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Diffstat (limited to 'Documentation/core-api/packing.rst')
| -rw-r--r-- | Documentation/core-api/packing.rst | 189 |
1 files changed, 184 insertions, 5 deletions
diff --git a/Documentation/core-api/packing.rst b/Documentation/core-api/packing.rst index 3ed13bc9a195..0ce2078c8e13 100644 --- a/Documentation/core-api/packing.rst +++ b/Documentation/core-api/packing.rst @@ -151,16 +151,195 @@ the more significant 4-byte word. We always think of our offsets as if there were no quirk, and we translate them afterwards, before accessing the memory region. +Note on buffer lengths not multiple of 4 +---------------------------------------- + +To deal with memory layout quirks where groups of 4 bytes are laid out "little +endian" relative to each other, but "big endian" within the group itself, the +concept of groups of 4 bytes is intrinsic to the packing API (not to be +confused with the memory access, which is performed byte by byte, though). + +With buffer lengths not multiple of 4, this means one group will be incomplete. +Depending on the quirks, this may lead to discontinuities in the bit fields +accessible through the buffer. The packing API assumes discontinuities were not +the intention of the memory layout, so it avoids them by effectively logically +shortening the most significant group of 4 octets to the number of octets +actually available. + +Example with a 31 byte sized buffer given below. Physical buffer offsets are +implicit, and increase from left to right within a group, and from top to +bottom within a column. + +No quirks: + +:: + + 31 29 28 | Group 7 (most significant) + 27 26 25 24 | Group 6 + 23 22 21 20 | Group 5 + 19 18 17 16 | Group 4 + 15 14 13 12 | Group 3 + 11 10 9 8 | Group 2 + 7 6 5 4 | Group 1 + 3 2 1 0 | Group 0 (least significant) + +QUIRK_LSW32_IS_FIRST: + +:: + + 3 2 1 0 | Group 0 (least significant) + 7 6 5 4 | Group 1 + 11 10 9 8 | Group 2 + 15 14 13 12 | Group 3 + 19 18 17 16 | Group 4 + 23 22 21 20 | Group 5 + 27 26 25 24 | Group 6 + 30 29 28 | Group 7 (most significant) + +QUIRK_LITTLE_ENDIAN: + +:: + + 30 28 29 | Group 7 (most significant) + 24 25 26 27 | Group 6 + 20 21 22 23 | Group 5 + 16 17 18 19 | Group 4 + 12 13 14 15 | Group 3 + 8 9 10 11 | Group 2 + 4 5 6 7 | Group 1 + 0 1 2 3 | Group 0 (least significant) + +QUIRK_LITTLE_ENDIAN | QUIRK_LSW32_IS_FIRST: + +:: + + 0 1 2 3 | Group 0 (least significant) + 4 5 6 7 | Group 1 + 8 9 10 11 | Group 2 + 12 13 14 15 | Group 3 + 16 17 18 19 | Group 4 + 20 21 22 23 | Group 5 + 24 25 26 27 | Group 6 + 28 29 30 | Group 7 (most significant) + 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. +describes. + +There are 3 supported usage patterns, detailed below. + +packing() +^^^^^^^^^ + +This API function is deprecated. The packing() function returns an int-encoded error code, which protects the programmer against incorrect API use. The errors are not expected to occur -during 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. +during runtime, therefore it is reasonable to wrap packing() into a custom +function which returns void and swallows those errors. Optionally it can +dump stack or print the error description. + +.. code-block:: c + + void my_packing(void *buf, u64 *val, int startbit, int endbit, + size_t len, enum packing_op op) + { + int err; + + /* Adjust quirks accordingly */ + err = packing(buf, val, startbit, endbit, len, op, QUIRK_LSW32_IS_FIRST); + if (likely(!err)) + return; + + if (err == -EINVAL) { + pr_err("Start bit (%d) expected to be larger than end (%d)\n", + startbit, endbit); + } else if (err == -ERANGE) { + if ((startbit - endbit + 1) > 64) + pr_err("Field %d-%d too large for 64 bits!\n", + startbit, endbit); + else + pr_err("Cannot store %llx inside bits %d-%d (would truncate)\n", + *val, startbit, endbit); + } + dump_stack(); + } + +pack() and unpack() +^^^^^^^^^^^^^^^^^^^ + +These are const-correct variants of packing(), and eliminate the last "enum +packing_op op" argument. + +Calling pack(...) is equivalent, and preferred, to calling packing(..., PACK). + +Calling unpack(...) is equivalent, and preferred, to calling packing(..., UNPACK). + +pack_fields() and unpack_fields() +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The library exposes optimized functions for the scenario where there are many +fields represented in a buffer, and it encourages consumer drivers to avoid +repetitive calls to pack() and unpack() for each field, but instead use +pack_fields() and unpack_fields(), which reduces the code footprint. + +These APIs use field definitions in arrays of ``struct packed_field_u8`` or +``struct packed_field_u16``, allowing consumer drivers to minimize the size +of these arrays according to their custom requirements. + +The pack_fields() and unpack_fields() API functions are actually macros which +automatically select the appropriate function at compile time, based on the +type of the fields array passed in. + +An additional benefit over pack() and unpack() is that sanity checks on the +field definitions are handled at compile time with ``BUILD_BUG_ON`` rather +than only when the offending code is executed. These functions return void and +wrapping them to handle unexpected errors is not necessary. + +It is recommended, but not required, that you wrap your packed buffer into a +structured type with a fixed size. This generally makes it easier for the +compiler to enforce that the correct size buffer is used. + +Here is an example of how to use the fields APIs: + +.. code-block:: c + + /* Ordering inside the unpacked structure is flexible and can be different + * from the packed buffer. Here, it is optimized to reduce padding. + */ + struct data { + u64 field3; + u32 field4; + u16 field1; + u8 field2; + }; + + #define SIZE 13 + + typdef struct __packed { u8 buf[SIZE]; } packed_buf_t; + + static const struct packed_field_u8 fields[] = { + PACKED_FIELD(100, 90, struct data, field1), + PACKED_FIELD(90, 87, struct data, field2), + PACKED_FIELD(86, 30, struct data, field3), + PACKED_FIELD(29, 0, struct data, field4), + }; + + void unpack_your_data(const packed_buf_t *buf, struct data *unpacked) + { + BUILD_BUG_ON(sizeof(*buf) != SIZE; + + unpack_fields(buf, sizeof(*buf), unpacked, fields, + QUIRK_LITTLE_ENDIAN); + } + + void pack_your_data(const struct data *unpacked, packed_buf_t *buf) + { + BUILD_BUG_ON(sizeof(*buf) != SIZE; + + pack_fields(buf, sizeof(*buf), unpacked, fields, + QUIRK_LITTLE_ENDIAN); + } |