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
copy_from_registers(char * buf,struct arm_smccc_res * res,size_t bytes)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
smccc_trng_read(struct hwrng * rng,void * data,size_t max,bool wait)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
smccc_trng_probe(struct platform_device * pdev)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_LICENSE("GPL");
122