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 		if ((int)res.a0 < 0)
75 			return (int)res.a0;
76 
77 		switch ((int)res.a0) {
78 		case SMCCC_RET_SUCCESS:
79 			copied += copy_from_registers(buf + copied, &res,
80 						      bits / BITS_PER_BYTE);
81 			tries = 0;
82 			break;
83 		case SMCCC_RET_TRNG_NO_ENTROPY:
84 			if (!wait)
85 				return copied;
86 			tries++;
87 			if (tries >= SMCCC_TRNG_MAX_TRIES)
88 				return copied;
89 			cond_resched();
90 			break;
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 	platform_set_drvdata(pdev, trng);
109 
110 	return devm_hwrng_register(&pdev->dev, trng);
111 }
112 
113 static struct platform_driver smccc_trng_driver = {
114 	.driver = {
115 		.name		= "smccc_trng",
116 	},
117 	.probe		= smccc_trng_probe,
118 };
119 module_platform_driver(smccc_trng_driver);
120 
121 MODULE_ALIAS("platform:smccc_trng");
122 MODULE_AUTHOR("Andre Przywara");
123 MODULE_LICENSE("GPL");
124