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Diffstat (limited to '')
| -rw-r--r-- | lib/packing.c | 214 |
1 files changed, 214 insertions, 0 deletions
diff --git a/lib/packing.c b/lib/packing.c new file mode 100644 index 000000000..6ed72dccf --- /dev/null +++ b/lib/packing.c @@ -0,0 +1,214 @@ +// SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0 +/* Copyright (c) 2016-2018, NXP Semiconductors + * Copyright (c) 2018-2019, Vladimir Oltean <olteanv@gmail.com> + */ +#include <linux/packing.h> +#include <linux/module.h> +#include <linux/bitops.h> +#include <linux/errno.h> +#include <linux/types.h> + +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 u64 bit_reverse(u64 val, unsigned int width) +{ + u64 new_val = 0; + unsigned int bit; + unsigned int i; + + for (i = 0; i < width; i++) { + bit = (val & (1 << i)) != 0; + new_val |= (bit << (width - i - 1)); + } + return new_val; +} + +static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit, + int *box_end_bit, u8 *box_mask) +{ + int box_bit_width = *box_start_bit - *box_end_bit + 1; + int new_box_start_bit, new_box_end_bit; + + *to_write >>= *box_end_bit; + *to_write = bit_reverse(*to_write, box_bit_width); + *to_write <<= *box_end_bit; + + new_box_end_bit = box_bit_width - *box_start_bit - 1; + new_box_start_bit = box_bit_width - *box_end_bit - 1; + *box_mask = GENMASK_ULL(new_box_start_bit, new_box_end_bit); + *box_start_bit = new_box_start_bit; + *box_end_bit = new_box_end_bit; +} + +/** + * packing - Convert numbers (currently u64) between a packed and an unpacked + * format. Unpacked means laid out in memory in the CPU's native + * understanding of integers, while packed means anything else that + * requires translation. + * + * @pbuf: Pointer to a buffer holding the packed value. + * @uval: Pointer to an u64 holding the unpacked value. + * @startbit: The index (in logical notation, compensated for quirks) where + * the packed value starts within pbuf. Must be larger than, or + * equal to, endbit. + * @endbit: The index (in logical notation, compensated for quirks) where + * the packed value ends within pbuf. Must be smaller than, or equal + * to, startbit. + * @pbuflen: The length in bytes of the packed buffer pointed to by @pbuf. + * @op: If PACK, then uval will be treated as const pointer and copied (packed) + * into pbuf, between startbit and endbit. + * If UNPACK, then pbuf will be treated as const pointer and the logical + * value between startbit and endbit will be copied (unpacked) to uval. + * @quirks: A bit mask of QUIRK_LITTLE_ENDIAN, QUIRK_LSW32_IS_FIRST and + * QUIRK_MSB_ON_THE_RIGHT. + * + * Return: 0 on success, EINVAL or ERANGE if called incorrectly. Assuming + * correct usage, return code may be discarded. + * If op is PACK, pbuf is modified. + * If op is UNPACK, uval is modified. + */ +int packing(void *pbuf, u64 *uval, int startbit, int endbit, size_t pbuflen, + enum packing_op op, u8 quirks) +{ + /* Number of bits for storing "uval" + * also width of the field to access in the pbuf + */ + u64 value_width; + /* Logical byte indices corresponding to the + * start and end of the field. + */ + int plogical_first_u8, plogical_last_u8, box; + + /* startbit is expected to be larger than endbit */ + if (startbit < endbit) + /* Invalid function call */ + return -EINVAL; + + value_width = startbit - endbit + 1; + if (value_width > 64) + return -ERANGE; + + /* Check if "uval" fits in "value_width" bits. + * If value_width is 64, the check will fail, but any + * 64-bit uval will surely fit. + */ + if (op == PACK && value_width < 64 && (*uval >= (1ull << value_width))) + /* Cannot store "uval" inside "value_width" bits. + * Truncating "uval" is most certainly not desirable, + * so simply erroring out is appropriate. + */ + return -ERANGE; + + /* Initialize parameter */ + if (op == UNPACK) + *uval = 0; + + /* Iterate through an idealistic view of the pbuf as an u64 with + * no quirks, u8 by u8 (aligned at u8 boundaries), from high to low + * logical bit significance. "box" denotes the current logical u8. + */ + plogical_first_u8 = startbit / 8; + plogical_last_u8 = endbit / 8; + + for (box = plogical_first_u8; box >= plogical_last_u8; box--) { + /* Bit indices into the currently accessed 8-bit box */ + int box_start_bit, box_end_bit, box_addr; + u8 box_mask; + /* Corresponding bits from the unpacked u64 parameter */ + int proj_start_bit, proj_end_bit; + u64 proj_mask; + + /* This u8 may need to be accessed in its entirety + * (from bit 7 to bit 0), or not, depending on the + * input arguments startbit and endbit. + */ + if (box == plogical_first_u8) + box_start_bit = startbit % 8; + else + box_start_bit = 7; + if (box == plogical_last_u8) + box_end_bit = endbit % 8; + else + box_end_bit = 0; + + /* We have determined the box bit start and end. + * Now we calculate where this (masked) u8 box would fit + * in the unpacked (CPU-readable) u64 - the u8 box's + * projection onto the unpacked u64. Though the + * box is u8, the projection is u64 because it may fall + * anywhere within the unpacked u64. + */ + proj_start_bit = ((box * 8) + box_start_bit) - endbit; + proj_end_bit = ((box * 8) + box_end_bit) - endbit; + proj_mask = GENMASK_ULL(proj_start_bit, proj_end_bit); + box_mask = GENMASK_ULL(box_start_bit, box_end_bit); + + /* Determine the offset of the u8 box inside the pbuf, + * adjusted for quirks. The adjusted box_addr will be used for + * 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); + + if (op == UNPACK) { + u64 pval; + + /* Read from pbuf, write to uval */ + pval = ((u8 *)pbuf)[box_addr] & box_mask; + if (quirks & QUIRK_MSB_ON_THE_RIGHT) + adjust_for_msb_right_quirk(&pval, + &box_start_bit, + &box_end_bit, + &box_mask); + + pval >>= box_end_bit; + pval <<= proj_end_bit; + *uval &= ~proj_mask; + *uval |= pval; + } else { + u64 pval; + + /* Write to pbuf, read from uval */ + pval = (*uval) & proj_mask; + pval >>= proj_end_bit; + if (quirks & QUIRK_MSB_ON_THE_RIGHT) + adjust_for_msb_right_quirk(&pval, + &box_start_bit, + &box_end_bit, + &box_mask); + + pval <<= box_end_bit; + ((u8 *)pbuf)[box_addr] &= ~box_mask; + ((u8 *)pbuf)[box_addr] |= pval; + } + } + return 0; +} +EXPORT_SYMBOL(packing); + +MODULE_LICENSE("GPL v2"); +MODULE_DESCRIPTION("Generic bitfield packing and unpacking"); |