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