diff --git a/Documentation/core-api/packing.rst b/Documentation/core-api/packing.rst index 3ed13bc9a195..821691f23c54 100644 --- a/Documentation/core-api/packing.rst +++ b/Documentation/core-api/packing.rst @@ -151,6 +151,77 @@ 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 ------------ diff --git a/lib/packing.c b/lib/packing.c index 439125286d2b..435236a914fe 100644 --- a/lib/packing.c +++ b/lib/packing.c @@ -9,27 +9,6 @@ #include #include -static int get_le_offset(int offset) -{ - int closest_multiple_of_4; - - closest_multiple_of_4 = (offset / 4) * 4; - offset -= closest_multiple_of_4; - return closest_multiple_of_4 + (3 - offset); -} - -static int get_reverse_lsw32_offset(int offset, size_t len) -{ - int closest_multiple_of_4; - int word_index; - - word_index = offset / 4; - closest_multiple_of_4 = word_index * 4; - offset -= closest_multiple_of_4; - word_index = (len / 4) - word_index - 1; - return word_index * 4 + offset; -} - static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit, int *box_end_bit, u8 *box_mask) { @@ -47,6 +26,48 @@ static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit, *box_end_bit = new_box_end_bit; } +/** + * calculate_box_addr - Determine physical location of byte in buffer + * @box: Index of byte within buffer seen as a logical big-endian big number + * @len: Size of buffer in bytes + * @quirks: mask of QUIRK_LSW32_IS_FIRST and QUIRK_LITTLE_ENDIAN + * + * Function interprets the buffer as a @len byte sized big number, and returns + * the physical offset of the @box logical octet within it. Internally, it + * treats the big number as groups of 4 bytes. If @len is not a multiple of 4, + * the last group may be shorter. + * + * @QUIRK_LSW32_IS_FIRST gives the ordering of groups of 4 octets relative to + * each other. If set, the most significant group of 4 octets is last in the + * buffer (and may be truncated if @len is not a multiple of 4). + * + * @QUIRK_LITTLE_ENDIAN gives the ordering of bytes within each group of 4. + * If set, the most significant byte is last in the group. If @len takes the + * form of 4k+3, the last group will only be able to represent 24 bits, and its + * most significant octet is byte 2. + * + * Return: the physical offset into the buffer corresponding to the logical box. + */ +static int calculate_box_addr(int box, size_t len, u8 quirks) +{ + size_t offset_of_group, offset_in_group, this_group = box / 4; + size_t group_size; + + if (quirks & QUIRK_LSW32_IS_FIRST) + offset_of_group = this_group * 4; + else + offset_of_group = len - ((this_group + 1) * 4); + + group_size = min(4, len - offset_of_group); + + if (quirks & QUIRK_LITTLE_ENDIAN) + offset_in_group = box - this_group * 4; + else + offset_in_group = group_size - (box - this_group * 4) - 1; + + return offset_of_group + offset_in_group; +} + /** * packing - Convert numbers (currently u64) between a packed and an unpacked * format. Unpacked means laid out in memory in the CPU's native @@ -157,12 +178,7 @@ int packing(void *pbuf, u64 *uval, int startbit, int endbit, size_t pbuflen, * effective addressing inside the pbuf (so it's not * logical any longer). */ - box_addr = pbuflen - box - 1; - if (quirks & QUIRK_LITTLE_ENDIAN) - box_addr = get_le_offset(box_addr); - if (quirks & QUIRK_LSW32_IS_FIRST) - box_addr = get_reverse_lsw32_offset(box_addr, - pbuflen); + box_addr = calculate_box_addr(box, pbuflen, quirks); if (op == UNPACK) { u64 pval;