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