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1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Randomness driver for the ARM SMCCC TRNG Firmware Interface
4  * https://developer.arm.com/documentation/den0098/latest/
5  *
6  *  Copyright (C) 2020 Arm Ltd.
7  *
8  * The ARM TRNG firmware interface specifies a protocol to read entropy
9  * from a higher exception level, to abstract from any machine specific
10  * implemenations and allow easier use in hypervisors.
11  *
12  * The firmware interface is realised using the SMCCC specification.
13  */
14
15 #include <linux/bits.h>
16 #include <linux/device.h>
17 #include <linux/hw_random.h>
18 #include <linux/module.h>
19 #include <linux/platform_device.h>
20 #include <linux/arm-smccc.h>
21
22 #ifdef CONFIG_ARM64
23 #define ARM_SMCCC_TRNG_RND      ARM_SMCCC_TRNG_RND64
24 #define MAX_BITS_PER_CALL       (3 * 64UL)
25 #else
26 #define ARM_SMCCC_TRNG_RND      ARM_SMCCC_TRNG_RND32
27 #define MAX_BITS_PER_CALL       (3 * 32UL)
28 #endif
29
30 /* We don't want to allow the firmware to stall us forever. */
31 #define SMCCC_TRNG_MAX_TRIES    20
32
33 #define SMCCC_RET_TRNG_INVALID_PARAMETER        -2
34 #define SMCCC_RET_TRNG_NO_ENTROPY               -3
35
36 static int copy_from_registers(char *buf, struct arm_smccc_res *res,
37                                size_t bytes)
38 {
39         unsigned int chunk, copied;
40
41         if (bytes == 0)
42                 return 0;
43
44         chunk = min(bytes, sizeof(long));
45         memcpy(buf, &res->a3, chunk);
46         copied = chunk;
47         if (copied >= bytes)
48                 return copied;
49
50         chunk = min((bytes - copied), sizeof(long));
51         memcpy(&buf[copied], &res->a2, chunk);
52         copied += chunk;
53         if (copied >= bytes)
54                 return copied;
55
56         chunk = min((bytes - copied), sizeof(long));
57         memcpy(&buf[copied], &res->a1, chunk);
58
59         return copied + chunk;
60 }
61
62 static int smccc_trng_read(struct hwrng *rng, void *data, size_t max, bool wait)
63 {
64         struct arm_smccc_res res;
65         u8 *buf = data;
66         unsigned int copied = 0;
67         int tries = 0;
68
69         while (copied < max) {
70                 size_t bits = min_t(size_t, (max - copied) * BITS_PER_BYTE,
71                                   MAX_BITS_PER_CALL);
72
73                 arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND, bits, &res);
74
75                 switch ((int)res.a0) {
76                 case SMCCC_RET_SUCCESS:
77                         copied += copy_from_registers(buf + copied, &res,
78                                                       bits / BITS_PER_BYTE);
79                         tries = 0;
80                         break;
81                 case SMCCC_RET_TRNG_NO_ENTROPY:
82                         if (!wait)
83                                 return copied;
84                         tries++;
85                         if (tries >= SMCCC_TRNG_MAX_TRIES)
86                                 return copied;
87                         cond_resched();
88                         break;
89                 default:
90                         return -EIO;
91                 }
92         }
93
94         return copied;
95 }
96
97 static int smccc_trng_probe(struct platform_device *pdev)
98 {
99         struct hwrng *trng;
100
101         trng = devm_kzalloc(&pdev->dev, sizeof(*trng), GFP_KERNEL);
102         if (!trng)
103                 return -ENOMEM;
104
105         trng->name = "smccc_trng";
106         trng->read = smccc_trng_read;
107
108         return devm_hwrng_register(&pdev->dev, trng);
109 }
110
111 static struct platform_driver smccc_trng_driver = {
112         .driver = {
113                 .name           = "smccc_trng",
114         },
115         .probe          = smccc_trng_probe,
116 };
117 module_platform_driver(smccc_trng_driver);
118
119 MODULE_ALIAS("platform:smccc_trng");
120 MODULE_AUTHOR("Andre Przywara");
121 MODULE_DESCRIPTION("Arm SMCCC TRNG firmware interface support");
122 MODULE_LICENSE("GPL");
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