drivers/entropy/entropy_nrf5.c

/*
 * Copyright (c) 2018 Nordic Semiconductor ASA
 * Copyright (c) 2017 Exati Tecnologia Ltda.
 *
 * SPDX-License-Identifier: Apache-2.0
 */

#include <entropy.h>
#include <atomic.h>
#include <soc.h>

/*
 * The nRF5 RNG HW has several characteristics that need to be taken
 * into account by the driver to achieve energy efficient generation
 * of entropy.
 *
 * The RNG does not support continuously DMA'ing entropy into RAM,
 * values must be read out by the CPU byte-by-byte. But once started,
 * it will continue to generate bytes until stopped.
 *
 * The generation time for byte 0 after starting generation (with BIAS
 * correction) is:
 *
 * nRF51822 - 677us
 * nRF52810 - 248us
 * nRF52840 - 248us
 *
 * The generation time for byte N >= 1 after starting generation (with
 * BIAS correction) is:
 *
 * nRF51822 - 677us
 * nRF52810 - 120us
 * nRF52840 - 120us
 *
 * Due to the first byte in a stream of bytes being more costly on
 * some platforms a "water system" inspired algorithm is used to
 * ammortize the cost of the first byte.
 *
 * The algorithm will delay generation of entropy until the amount of
 * bytes goes below THRESHOLD, at which point it will generate entropy
 * until the BUF_LEN limit is reached.
 *
 * The entropy level is checked at the end of every consumption of
 * entropy.
 *
 * The algorithm and HW together has these characteristics:
 *
 * Setting a low threshold will highly ammortize the extra 120us cost
 * of the first byte on nRF52.
 *
 * Setting a high threshold will minimize the time spent waiting for
 * entropy.
 *
 * To minimize power consumption the threshold should either be set
 * low or high depending on the HFCLK-usage pattern of other
 * components.
 *
 * If the threshold is set close to the BUF_LEN, and the system
 * happens to anyway be using the HFCLK for several hundred us after
 * entropy is requested there will be no extra current-consumption for
 * keeping clocks running for entropy generation.
 *
 */

struct rand {
	u8_t count;
	u8_t threshold;
	u8_t first;
	u8_t last;
	u8_t rand[0];
};

#define RAND_DEFINE(name, len) u8_t name[sizeof(struct rand) + len] __aligned(4)

#define RAND_THREAD_LEN (CONFIG_ENTROPY_NRF5_THR_BUF_LEN + 1)
#define RAND_ISR_LEN    (CONFIG_ENTROPY_NRF5_ISR_BUF_LEN + 1)

struct entropy_nrf5_dev_data {
	struct k_sem sem_lock;
	struct k_sem sem_sync;

	RAND_DEFINE(thr, RAND_THREAD_LEN);
	RAND_DEFINE(isr, RAND_ISR_LEN);
};

#define DEV_DATA(dev) \
	((struct entropy_nrf5_dev_data *)(dev)->driver_data)

#pragma GCC push_options
#if defined(CONFIG_BT_CTLR_FAST_ENC)
#pragma GCC optimize ("Ofast")
#endif
static inline u8_t get(struct rand *rng, u8_t octets, u8_t *rand)
{
	u8_t first, last, avail, remaining, *d, *s;

	__ASSERT_NO_MSG(rng);

	first = rng->first;
	last = rng->last;

	d = &rand[octets];
	s = &rng->rand[first];

	if (first <= last) {
		/* copy octets from contiguous memory */
		avail = last - first;
		if (octets < avail) {
			remaining = avail - octets;
			avail = octets;
		} else {
			remaining = 0;
		}

		first += avail;
		octets -= avail;

		while (avail--) {
			*(--d) = *s++;
		}

		rng->first = first;
	} else {
		/* copy octets from split halves - until end of array */
		avail = rng->count - first;
		if (octets < avail) {
			remaining = avail + last - octets;
			avail = octets;
			first += avail;
		} else {
			remaining = last;
			first = 0;
		}

		octets -= avail;

		while (avail--) {
			*(--d) = *s++;
		}

		/* copy from beginning of array - until ring buffer last idx */
		if (octets && last) {
			s = &rng->rand[0];

			if (octets < last) {
				remaining = last - octets;
				last = octets;
			} else {
				remaining = 0;
			}

			first = last;
			octets -= last;

			while (last--) {
				*(--d) = *s++;
			}
		}

		rng->first = first;
	}

	if (remaining < rng->threshold) {
		NRF_RNG->TASKS_START = 1;
#if defined(CONFIG_BOARD_NRFXX_NWTSIM)
		NRF_RNG_regw_sideeffects();
#endif
	}

	return octets;
}
#pragma GCC pop_options

static int isr(struct rand *rng, bool store)
{
	u8_t last;

	if (!rng) {
		return -ENOBUFS;
	}

	last = rng->last + 1;
	if (last == rng->count) {
		last = 0;
	}

	if (last == rng->first) {
		/* this condition should not happen, but due to probable race,
		 * new value could be generated before NRF_RNG task is stopped.
		 */
		return -ENOBUFS;
	}

	if (!store) {
		return -EBUSY;
	}

	rng->rand[rng->last] = NRF_RNG->VALUE;
	rng->last = last;

	last = rng->last + 1;
	if (last == rng->count) {
		last = 0;
	}

	if (last == rng->first) {
		return 0;
	}

	return -EBUSY;
}

static void isr_rand(void *arg)
{
	struct device *device = arg;

	if (NRF_RNG->EVENTS_VALRDY) {
		struct entropy_nrf5_dev_data *dev_data = DEV_DATA(device);
		int ret;

		ret = isr((struct rand *)dev_data->isr, true);
		if (ret != -EBUSY) {
			ret = isr((struct rand *)dev_data->thr,
				  (ret == -ENOBUFS));
			k_sem_give(&dev_data->sem_sync);
		}

		NRF_RNG->EVENTS_VALRDY = 0;

		if (ret != -EBUSY) {
			NRF_RNG->TASKS_STOP = 1;
#if defined(CONFIG_BOARD_NRFXX_NWTSIM)
			NRF_RNG_regw_sideeffects();
#endif
		}
	}
}

static void init(struct rand *rng, u8_t len, u8_t threshold)
{
	rng->count = len;
	rng->threshold = threshold;
	rng->first = rng->last = 0;
}

static int entropy_nrf5_get_entropy(struct device *device, u8_t *buf, u16_t len)
{
	struct entropy_nrf5_dev_data *dev_data = DEV_DATA(device);

	while (len) {
		u8_t len8;

		if (len > UINT8_MAX) {
			len8 = UINT8_MAX;
		} else {
			len8 = len;
		}
		len -= len8;

		while (len8) {
			k_sem_take(&dev_data->sem_lock, K_FOREVER);
			len8 = get((struct rand *)dev_data->thr, len8, buf);
			k_sem_give(&dev_data->sem_lock);
			if (len8) {
				/* Sleep until next interrupt */
				k_sem_take(&dev_data->sem_sync, K_FOREVER);
			}
		}
	}

	return 0;
}

static struct entropy_nrf5_dev_data entropy_nrf5_data;
static int entropy_nrf5_init(struct device *device);

static const struct entropy_driver_api entropy_nrf5_api_funcs = {
	.get_entropy = entropy_nrf5_get_entropy
};

DEVICE_AND_API_INIT(entropy_nrf5, CONFIG_ENTROPY_NAME,
		    entropy_nrf5_init, &entropy_nrf5_data, NULL,
		    PRE_KERNEL_1, CONFIG_KERNEL_INIT_PRIORITY_DEVICE,
		    &entropy_nrf5_api_funcs);

static int entropy_nrf5_init(struct device *device)
{
	struct entropy_nrf5_dev_data *dev_data = DEV_DATA(device);

	/* Locking semaphore initialized to 1 (unlocked) */
	k_sem_init(&dev_data->sem_lock, 1, 1);
	/* Synching semaphore */
	k_sem_init(&dev_data->sem_sync, 0, 1);

	init((struct rand *)dev_data->thr, RAND_THREAD_LEN,
	     CONFIG_ENTROPY_NRF5_THR_THRESHOLD);
	init((struct rand *)dev_data->isr, RAND_ISR_LEN,
	     CONFIG_ENTROPY_NRF5_ISR_THRESHOLD);

	/* Enable or disable bias correction */
	if (IS_ENABLED(CONFIG_ENTROPY_NRF5_BIAS_CORRECTION)) {
		NRF_RNG->CONFIG |= RNG_CONFIG_DERCEN_Msk;
	} else {
		NRF_RNG->CONFIG &= ~RNG_CONFIG_DERCEN_Msk;
	}

	NRF_RNG->EVENTS_VALRDY = 0;
	NRF_RNG->INTENSET = RNG_INTENSET_VALRDY_Msk;

	NRF_RNG->TASKS_START = 1;
#if defined(CONFIG_BOARD_NRFXX_NWTSIM)
	NRF_RNG_regw_sideeffects();
#endif

	IRQ_CONNECT(NRF5_IRQ_RNG_IRQn, CONFIG_ENTROPY_NRF5_PRI, isr_rand,
		    DEVICE_GET(entropy_nrf5), 0);
	irq_enable(NRF5_IRQ_RNG_IRQn);

	return 0;
}

u8_t entropy_get_entropy_isr(struct device *dev, u8_t *buf, u8_t len)
{
	ARG_UNUSED(dev);
	return get((struct rand *)entropy_nrf5_data.isr, len, buf);
}