drivers/char/hw_random/arm_smccc_trng.c

   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");