// 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_DESCRIPTION("Arm SMCCC TRNG firmware interface support");
MODULE_LICENSE("GPL");