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// SPDX-License-Identifier: GPL-2.0
/*
* Randomness driver for the ARM SMCCC TRNG Firmware Interface
* https://developer.arm.com/documentation/den0098/latest/
*
* Copyright (C) 2020 Arm Ltd.
*
* The ARM TRNG firmware interface specifies a protocol to read entropy
* from a higher exception level, to abstract from any machine specific
* implemenations and allow easier use in hypervisors.
*
* The firmware interface is realised using the SMCCC specification.
*/
#include <linux/bits.h>
#include <linux/device.h>
#include <linux/hw_random.h>
#include <linux/module.h>
#include <linux/platform_device.h>
#include <linux/arm-smccc.h>
#ifdef CONFIG_ARM64
#define ARM_SMCCC_TRNG_RND ARM_SMCCC_TRNG_RND64
#define MAX_BITS_PER_CALL (3 * 64UL)
#else
#define ARM_SMCCC_TRNG_RND ARM_SMCCC_TRNG_RND32
#define MAX_BITS_PER_CALL (3 * 32UL)
#endif
/* We don't want to allow the firmware to stall us forever. */
#define SMCCC_TRNG_MAX_TRIES 20
#define SMCCC_RET_TRNG_INVALID_PARAMETER -2
#define SMCCC_RET_TRNG_NO_ENTROPY -3
static int copy_from_registers(char *buf, struct arm_smccc_res *res,
size_t bytes)
{
unsigned int chunk, copied;
if (bytes == 0)
return 0;
chunk = min(bytes, sizeof(long));
memcpy(buf, &res->a3, chunk);
copied = chunk;
if (copied >= bytes)
return copied;
chunk = min((bytes - copied), sizeof(long));
memcpy(&buf[copied], &res->a2, chunk);
copied += chunk;
if (copied >= bytes)
return copied;
chunk = min((bytes - copied), sizeof(long));
memcpy(&buf[copied], &res->a1, chunk);
return copied + chunk;
}
static int smccc_trng_read(struct hwrng *rng, void *data, size_t max, bool wait)
{
struct arm_smccc_res res;
u8 *buf = data;
unsigned int copied = 0;
int tries = 0;
while (copied < max) {
size_t bits = min_t(size_t, (max - copied) * BITS_PER_BYTE,
MAX_BITS_PER_CALL);
arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND, bits, &res);
switch ((int)res.a0) {
case SMCCC_RET_SUCCESS:
copied += copy_from_registers(buf + copied, &res,
bits / BITS_PER_BYTE);
tries = 0;
break;
case SMCCC_RET_TRNG_NO_ENTROPY:
if (!wait)
return copied;
tries++;
if (tries >= SMCCC_TRNG_MAX_TRIES)
return copied;
cond_resched();
break;
default:
return -EIO;
}
}
return copied;
}
static int smccc_trng_probe(struct platform_device *pdev)
{
struct hwrng *trng;
trng = devm_kzalloc(&pdev->dev, sizeof(*trng), GFP_KERNEL);
if (!trng)
return -ENOMEM;
trng->name = "smccc_trng";
trng->read = smccc_trng_read;
return devm_hwrng_register(&pdev->dev, trng);
}
static struct platform_driver smccc_trng_driver = {
.driver = {
.name = "smccc_trng",
},
.probe = smccc_trng_probe,
};
module_platform_driver(smccc_trng_driver);
MODULE_ALIAS("platform:smccc_trng");
MODULE_AUTHOR("Andre Przywara");
MODULE_LICENSE("GPL");
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