1 // SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0
2 /* Copyright 2016-2018 NXP
3 * Copyright (c) 2018-2019, Vladimir Oltean <olteanv@gmail.com>
4 */
5 #include <linux/packing.h>
6 #include <linux/module.h>
7 #include <linux/bitops.h>
8 #include <linux/errno.h>
9 #include <linux/types.h>
10 #include <linux/bitrev.h>
11
12 static int get_le_offset(int offset)
13 {
14 int closest_multiple_of_4;
15
16 closest_multiple_of_4 = (offset / 4) * 4;
17 offset -= closest_multiple_of_4;
18 return closest_multiple_of_4 + (3 - offset);
19 }
20
21 static int get_reverse_lsw32_offset(int offset, size_t len)
22 {
23 int closest_multiple_of_4;
24 int word_index;
25
26 word_index = offset / 4;
27 closest_multiple_of_4 = word_index * 4;
28 offset -= closest_multiple_of_4;
29 word_index = (len / 4) - word_index - 1;
30 return word_index * 4 + offset;
31 }
32
33 static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit,
34 int *box_end_bit, u8 *box_mask)
35 {
36 int box_bit_width = *box_start_bit - *box_end_bit + 1;
37 int new_box_start_bit, new_box_end_bit;
38
39 *to_write >>= *box_end_bit;
40 *to_write = bitrev8(*to_write) >> (8 - box_bit_width);
41 *to_write <<= *box_end_bit;
42
43 new_box_end_bit = box_bit_width - *box_start_bit - 1;
44 new_box_start_bit = box_bit_width - *box_end_bit - 1;
45 *box_mask = GENMASK_ULL(new_box_start_bit, new_box_end_bit);
46 *box_start_bit = new_box_start_bit;
47 *box_end_bit = new_box_end_bit;
48 }
49
50 /**
51 * packing - Convert numbers (currently u64) between a packed and an unpacked
52 * format. Unpacked means laid out in memory in the CPU's native
53 * understanding of integers, while packed means anything else that
54 * requires translation.
55 *
56 * @pbuf: Pointer to a buffer holding the packed value.
57 * @uval: Pointer to an u64 holding the unpacked value.
58 * @startbit: The index (in logical notation, compensated for quirks) where
59 * the packed value starts within pbuf. Must be larger than, or
60 * equal to, endbit.
61 * @endbit: The index (in logical notation, compensated for quirks) where
62 * the packed value ends within pbuf. Must be smaller than, or equal
63 * to, startbit.
64 * @pbuflen: The length in bytes of the packed buffer pointed to by @pbuf.
65 * @op: If PACK, then uval will be treated as const pointer and copied (packed)
66 * into pbuf, between startbit and endbit.
67 * If UNPACK, then pbuf will be treated as const pointer and the logical
68 * value between startbit and endbit will be copied (unpacked) to uval.
69 * @quirks: A bit mask of QUIRK_LITTLE_ENDIAN, QUIRK_LSW32_IS_FIRST and
70 * QUIRK_MSB_ON_THE_RIGHT.
71 *
72 * Return: 0 on success, EINVAL or ERANGE if called incorrectly. Assuming
73 * correct usage, return code may be discarded.
74 * If op is PACK, pbuf is modified.
75 * If op is UNPACK, uval is modified.
76 */
77 int packing(void *pbuf, u64 *uval, int startbit, int endbit, size_t pbuflen,
78 enum packing_op op, u8 quirks)
79 {
80 /* Number of bits for storing "uval"
81 * also width of the field to access in the pbuf
82 */
83 u64 value_width;
84 /* Logical byte indices corresponding to the
85 * start and end of the field.
86 */
87 int plogical_first_u8, plogical_last_u8, box;
88
89 /* startbit is expected to be larger than endbit */
90 if (startbit < endbit)
91 /* Invalid function call */
92 return -EINVAL;
93
94 value_width = startbit - endbit + 1;
95 if (value_width > 64)
96 return -ERANGE;
97
98 /* Check if "uval" fits in "value_width" bits.
99 * If value_width is 64, the check will fail, but any
100 * 64-bit uval will surely fit.
101 */
102 if (op == PACK && value_width < 64 && (*uval >= (1ull << value_width)))
103 /* Cannot store "uval" inside "value_width" bits.
104 * Truncating "uval" is most certainly not desirable,
105 * so simply erroring out is appropriate.
106 */
107 return -ERANGE;
108
109 /* Initialize parameter */
110 if (op == UNPACK)
111 *uval = 0;
112
113 /* Iterate through an idealistic view of the pbuf as an u64 with
114 * no quirks, u8 by u8 (aligned at u8 boundaries), from high to low
115 * logical bit significance. "box" denotes the current logical u8.
116 */
117 plogical_first_u8 = startbit / 8;
118 plogical_last_u8 = endbit / 8;
119
120 for (box = plogical_first_u8; box >= plogical_last_u8; box--) {
121 /* Bit indices into the currently accessed 8-bit box */
122 int box_start_bit, box_end_bit, box_addr;
123 u8 box_mask;
124 /* Corresponding bits from the unpacked u64 parameter */
125 int proj_start_bit, proj_end_bit;
126 u64 proj_mask;
127
128 /* This u8 may need to be accessed in its entirety
129 * (from bit 7 to bit 0), or not, depending on the
130 * input arguments startbit and endbit.
131 */
132 if (box == plogical_first_u8)
133 box_start_bit = startbit % 8;
134 else
135 box_start_bit = 7;
136 if (box == plogical_last_u8)
137 box_end_bit = endbit % 8;
138 else
139 box_end_bit = 0;
140
141 /* We have determined the box bit start and end.
142 * Now we calculate where this (masked) u8 box would fit
143 * in the unpacked (CPU-readable) u64 - the u8 box's
144 * projection onto the unpacked u64. Though the
145 * box is u8, the projection is u64 because it may fall
146 * anywhere within the unpacked u64.
147 */
148 proj_start_bit = ((box * 8) + box_start_bit) - endbit;
149 proj_end_bit = ((box * 8) + box_end_bit) - endbit;
150 proj_mask = GENMASK_ULL(proj_start_bit, proj_end_bit);
151 box_mask = GENMASK_ULL(box_start_bit, box_end_bit);
152
153 /* Determine the offset of the u8 box inside the pbuf,
154 * adjusted for quirks. The adjusted box_addr will be used for
155 * effective addressing inside the pbuf (so it's not
156 * logical any longer).
157 */
158 box_addr = pbuflen - box - 1;
159 if (quirks & QUIRK_LITTLE_ENDIAN)
160 box_addr = get_le_offset(box_addr);
161 if (quirks & QUIRK_LSW32_IS_FIRST)
162 box_addr = get_reverse_lsw32_offset(box_addr,
163 pbuflen);
164
165 if (op == UNPACK) {
166 u64 pval;
167
168 /* Read from pbuf, write to uval */
169 pval = ((u8 *)pbuf)[box_addr] & box_mask;
170 if (quirks & QUIRK_MSB_ON_THE_RIGHT)
171 adjust_for_msb_right_quirk(&pval,
172 &box_start_bit,
173 &box_end_bit,
174 &box_mask);
175
176 pval >>= box_end_bit;
177 pval <<= proj_end_bit;
178 *uval &= ~proj_mask;
179 *uval |= pval;
180 } else {
181 u64 pval;
182
183 /* Write to pbuf, read from uval */
184 pval = (*uval) & proj_mask;
185 pval >>= proj_end_bit;
186 if (quirks & QUIRK_MSB_ON_THE_RIGHT)
187 adjust_for_msb_right_quirk(&pval,
188 &box_start_bit,
189 &box_end_bit,
190 &box_mask);
191
192 pval <<= box_end_bit;
193 ((u8 *)pbuf)[box_addr] &= ~box_mask;
194 ((u8 *)pbuf)[box_addr] |= pval;
195 }
196 }
197 return 0;
198 }
199 EXPORT_SYMBOL(packing);
200
201 MODULE_DESCRIPTION("Generic bitfield packing and unpacking");
202
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