// SPDX-License-Identifier: GPL-2.0
// Copyright (C) 2016, Linaro Ltd - Daniel Lezcano <daniel.lezcano@linaro.org>

#include <linux/kernel.h>
#include <linux/percpu.h>
#include <linux/slab.h>
#include <linux/static_key.h>
#include <linux/interrupt.h>
#include <linux/idr.h>
#include <linux/irq.h>
#include <linux/math64.h>

#include <trace/events/irq.h>

#include "internals.h"

DEFINE_STATIC_KEY_FALSE(irq_timing_enabled);

DEFINE_PER_CPU(struct irq_timings, irq_timings);

struct irqt_stat {
        u64     next_evt;
        u64     last_ts;
        u64     variance;
        u32     avg;
        u32     nr_samples;
        int     anomalies;
        int     valid;
};

static DEFINE_IDR(irqt_stats);

void irq_timings_enable(void)
{
        static_branch_enable(&irq_timing_enabled);
}

void irq_timings_disable(void)
{
        static_branch_disable(&irq_timing_enabled);
}

/**
 * irqs_update - update the irq timing statistics with a new timestamp
 *
 * @irqs: an irqt_stat struct pointer
 * @ts: the new timestamp
 *
 * The statistics are computed online, in other words, the code is
 * designed to compute the statistics on a stream of values rather
 * than doing multiple passes on the values to compute the average,
 * then the variance. The integer division introduces a loss of
 * precision but with an acceptable error margin regarding the results
 * we would have with the double floating precision: we are dealing
 * with nanosec, so big numbers, consequently the mantisse is
 * negligeable, especially when converting the time in usec
 * afterwards.
 *
 * The computation happens at idle time. When the CPU is not idle, the
 * interrupts' timestamps are stored in the circular buffer, when the
 * CPU goes idle and this routine is called, all the buffer's values
 * are injected in the statistical model continuying to extend the
 * statistics from the previous busy-idle cycle.
 *
 * The observations showed a device will trigger a burst of periodic
 * interrupts followed by one or two peaks of longer time, for
 * instance when a SD card device flushes its cache, then the periodic
 * intervals occur again. A one second inactivity period resets the
 * stats, that gives us the certitude the statistical values won't
 * exceed 1x10^9, thus the computation won't overflow.
 *
 * Basically, the purpose of the algorithm is to watch the periodic
 * interrupts and eliminate the peaks.
 *
 * An interrupt is considered periodically stable if the interval of
 * its occurences follow the normal distribution, thus the values
 * comply with:
 *
 *      avg - 3 x stddev < value < avg + 3 x stddev
 *
 * Which can be simplified to:
 *
 *      -3 x stddev < value - avg < 3 x stddev
 *
 *      abs(value - avg) < 3 x stddev
 *
 * In order to save a costly square root computation, we use the
 * variance. For the record, stddev = sqrt(variance). The equation
 * above becomes:
 *
 *      abs(value - avg) < 3 x sqrt(variance)
 *
 * And finally we square it:
 *
 *      (value - avg) ^ 2 < (3 x sqrt(variance)) ^ 2
 *
 *      (value - avg) x (value - avg) < 9 x variance
 *
 * Statistically speaking, any values out of this interval is
 * considered as an anomaly and is discarded. However, a normal
 * distribution appears when the number of samples is 30 (it is the
 * rule of thumb in statistics, cf. "30 samples" on Internet). When
 * there are three consecutive anomalies, the statistics are resetted.
 *
 */
static void irqs_update(struct irqt_stat *irqs, u64 ts)
{
        u64 old_ts = irqs->last_ts;
        u64 variance = 0;
        u64 interval;
        s64 diff;

        /*
         * The timestamps are absolute time values, we need to compute
         * the timing interval between two interrupts.
         */
        irqs->last_ts = ts;

        /*
         * The interval type is u64 in order to deal with the same
         * type in our computation, that prevent mindfuck issues with
         * overflow, sign and division.
         */
        interval = ts - old_ts;

        /*
         * The interrupt triggered more than one second apart, that
         * ends the sequence as predictible for our purpose. In this
         * case, assume we have the beginning of a sequence and the
         * timestamp is the first value. As it is impossible to
         * predict anything at this point, return.
         *
         * Note the first timestamp of the sequence will always fall
         * in this test because the old_ts is zero. That is what we
         * want as we need another timestamp to compute an interval.
         */
        if (interval >= NSEC_PER_SEC) {
                memset(irqs, 0, sizeof(*irqs));
                irqs->last_ts = ts;
                return;
        }

        /*
         * Pre-compute the delta with the average as the result is
         * used several times in this function.
         */
        diff = interval - irqs->avg;

        /*
         * Increment the number of samples.
         */
        irqs->nr_samples++;

        /*
         * Online variance divided by the number of elements if there
         * is more than one sample.  Normally the formula is division
         * by nr_samples - 1 but we assume the number of element will be
         * more than 32 and dividing by 32 instead of 31 is enough
         * precise.
         */
        if (likely(irqs->nr_samples > 1))
                variance = irqs->variance >> IRQ_TIMINGS_SHIFT;

        /*
         * The rule of thumb in statistics for the normal distribution
         * is having at least 30 samples in order to have the model to
         * apply. Values outside the interval are considered as an
         * anomaly.
         */
        if ((irqs->nr_samples >= 30) && ((diff * diff) > (9 * variance))) {
                /*
                 * After three consecutive anomalies, we reset the
                 * stats as it is no longer stable enough.
                 */
                if (irqs->anomalies++ >= 3) {
                        memset(irqs, 0, sizeof(*irqs));
                        irqs->last_ts = ts;
                        return;
                }
        } else {
                /*
                 * The anomalies must be consecutives, so at this
                 * point, we reset the anomalies counter.
                 */
                irqs->anomalies = 0;
        }

        /*
         * The interrupt is considered stable enough to try to predict
         * the next event on it.
         */
        irqs->valid = 1;

        /*
         * Online average algorithm:
         *
         *  new_average = average + ((value - average) / count)
         *
         * The variance computation depends on the new average
         * to be computed here first.
         *
         */
        irqs->avg = irqs->avg + (diff >> IRQ_TIMINGS_SHIFT);

        /*
         * Online variance algorithm:
         *
         *  new_variance = variance + (value - average) x (value - new_average)
         *
         * Warning: irqs->avg is updated with the line above, hence
         * 'interval - irqs->avg' is no longer equal to 'diff'
         */
        irqs->variance = irqs->variance + (diff * (interval - irqs->avg));

        /*
         * Update the next event
         */
        irqs->next_evt = ts + irqs->avg;
}

/**
 * irq_timings_next_event - Return when the next event is supposed to arrive
 *
 * During the last busy cycle, the number of interrupts is incremented
 * and stored in the irq_timings structure. This information is
 * necessary to:
 *
 * - know if the index in the table wrapped up:
 *
 *      If more than the array size interrupts happened during the
 *      last busy/idle cycle, the index wrapped up and we have to
 *      begin with the next element in the array which is the last one
 *      in the sequence, otherwise it is a the index 0.
 *
 * - have an indication of the interrupts activity on this CPU
 *   (eg. irq/sec)
 *
 * The values are 'consumed' after inserting in the statistical model,
 * thus the count is reinitialized.
 *
 * The array of values **must** be browsed in the time direction, the
 * timestamp must increase between an element and the next one.
 *
 * Returns a nanosec time based estimation of the earliest interrupt,
 * U64_MAX otherwise.
 */
u64 irq_timings_next_event(u64 now)
{
        struct irq_timings *irqts = this_cpu_ptr(&irq_timings);
        struct irqt_stat *irqs;
        struct irqt_stat __percpu *s;
        u64 ts, next_evt = U64_MAX;
        int i, irq = 0;

        /*
         * This function must be called with the local irq disabled in
         * order to prevent the timings circular buffer to be updated
         * while we are reading it.
         */
        lockdep_assert_irqs_disabled();

        /*
         * Number of elements in the circular buffer: If it happens it
         * was flushed before, then the number of elements could be
         * smaller than IRQ_TIMINGS_SIZE, so the count is used,
         * otherwise the array size is used as we wrapped. The index
         * begins from zero when we did not wrap. That could be done
         * in a nicer way with the proper circular array structure
         * type but with the cost of extra computation in the
         * interrupt handler hot path. We choose efficiency.
         *
         * Inject measured irq/timestamp to the statistical model
         * while decrementing the counter because we consume the data
         * from our circular buffer.
         */
        for (i = irqts->count & IRQ_TIMINGS_MASK,
                     irqts->count = min(IRQ_TIMINGS_SIZE, irqts->count);
             irqts->count > 0; irqts->count--, i = (i + 1) & IRQ_TIMINGS_MASK) {

                irq = irq_timing_decode(irqts->values[i], &ts);

                s = idr_find(&irqt_stats, irq);
                if (s) {
                        irqs = this_cpu_ptr(s);
                        irqs_update(irqs, ts);
                }
        }

        /*
         * Look in the list of interrupts' statistics, the earliest
         * next event.
         */
        idr_for_each_entry(&irqt_stats, s, i) {

                irqs = this_cpu_ptr(s);

                if (!irqs->valid)
                        continue;

                if (irqs->next_evt <= now) {
                        irq = i;
                        next_evt = now;

                        /*
                         * This interrupt mustn't use in the future
                         * until new events occur and update the
                         * statistics.
                         */
                        irqs->valid = 0;
                        break;
                }

                if (irqs->next_evt < next_evt) {
                        irq = i;
                        next_evt = irqs->next_evt;
                }
        }

        return next_evt;
}

void irq_timings_free(int irq)
{
        struct irqt_stat __percpu *s;

        s = idr_find(&irqt_stats, irq);
        if (s) {
                free_percpu(s);
                idr_remove(&irqt_stats, irq);
        }
}

int irq_timings_alloc(int irq)
{
        struct irqt_stat __percpu *s;
        int id;

        /*
         * Some platforms can have the same private interrupt per cpu,
         * so this function may be be called several times with the
         * same interrupt number. Just bail out in case the per cpu
         * stat structure is already allocated.
         */
        s = idr_find(&irqt_stats, irq);
        if (s)
                return 0;

        s = alloc_percpu(*s);
        if (!s)
                return -ENOMEM;

        idr_preload(GFP_KERNEL);
        id = idr_alloc(&irqt_stats, s, irq, irq + 1, GFP_NOWAIT);
        idr_preload_end();

        if (id < 0) {
                free_percpu(s);
                return id;
        }

        return 0;
}