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
34  - Packing a CPU-usable number into a memory buffer (with hardware
35    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 between
41the two.
42
43The basic unit of these API functions is the u64. From the CPU's
44perspective, bit 63 always means bit offset 7 of byte 7, albeit only
45logically. 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 32
56  7                       6                       5                        4
57  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
58  3                       2                       1                        0
59
60That is, the MSByte (7) of the CPU-usable u64 sits at memory offset 0, and the
61LSByte (0) of the u64 sits at memory offset 7.
62This corresponds to what most folks would regard to as "big endian", where
63bit i corresponds to the number 2^i. This is also referred to in the code
64comments 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 39
72  7                       6                        5                       4
73  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
74  3                       2                        1                       0
75
76That is, QUIRK_MSB_ON_THE_RIGHT does not affect byte positioning, but
77inverts 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 56
85  4                       5                       6                       7
86  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
87  0                       1                       2                       3
88
89Therefore, QUIRK_LITTLE_ENDIAN means that inside the memory region, every
90byte from each 4-byte word is placed at its mirrored position compared to
91the boundary of that word.
92
934. If QUIRK_MSB_ON_THE_RIGHT and QUIRK_LITTLE_ENDIAN are both set, we do it
94   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 63
99  4                       5                       6                       7
100  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
101  0                       1                       2                       3
102
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  0
109  3                       2                       1                        0
110  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
111  7                       6                       5                        4
112
113In this case the 8 byte memory region is interpreted as follows: first
1144 bytes correspond to the least significant 4-byte word, next 4 bytes to
115the more significant 4-byte word.
116
117
1186. If QUIRK_LSW32_IS_FIRST and QUIRK_MSB_ON_THE_RIGHT are set, we do it like
119   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  7
124  3                       2                        1                       0
125  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
126  7                       6                        5                       4
127
128
1297. If QUIRK_LSW32_IS_FIRST and QUIRK_LITTLE_ENDIAN are set, it looks like
130   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 24
135  0                       1                       2                       3
136  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
137  4                       5                       6                       7
138
139
1408. If QUIRK_LSW32_IS_FIRST, QUIRK_LITTLE_ENDIAN and QUIRK_MSB_ON_THE_RIGHT
141   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 31
146  0                       1                       2                       3
147  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
148  4                       5                       6                       7
149
150
151We always think of our offsets as if there were no quirk, and we translate
152them afterwards, before accessing the memory region.
153
154Intended use
155------------
156
157Drivers that opt to use this API first need to identify which of the above 3
158quirk combinations (for a total of 8) match what the hardware documentation
159describes. Then they should wrap the packing() function, creating a new
160xxx_packing() that calls it using the proper QUIRK_* one-hot bits set.
161
162The packing() function returns an int-encoded error code, which protects the
163programmer against incorrect API use.  The errors are not expected to occur
164durring runtime, therefore it is reasonable for xxx_packing() to return void
165and simply swallow those errors. Optionally it can dump stack or print the
166error description.
167