167 lines
7.3 KiB
ReStructuredText
167 lines
7.3 KiB
ReStructuredText
|
================================================
|
||
|
|
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.
|