1 // SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0
2 /* Copyright (c) 2016-2018, NXP Semiconductors
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
get_le_offset(int offset)11 static int get_le_offset(int offset)
12 {
13 int closest_multiple_of_4;
14
15 closest_multiple_of_4 = (offset / 4) * 4;
16 offset -= closest_multiple_of_4;
17 return closest_multiple_of_4 + (3 - offset);
18 }
19
get_reverse_lsw32_offset(int offset,size_t len)20 static int get_reverse_lsw32_offset(int offset, size_t len)
21 {
22 int closest_multiple_of_4;
23 int word_index;
24
25 word_index = offset / 4;
26 closest_multiple_of_4 = word_index * 4;
27 offset -= closest_multiple_of_4;
28 word_index = (len / 4) - word_index - 1;
29 return word_index * 4 + offset;
30 }
31
bit_reverse(u64 val,unsigned int width)32 static u64 bit_reverse(u64 val, unsigned int width)
33 {
34 u64 new_val = 0;
35 unsigned int bit;
36 unsigned int i;
37
38 for (i = 0; i < width; i++) {
39 bit = (val & (1 << i)) != 0;
40 new_val |= (bit << (width - i - 1));
41 }
42 return new_val;
43 }
44
adjust_for_msb_right_quirk(u64 * to_write,int * box_start_bit,int * box_end_bit,u8 * box_mask)45 static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit,
46 int *box_end_bit, u8 *box_mask)
47 {
48 int box_bit_width = *box_start_bit - *box_end_bit + 1;
49 int new_box_start_bit, new_box_end_bit;
50
51 *to_write >>= *box_end_bit;
52 *to_write = bit_reverse(*to_write, box_bit_width);
53 *to_write <<= *box_end_bit;
54
55 new_box_end_bit = box_bit_width - *box_start_bit - 1;
56 new_box_start_bit = box_bit_width - *box_end_bit - 1;
57 *box_mask = GENMASK_ULL(new_box_start_bit, new_box_end_bit);
58 *box_start_bit = new_box_start_bit;
59 *box_end_bit = new_box_end_bit;
60 }
61
62 /**
63 * packing - Convert numbers (currently u64) between a packed and an unpacked
64 * format. Unpacked means laid out in memory in the CPU's native
65 * understanding of integers, while packed means anything else that
66 * requires translation.
67 *
68 * @pbuf: Pointer to a buffer holding the packed value.
69 * @uval: Pointer to an u64 holding the unpacked value.
70 * @startbit: The index (in logical notation, compensated for quirks) where
71 * the packed value starts within pbuf. Must be larger than, or
72 * equal to, endbit.
73 * @endbit: The index (in logical notation, compensated for quirks) where
74 * the packed value ends within pbuf. Must be smaller than, or equal
75 * to, startbit.
76 * @pbuflen: The length in bytes of the packed buffer pointed to by @pbuf.
77 * @op: If PACK, then uval will be treated as const pointer and copied (packed)
78 * into pbuf, between startbit and endbit.
79 * If UNPACK, then pbuf will be treated as const pointer and the logical
80 * value between startbit and endbit will be copied (unpacked) to uval.
81 * @quirks: A bit mask of QUIRK_LITTLE_ENDIAN, QUIRK_LSW32_IS_FIRST and
82 * QUIRK_MSB_ON_THE_RIGHT.
83 *
84 * Return: 0 on success, EINVAL or ERANGE if called incorrectly. Assuming
85 * correct usage, return code may be discarded.
86 * If op is PACK, pbuf is modified.
87 * If op is UNPACK, uval is modified.
88 */
packing(void * pbuf,u64 * uval,int startbit,int endbit,size_t pbuflen,enum packing_op op,u8 quirks)89 int packing(void *pbuf, u64 *uval, int startbit, int endbit, size_t pbuflen,
90 enum packing_op op, u8 quirks)
91 {
92 /* Number of bits for storing "uval"
93 * also width of the field to access in the pbuf
94 */
95 u64 value_width;
96 /* Logical byte indices corresponding to the
97 * start and end of the field.
98 */
99 int plogical_first_u8, plogical_last_u8, box;
100
101 /* startbit is expected to be larger than endbit */
102 if (startbit < endbit)
103 /* Invalid function call */
104 return -EINVAL;
105
106 value_width = startbit - endbit + 1;
107 if (value_width > 64)
108 return -ERANGE;
109
110 /* Check if "uval" fits in "value_width" bits.
111 * If value_width is 64, the check will fail, but any
112 * 64-bit uval will surely fit.
113 */
114 if (op == PACK && value_width < 64 && (*uval >= (1ull << value_width)))
115 /* Cannot store "uval" inside "value_width" bits.
116 * Truncating "uval" is most certainly not desirable,
117 * so simply erroring out is appropriate.
118 */
119 return -ERANGE;
120
121 /* Initialize parameter */
122 if (op == UNPACK)
123 *uval = 0;
124
125 /* Iterate through an idealistic view of the pbuf as an u64 with
126 * no quirks, u8 by u8 (aligned at u8 boundaries), from high to low
127 * logical bit significance. "box" denotes the current logical u8.
128 */
129 plogical_first_u8 = startbit / 8;
130 plogical_last_u8 = endbit / 8;
131
132 for (box = plogical_first_u8; box >= plogical_last_u8; box--) {
133 /* Bit indices into the currently accessed 8-bit box */
134 int box_start_bit, box_end_bit, box_addr;
135 u8 box_mask;
136 /* Corresponding bits from the unpacked u64 parameter */
137 int proj_start_bit, proj_end_bit;
138 u64 proj_mask;
139
140 /* This u8 may need to be accessed in its entirety
141 * (from bit 7 to bit 0), or not, depending on the
142 * input arguments startbit and endbit.
143 */
144 if (box == plogical_first_u8)
145 box_start_bit = startbit % 8;
146 else
147 box_start_bit = 7;
148 if (box == plogical_last_u8)
149 box_end_bit = endbit % 8;
150 else
151 box_end_bit = 0;
152
153 /* We have determined the box bit start and end.
154 * Now we calculate where this (masked) u8 box would fit
155 * in the unpacked (CPU-readable) u64 - the u8 box's
156 * projection onto the unpacked u64. Though the
157 * box is u8, the projection is u64 because it may fall
158 * anywhere within the unpacked u64.
159 */
160 proj_start_bit = ((box * 8) + box_start_bit) - endbit;
161 proj_end_bit = ((box * 8) + box_end_bit) - endbit;
162 proj_mask = GENMASK_ULL(proj_start_bit, proj_end_bit);
163 box_mask = GENMASK_ULL(box_start_bit, box_end_bit);
164
165 /* Determine the offset of the u8 box inside the pbuf,
166 * adjusted for quirks. The adjusted box_addr will be used for
167 * effective addressing inside the pbuf (so it's not
168 * logical any longer).
169 */
170 box_addr = pbuflen - box - 1;
171 if (quirks & QUIRK_LITTLE_ENDIAN)
172 box_addr = get_le_offset(box_addr);
173 if (quirks & QUIRK_LSW32_IS_FIRST)
174 box_addr = get_reverse_lsw32_offset(box_addr,
175 pbuflen);
176
177 if (op == UNPACK) {
178 u64 pval;
179
180 /* Read from pbuf, write to uval */
181 pval = ((u8 *)pbuf)[box_addr] & box_mask;
182 if (quirks & QUIRK_MSB_ON_THE_RIGHT)
183 adjust_for_msb_right_quirk(&pval,
184 &box_start_bit,
185 &box_end_bit,
186 &box_mask);
187
188 pval >>= box_end_bit;
189 pval <<= proj_end_bit;
190 *uval &= ~proj_mask;
191 *uval |= pval;
192 } else {
193 u64 pval;
194
195 /* Write to pbuf, read from uval */
196 pval = (*uval) & proj_mask;
197 pval >>= proj_end_bit;
198 if (quirks & QUIRK_MSB_ON_THE_RIGHT)
199 adjust_for_msb_right_quirk(&pval,
200 &box_start_bit,
201 &box_end_bit,
202 &box_mask);
203
204 pval <<= box_end_bit;
205 ((u8 *)pbuf)[box_addr] &= ~box_mask;
206 ((u8 *)pbuf)[box_addr] |= pval;
207 }
208 }
209 return 0;
210 }
211 EXPORT_SYMBOL(packing);
212
213 MODULE_LICENSE("GPL v2");
214 MODULE_DESCRIPTION("Generic bitfield packing and unpacking");
215
