation/core-api/packing.rst @@ -0,0 +1,166 @@ +================================================ +Generic bitfield packing and unpacking functions +================================================ + +Problem statement +----------------- + +When working with hardware, one has to choose between several approaches of +interfacing with it. +One can memory-map a pointer to a carefully crafted struct over the hardware +device's memory region, and access its fields as struct members (potentially +declared as bitfields). But writing code this way would make it less portable, +due to potential endianness mismatches between the CPU and the hardware device. +Additionally, one has to pay close attention when translating register +definitions from the hardware documentation into bit field indices for the +structs. Also, some hardware (typically networking equipment) tends to group +its register fields in ways that violate any reasonable word boundaries +(sometimes even 64 bit ones). This creates the inconvenience of having to +define "high" and "low" portions of register fields within the struct. +A more robust alternative to struct field definitions would be to extract the +required fields by shifting the appropriate number of bits. But this would +still not protect from endianness mismatches, except if all memory accesses +were performed byte-by-byte. Also the code can easily get cluttered, and the +high-level idea might get lost among the many bit shifts required. +Many drivers take the bit-shifting approach and then attempt to reduce the +clutter with tailored macros, but more often than not these macros take +shortcuts that still prevent the code from being truly portable. + +The solution +------------ + +This API deals with 2 basic operations: + + - Packing a CPU-usable number into a memory buffer (with hardware + constraints/quirks) + - Unpacking a memory buffer (which has hardware constraints/quirks) + into a CPU-usable number. + +The API offers an abstraction over said hardware constraints and quirks, +over CPU endianness and therefore between possible mismatches between +the two. + +The basic unit of these API functions is the u64. From the CPU's +perspective, bit 63 always means bit offset 7 of byte 7, albeit only +logically. The question is: where do we lay this bit out in memory? + +The following examples cover the memory layout of a packed u64 field. +The byte offsets in the packed buffer are always implicitly 0, 1, ... 7. +What the examples show is where the logical bytes and bits sit. + +1. Normally (no quirks), we would do it like this: + +:: + + 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 32 + 7 6 5 4 + 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 0 + 3 2 1 0 + +That is, the MSByte (7) of the CPU-usable u64 sits at memory offset 0, and the +LSByte (0) of the u64 sits at memory offset 7. +This corresponds to what most folks would regard to as "big endian", where +bit i corresponds to the number 2^i. This is also referred to in the code +comments as "logical" notation. + + +2. If QUIRK_MSB_ON_THE_RIGHT is set, we do it like this: + +:: + + 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 39 + 7 6 5 4 + 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 7 + 3 2 1 0 + +That is, QUIRK_MSB_ON_THE_RIGHT does not affect byte positioning, but +inverts bit offsets inside a byte. + + +3. If QUIRK_LITTLE_ENDIAN is set, we do it like this: + +:: + + 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 56 + 4 5 6 7 + 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 24 + 0 1 2 3 + +Therefore, QUIRK_LITTLE_ENDIAN means that inside the memory region, every +byte from each 4-byte word is placed at its mirrored position compared to +the boundary of that word. + +4. If QUIRK_MSB_ON_THE_RIGHT and QUIRK_LITTLE_ENDIAN are both set, we do it + like this: + +:: + + 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 63 + 4 5 6 7 + 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 31 + 0 1 2 3 + + +5. If just QUIRK_LSW32_IS_FIRST is set, we do it like this: + +:: + + 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 0 + 3 2 1 0 + 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 32 + 7 6 5 4 + +In this case the 8 byte memory region is interpreted as follows: first +4 bytes correspond to the least significant 4-byte word, next 4 bytes to +the more significant 4-byte word. + + +6. If QUIRK_LSW32_IS_FIRST and QUIRK_MSB_ON_THE_RIGHT are set, we do it like + this: + +:: + + 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 7 + 3 2 1 0 + 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 39 + 7 6 5 4 + + +7. If QUIRK_LSW32_IS_FIRST and QUIRK_LITTLE_ENDIAN are set, it looks like + this: + +:: + + 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 24 + 0 1 2 3 + 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 56 + 4 5 6 7 + + +8. If QUIRK_LSW32_IS_FIRST, QUIRK_LITTLE_ENDIAN and QUIRK_MSB_ON_THE_RIGHT + are set, it looks like this: + +:: + + 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 31 + 0 1 2 3 + 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 63 + 4 5 6 7 + + +We always think of our offsets as if there were no quirk, and we translate +them afterwards, before accessing the memory region. + +Intended use +------------ + +Drivers that opt to use this API first need to identify which of the above 3 +quirk combinations (for a total of 8) match what the hardware documentation +describes. Then they should wrap the packing() function, creating a new +xxx_packing() that calls it using the proper QUIRK_* one-hot bits set. + +The packing() function returns an int-encoded error code, which protects the +programmer against incorrect API use. The errors are not expected to occur +durring runtime, therefore it is reasonable for xxx_packing() to return void +and simply swallow those errors. Optionally it can dump stack or print the +error description. |