Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel...

[cris-mirror.git] / drivers / gpu / drm / i915 / intel_breadcrumbs.c

/*
 * Copyright © 2015 Intel Corporation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 */

#include <linux/kthread.h>
#include <uapi/linux/sched/types.h>

#include "i915_drv.h"

#ifdef CONFIG_SMP
#define task_asleep(tsk) ((tsk)->state & TASK_NORMAL && !(tsk)->on_cpu)
#else
#define task_asleep(tsk) ((tsk)->state & TASK_NORMAL)
#endif

static unsigned int __intel_breadcrumbs_wakeup(struct intel_breadcrumbs *b)
{
        struct intel_wait *wait;
        unsigned int result = 0;

        lockdep_assert_held(&b->irq_lock);

        wait = b->irq_wait;
        if (wait) {
                /*
                 * N.B. Since task_asleep() and ttwu are not atomic, the
                 * waiter may actually go to sleep after the check, causing
                 * us to suppress a valid wakeup. We prefer to reduce the
                 * number of false positive missed_breadcrumb() warnings
                 * at the expense of a few false negatives, as it it easy
                 * to trigger a false positive under heavy load. Enough
                 * signal should remain from genuine missed_breadcrumb()
                 * for us to detect in CI.
                 */
                bool was_asleep = task_asleep(wait->tsk);

                result = ENGINE_WAKEUP_WAITER;
                if (wake_up_process(wait->tsk) && was_asleep)
                        result |= ENGINE_WAKEUP_ASLEEP;
        }

        return result;
}

unsigned int intel_engine_wakeup(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        unsigned long flags;
        unsigned int result;

        spin_lock_irqsave(&b->irq_lock, flags);
        result = __intel_breadcrumbs_wakeup(b);
        spin_unlock_irqrestore(&b->irq_lock, flags);

        return result;
}

static unsigned long wait_timeout(void)
{
        return round_jiffies_up(jiffies + DRM_I915_HANGCHECK_JIFFIES);
}

static noinline void missed_breadcrumb(struct intel_engine_cs *engine)
{
        if (drm_debug & DRM_UT_DRIVER) {
                struct drm_printer p = drm_debug_printer(__func__);

                intel_engine_dump(engine, &p,
                                  "%s missed breadcrumb at %pS\n",
                                  engine->name, __builtin_return_address(0));
        }

        set_bit(engine->id, &engine->i915->gpu_error.missed_irq_rings);
}

static void intel_breadcrumbs_hangcheck(struct timer_list *t)
{
        struct intel_engine_cs *engine =
                from_timer(engine, t, breadcrumbs.hangcheck);
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        if (!b->irq_armed)
                return;

        if (b->hangcheck_interrupts != atomic_read(&engine->irq_count)) {
                b->hangcheck_interrupts = atomic_read(&engine->irq_count);
                mod_timer(&b->hangcheck, wait_timeout());
                return;
        }

        /* We keep the hangcheck timer alive until we disarm the irq, even
         * if there are no waiters at present.
         *
         * If the waiter was currently running, assume it hasn't had a chance
         * to process the pending interrupt (e.g, low priority task on a loaded
         * system) and wait until it sleeps before declaring a missed interrupt.
         *
         * If the waiter was asleep (and not even pending a wakeup), then we
         * must have missed an interrupt as the GPU has stopped advancing
         * but we still have a waiter. Assuming all batches complete within
         * DRM_I915_HANGCHECK_JIFFIES [1.5s]!
         */
        if (intel_engine_wakeup(engine) & ENGINE_WAKEUP_ASLEEP) {
                missed_breadcrumb(engine);
                mod_timer(&b->fake_irq, jiffies + 1);
        } else {
                mod_timer(&b->hangcheck, wait_timeout());
        }
}

static void intel_breadcrumbs_fake_irq(struct timer_list *t)
{
        struct intel_engine_cs *engine = from_timer(engine, t,
                                                    breadcrumbs.fake_irq);
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        /* The timer persists in case we cannot enable interrupts,
         * or if we have previously seen seqno/interrupt incoherency
         * ("missed interrupt" syndrome, better known as a "missed breadcrumb").
         * Here the worker will wake up every jiffie in order to kick the
         * oldest waiter to do the coherent seqno check.
         */

        spin_lock_irq(&b->irq_lock);
        if (b->irq_armed && !__intel_breadcrumbs_wakeup(b))
                __intel_engine_disarm_breadcrumbs(engine);
        spin_unlock_irq(&b->irq_lock);
        if (!b->irq_armed)
                return;

        mod_timer(&b->fake_irq, jiffies + 1);
}

static void irq_enable(struct intel_engine_cs *engine)
{
        /*
         * FIXME: Ideally we want this on the API boundary, but for the
         * sake of testing with mock breadcrumbs (no HW so unable to
         * enable irqs) we place it deep within the bowels, at the point
         * of no return.
         */
        GEM_BUG_ON(!intel_irqs_enabled(engine->i915));

        /* Enabling the IRQ may miss the generation of the interrupt, but
         * we still need to force the barrier before reading the seqno,
         * just in case.
         */
        set_bit(ENGINE_IRQ_BREADCRUMB, &engine->irq_posted);

        /* Caller disables interrupts */
        spin_lock(&engine->i915->irq_lock);
        engine->irq_enable(engine);
        spin_unlock(&engine->i915->irq_lock);
}

static void irq_disable(struct intel_engine_cs *engine)
{
        /* Caller disables interrupts */
        spin_lock(&engine->i915->irq_lock);
        engine->irq_disable(engine);
        spin_unlock(&engine->i915->irq_lock);
}

void __intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        lockdep_assert_held(&b->irq_lock);
        GEM_BUG_ON(b->irq_wait);
        GEM_BUG_ON(!b->irq_armed);

        GEM_BUG_ON(!b->irq_enabled);
        if (!--b->irq_enabled)
                irq_disable(engine);

        b->irq_armed = false;
}

void intel_engine_pin_breadcrumbs_irq(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        spin_lock_irq(&b->irq_lock);
        if (!b->irq_enabled++)
                irq_enable(engine);
        GEM_BUG_ON(!b->irq_enabled); /* no overflow! */
        spin_unlock_irq(&b->irq_lock);
}

void intel_engine_unpin_breadcrumbs_irq(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        spin_lock_irq(&b->irq_lock);
        GEM_BUG_ON(!b->irq_enabled); /* no underflow! */
        if (!--b->irq_enabled)
                irq_disable(engine);
        spin_unlock_irq(&b->irq_lock);
}

void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        struct intel_wait *wait, *n;

        if (!b->irq_armed)
                goto wakeup_signaler;

        /*
         * We only disarm the irq when we are idle (all requests completed),
         * so if the bottom-half remains asleep, it missed the request
         * completion.
         */
        if (intel_engine_wakeup(engine) & ENGINE_WAKEUP_ASLEEP)
                missed_breadcrumb(engine);

        spin_lock_irq(&b->rb_lock);

        spin_lock(&b->irq_lock);
        b->irq_wait = NULL;
        if (b->irq_armed)
                __intel_engine_disarm_breadcrumbs(engine);
        spin_unlock(&b->irq_lock);

        rbtree_postorder_for_each_entry_safe(wait, n, &b->waiters, node) {
                RB_CLEAR_NODE(&wait->node);
                wake_up_process(wait->tsk);
        }
        b->waiters = RB_ROOT;

        spin_unlock_irq(&b->rb_lock);

        /*
         * The signaling thread may be asleep holding a reference to a request,
         * that had its signaling cancelled prior to being preempted. We need
         * to kick the signaler, just in case, to release any such reference.
         */
wakeup_signaler:
        wake_up_process(b->signaler);
}

static bool use_fake_irq(const struct intel_breadcrumbs *b)
{
        const struct intel_engine_cs *engine =
                container_of(b, struct intel_engine_cs, breadcrumbs);

        if (!test_bit(engine->id, &engine->i915->gpu_error.missed_irq_rings))
                return false;

        /* Only start with the heavy weight fake irq timer if we have not
         * seen any interrupts since enabling it the first time. If the
         * interrupts are still arriving, it means we made a mistake in our
         * engine->seqno_barrier(), a timing error that should be transient
         * and unlikely to reoccur.
         */
        return atomic_read(&engine->irq_count) == b->hangcheck_interrupts;
}

static void enable_fake_irq(struct intel_breadcrumbs *b)
{
        /* Ensure we never sleep indefinitely */
        if (!b->irq_enabled || use_fake_irq(b))
                mod_timer(&b->fake_irq, jiffies + 1);
        else
                mod_timer(&b->hangcheck, wait_timeout());
}

static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
{
        struct intel_engine_cs *engine =
                container_of(b, struct intel_engine_cs, breadcrumbs);
        struct drm_i915_private *i915 = engine->i915;
        bool enabled;

        lockdep_assert_held(&b->irq_lock);
        if (b->irq_armed)
                return false;

        /* The breadcrumb irq will be disarmed on the interrupt after the
         * waiters are signaled. This gives us a single interrupt window in
         * which we can add a new waiter and avoid the cost of re-enabling
         * the irq.
         */
        b->irq_armed = true;

        if (I915_SELFTEST_ONLY(b->mock)) {
                /* For our mock objects we want to avoid interaction
                 * with the real hardware (which is not set up). So
                 * we simply pretend we have enabled the powerwell
                 * and the irq, and leave it up to the mock
                 * implementation to call intel_engine_wakeup()
                 * itself when it wants to simulate a user interrupt,
                 */
                return true;
        }

        /* Since we are waiting on a request, the GPU should be busy
         * and should have its own rpm reference. This is tracked
         * by i915->gt.awake, we can forgo holding our own wakref
         * for the interrupt as before i915->gt.awake is released (when
         * the driver is idle) we disarm the breadcrumbs.
         */

        /* No interrupts? Kick the waiter every jiffie! */
        enabled = false;
        if (!b->irq_enabled++ &&
            !test_bit(engine->id, &i915->gpu_error.test_irq_rings)) {
                irq_enable(engine);
                enabled = true;
        }

        enable_fake_irq(b);
        return enabled;
}

static inline struct intel_wait *to_wait(struct rb_node *node)
{
        return rb_entry(node, struct intel_wait, node);
}

static inline void __intel_breadcrumbs_finish(struct intel_breadcrumbs *b,
                                              struct intel_wait *wait)
{
        lockdep_assert_held(&b->rb_lock);
        GEM_BUG_ON(b->irq_wait == wait);

        /* This request is completed, so remove it from the tree, mark it as
         * complete, and *then* wake up the associated task. N.B. when the
         * task wakes up, it will find the empty rb_node, discern that it
         * has already been removed from the tree and skip the serialisation
         * of the b->rb_lock and b->irq_lock. This means that the destruction
         * of the intel_wait is not serialised with the interrupt handler
         * by the waiter - it must instead be serialised by the caller.
         */
        rb_erase(&wait->node, &b->waiters);
        RB_CLEAR_NODE(&wait->node);

        wake_up_process(wait->tsk); /* implicit smp_wmb() */
}

static inline void __intel_breadcrumbs_next(struct intel_engine_cs *engine,
                                            struct rb_node *next)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        spin_lock(&b->irq_lock);
        GEM_BUG_ON(!b->irq_armed);
        GEM_BUG_ON(!b->irq_wait);
        b->irq_wait = to_wait(next);
        spin_unlock(&b->irq_lock);

        /* We always wake up the next waiter that takes over as the bottom-half
         * as we may delegate not only the irq-seqno barrier to the next waiter
         * but also the task of waking up concurrent waiters.
         */
        if (next)
                wake_up_process(to_wait(next)->tsk);
}

static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
                                    struct intel_wait *wait)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        struct rb_node **p, *parent, *completed;
        bool first, armed;
        u32 seqno;

        /* Insert the request into the retirement ordered list
         * of waiters by walking the rbtree. If we are the oldest
         * seqno in the tree (the first to be retired), then
         * set ourselves as the bottom-half.
         *
         * As we descend the tree, prune completed branches since we hold the
         * spinlock we know that the first_waiter must be delayed and can
         * reduce some of the sequential wake up latency if we take action
         * ourselves and wake up the completed tasks in parallel. Also, by
         * removing stale elements in the tree, we may be able to reduce the
         * ping-pong between the old bottom-half and ourselves as first-waiter.
         */
        armed = false;
        first = true;
        parent = NULL;
        completed = NULL;
        seqno = intel_engine_get_seqno(engine);

         /* If the request completed before we managed to grab the spinlock,
          * return now before adding ourselves to the rbtree. We let the
          * current bottom-half handle any pending wakeups and instead
          * try and get out of the way quickly.
          */
        if (i915_seqno_passed(seqno, wait->seqno)) {
                RB_CLEAR_NODE(&wait->node);
                return first;
        }

        p = &b->waiters.rb_node;
        while (*p) {
                parent = *p;
                if (wait->seqno == to_wait(parent)->seqno) {
                        /* We have multiple waiters on the same seqno, select
                         * the highest priority task (that with the smallest
                         * task->prio) to serve as the bottom-half for this
                         * group.
                         */
                        if (wait->tsk->prio > to_wait(parent)->tsk->prio) {
                                p = &parent->rb_right;
                                first = false;
                        } else {
                                p = &parent->rb_left;
                        }
                } else if (i915_seqno_passed(wait->seqno,
                                             to_wait(parent)->seqno)) {
                        p = &parent->rb_right;
                        if (i915_seqno_passed(seqno, to_wait(parent)->seqno))
                                completed = parent;
                        else
                                first = false;
                } else {
                        p = &parent->rb_left;
                }
        }
        rb_link_node(&wait->node, parent, p);
        rb_insert_color(&wait->node, &b->waiters);

        if (first) {
                spin_lock(&b->irq_lock);
                b->irq_wait = wait;
                /* After assigning ourselves as the new bottom-half, we must
                 * perform a cursory check to prevent a missed interrupt.
                 * Either we miss the interrupt whilst programming the hardware,
                 * or if there was a previous waiter (for a later seqno) they
                 * may be woken instead of us (due to the inherent race
                 * in the unlocked read of b->irq_seqno_bh in the irq handler)
                 * and so we miss the wake up.
                 */
                armed = __intel_breadcrumbs_enable_irq(b);
                spin_unlock(&b->irq_lock);
        }

        if (completed) {
                /* Advance the bottom-half (b->irq_wait) before we wake up
                 * the waiters who may scribble over their intel_wait
                 * just as the interrupt handler is dereferencing it via
                 * b->irq_wait.
                 */
                if (!first) {
                        struct rb_node *next = rb_next(completed);
                        GEM_BUG_ON(next == &wait->node);
                        __intel_breadcrumbs_next(engine, next);
                }

                do {
                        struct intel_wait *crumb = to_wait(completed);
                        completed = rb_prev(completed);
                        __intel_breadcrumbs_finish(b, crumb);
                } while (completed);
        }

        GEM_BUG_ON(!b->irq_wait);
        GEM_BUG_ON(!b->irq_armed);
        GEM_BUG_ON(rb_first(&b->waiters) != &b->irq_wait->node);

        return armed;
}

bool intel_engine_add_wait(struct intel_engine_cs *engine,
                           struct intel_wait *wait)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        bool armed;

        spin_lock_irq(&b->rb_lock);
        armed = __intel_engine_add_wait(engine, wait);
        spin_unlock_irq(&b->rb_lock);
        if (armed)
                return armed;

        /* Make the caller recheck if its request has already started. */
        return i915_seqno_passed(intel_engine_get_seqno(engine),
                                 wait->seqno - 1);
}

static inline bool chain_wakeup(struct rb_node *rb, int priority)
{
        return rb && to_wait(rb)->tsk->prio <= priority;
}

static inline int wakeup_priority(struct intel_breadcrumbs *b,
                                  struct task_struct *tsk)
{
        if (tsk == b->signaler)
                return INT_MIN;
        else
                return tsk->prio;
}

static void __intel_engine_remove_wait(struct intel_engine_cs *engine,
                                       struct intel_wait *wait)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        lockdep_assert_held(&b->rb_lock);

        if (RB_EMPTY_NODE(&wait->node))
                goto out;

        if (b->irq_wait == wait) {
                const int priority = wakeup_priority(b, wait->tsk);
                struct rb_node *next;

                /* We are the current bottom-half. Find the next candidate,
                 * the first waiter in the queue on the remaining oldest
                 * request. As multiple seqnos may complete in the time it
                 * takes us to wake up and find the next waiter, we have to
                 * wake up that waiter for it to perform its own coherent
                 * completion check.
                 */
                next = rb_next(&wait->node);
                if (chain_wakeup(next, priority)) {
                        /* If the next waiter is already complete,
                         * wake it up and continue onto the next waiter. So
                         * if have a small herd, they will wake up in parallel
                         * rather than sequentially, which should reduce
                         * the overall latency in waking all the completed
                         * clients.
                         *
                         * However, waking up a chain adds extra latency to
                         * the first_waiter. This is undesirable if that
                         * waiter is a high priority task.
                         */
                        u32 seqno = intel_engine_get_seqno(engine);

                        while (i915_seqno_passed(seqno, to_wait(next)->seqno)) {
                                struct rb_node *n = rb_next(next);

                                __intel_breadcrumbs_finish(b, to_wait(next));
                                next = n;
                                if (!chain_wakeup(next, priority))
                                        break;
                        }
                }

                __intel_breadcrumbs_next(engine, next);
        } else {
                GEM_BUG_ON(rb_first(&b->waiters) == &wait->node);
        }

        GEM_BUG_ON(RB_EMPTY_NODE(&wait->node));
        rb_erase(&wait->node, &b->waiters);
        RB_CLEAR_NODE(&wait->node);

out:
        GEM_BUG_ON(b->irq_wait == wait);
        GEM_BUG_ON(rb_first(&b->waiters) !=
                   (b->irq_wait ? &b->irq_wait->node : NULL));
}

void intel_engine_remove_wait(struct intel_engine_cs *engine,
                              struct intel_wait *wait)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        /* Quick check to see if this waiter was already decoupled from
         * the tree by the bottom-half to avoid contention on the spinlock
         * by the herd.
         */
        if (RB_EMPTY_NODE(&wait->node)) {
                GEM_BUG_ON(READ_ONCE(b->irq_wait) == wait);
                return;
        }

        spin_lock_irq(&b->rb_lock);
        __intel_engine_remove_wait(engine, wait);
        spin_unlock_irq(&b->rb_lock);
}

static bool signal_complete(const struct drm_i915_gem_request *request)
{
        if (!request)
                return false;

        /*
         * Carefully check if the request is complete, giving time for the
         * seqno to be visible or if the GPU hung.
         */
        return __i915_request_irq_complete(request);
}

static struct drm_i915_gem_request *to_signaler(struct rb_node *rb)
{
        return rb_entry(rb, struct drm_i915_gem_request, signaling.node);
}

static void signaler_set_rtpriority(void)
{
         struct sched_param param = { .sched_priority = 1 };

         sched_setscheduler_nocheck(current, SCHED_FIFO, &param);
}

static int intel_breadcrumbs_signaler(void *arg)
{
        struct intel_engine_cs *engine = arg;
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        struct drm_i915_gem_request *request;

        /* Install ourselves with high priority to reduce signalling latency */
        signaler_set_rtpriority();

        do {
                bool do_schedule = true;

                set_current_state(TASK_INTERRUPTIBLE);

                /* We are either woken up by the interrupt bottom-half,
                 * or by a client adding a new signaller. In both cases,
                 * the GPU seqno may have advanced beyond our oldest signal.
                 * If it has, propagate the signal, remove the waiter and
                 * check again with the next oldest signal. Otherwise we
                 * need to wait for a new interrupt from the GPU or for
                 * a new client.
                 */
                rcu_read_lock();
                request = rcu_dereference(b->first_signal);
                if (request)
                        request = i915_gem_request_get_rcu(request);
                rcu_read_unlock();
                if (signal_complete(request)) {
                        if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
                                      &request->fence.flags)) {
                                local_bh_disable();
                                dma_fence_signal(&request->fence);
                                GEM_BUG_ON(!i915_gem_request_completed(request));
                                local_bh_enable(); /* kick start the tasklets */
                        }

                        spin_lock_irq(&b->rb_lock);

                        /* Wake up all other completed waiters and select the
                         * next bottom-half for the next user interrupt.
                         */
                        __intel_engine_remove_wait(engine,
                                                   &request->signaling.wait);

                        /* Find the next oldest signal. Note that as we have
                         * not been holding the lock, another client may
                         * have installed an even older signal than the one
                         * we just completed - so double check we are still
                         * the oldest before picking the next one.
                         */
                        if (request == rcu_access_pointer(b->first_signal)) {
                                struct rb_node *rb =
                                        rb_next(&request->signaling.node);
                                rcu_assign_pointer(b->first_signal,
                                                   rb ? to_signaler(rb) : NULL);
                        }
                        rb_erase(&request->signaling.node, &b->signals);
                        RB_CLEAR_NODE(&request->signaling.node);

                        spin_unlock_irq(&b->rb_lock);

                        i915_gem_request_put(request);

                        /* If the engine is saturated we may be continually
                         * processing completed requests. This angers the
                         * NMI watchdog if we never let anything else
                         * have access to the CPU. Let's pretend to be nice
                         * and relinquish the CPU if we burn through the
                         * entire RT timeslice!
                         */
                        do_schedule = need_resched();
                }

                if (unlikely(do_schedule)) {
                        if (kthread_should_park())
                                kthread_parkme();

                        if (unlikely(kthread_should_stop())) {
                                i915_gem_request_put(request);
                                break;
                        }

                        schedule();
                }
                i915_gem_request_put(request);
        } while (1);
        __set_current_state(TASK_RUNNING);

        return 0;
}

void intel_engine_enable_signaling(struct drm_i915_gem_request *request,
                                   bool wakeup)
{
        struct intel_engine_cs *engine = request->engine;
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        u32 seqno;

        /* Note that we may be called from an interrupt handler on another
         * device (e.g. nouveau signaling a fence completion causing us
         * to submit a request, and so enable signaling). As such,
         * we need to make sure that all other users of b->rb_lock protect
         * against interrupts, i.e. use spin_lock_irqsave.
         */

        /* locked by dma_fence_enable_sw_signaling() (irqsafe fence->lock) */
        GEM_BUG_ON(!irqs_disabled());
        lockdep_assert_held(&request->lock);

        seqno = i915_gem_request_global_seqno(request);
        if (!seqno)
                return;

        request->signaling.wait.tsk = b->signaler;
        request->signaling.wait.request = request;
        request->signaling.wait.seqno = seqno;
        i915_gem_request_get(request);

        spin_lock(&b->rb_lock);

        /* First add ourselves into the list of waiters, but register our
         * bottom-half as the signaller thread. As per usual, only the oldest
         * waiter (not just signaller) is tasked as the bottom-half waking
         * up all completed waiters after the user interrupt.
         *
         * If we are the oldest waiter, enable the irq (after which we
         * must double check that the seqno did not complete).
         */
        wakeup &= __intel_engine_add_wait(engine, &request->signaling.wait);

        if (!__i915_gem_request_completed(request, seqno)) {
                struct rb_node *parent, **p;
                bool first;

                /* Now insert ourselves into the retirement ordered list of
                 * signals on this engine. We track the oldest seqno as that
                 * will be the first signal to complete.
                 */
                parent = NULL;
                first = true;
                p = &b->signals.rb_node;
                while (*p) {
                        parent = *p;
                        if (i915_seqno_passed(seqno,
                                              to_signaler(parent)->signaling.wait.seqno)) {
                                p = &parent->rb_right;
                                first = false;
                        } else {
                                p = &parent->rb_left;
                        }
                }
                rb_link_node(&request->signaling.node, parent, p);
                rb_insert_color(&request->signaling.node, &b->signals);
                if (first)
                        rcu_assign_pointer(b->first_signal, request);
        } else {
                __intel_engine_remove_wait(engine, &request->signaling.wait);
                i915_gem_request_put(request);
                wakeup = false;
        }

        spin_unlock(&b->rb_lock);

        if (wakeup)
                wake_up_process(b->signaler);
}

void intel_engine_cancel_signaling(struct drm_i915_gem_request *request)
{
        struct intel_engine_cs *engine = request->engine;
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        GEM_BUG_ON(!irqs_disabled());
        lockdep_assert_held(&request->lock);
        GEM_BUG_ON(!request->signaling.wait.seqno);

        spin_lock(&b->rb_lock);

        if (!RB_EMPTY_NODE(&request->signaling.node)) {
                if (request == rcu_access_pointer(b->first_signal)) {
                        struct rb_node *rb =
                                rb_next(&request->signaling.node);
                        rcu_assign_pointer(b->first_signal,
                                           rb ? to_signaler(rb) : NULL);
                }
                rb_erase(&request->signaling.node, &b->signals);
                RB_CLEAR_NODE(&request->signaling.node);
                i915_gem_request_put(request);
        }

        __intel_engine_remove_wait(engine, &request->signaling.wait);

        spin_unlock(&b->rb_lock);

        request->signaling.wait.seqno = 0;
}

int intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        struct task_struct *tsk;

        spin_lock_init(&b->rb_lock);
        spin_lock_init(&b->irq_lock);

        timer_setup(&b->fake_irq, intel_breadcrumbs_fake_irq, 0);
        timer_setup(&b->hangcheck, intel_breadcrumbs_hangcheck, 0);

        /* Spawn a thread to provide a common bottom-half for all signals.
         * As this is an asynchronous interface we cannot steal the current
         * task for handling the bottom-half to the user interrupt, therefore
         * we create a thread to do the coherent seqno dance after the
         * interrupt and then signal the waitqueue (via the dma-buf/fence).
         */
        tsk = kthread_run(intel_breadcrumbs_signaler, engine,
                          "i915/signal:%d", engine->id);
        if (IS_ERR(tsk))
                return PTR_ERR(tsk);

        b->signaler = tsk;

        return 0;
}

static void cancel_fake_irq(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        del_timer_sync(&b->hangcheck);
        del_timer_sync(&b->fake_irq);
        clear_bit(engine->id, &engine->i915->gpu_error.missed_irq_rings);
}

void intel_engine_reset_breadcrumbs(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        cancel_fake_irq(engine);
        spin_lock_irq(&b->irq_lock);

        if (b->irq_enabled)
                irq_enable(engine);
        else
                irq_disable(engine);

        /* We set the IRQ_BREADCRUMB bit when we enable the irq presuming the
         * GPU is active and may have already executed the MI_USER_INTERRUPT
         * before the CPU is ready to receive. However, the engine is currently
         * idle (we haven't started it yet), there is no possibility for a
         * missed interrupt as we enabled the irq and so we can clear the
         * immediate wakeup (until a real interrupt arrives for the waiter).
         */
        clear_bit(ENGINE_IRQ_BREADCRUMB, &engine->irq_posted);

        if (b->irq_armed)
                enable_fake_irq(b);

        spin_unlock_irq(&b->irq_lock);
}

void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;

        /* The engines should be idle and all requests accounted for! */
        WARN_ON(READ_ONCE(b->irq_wait));
        WARN_ON(!RB_EMPTY_ROOT(&b->waiters));
        WARN_ON(rcu_access_pointer(b->first_signal));
        WARN_ON(!RB_EMPTY_ROOT(&b->signals));

        if (!IS_ERR_OR_NULL(b->signaler))
                kthread_stop(b->signaler);

        cancel_fake_irq(engine);
}

bool intel_breadcrumbs_busy(struct intel_engine_cs *engine)
{
        struct intel_breadcrumbs *b = &engine->breadcrumbs;
        bool busy = false;

        spin_lock_irq(&b->rb_lock);

        if (b->irq_wait) {
                wake_up_process(b->irq_wait->tsk);
                busy = true;
        }

        if (rcu_access_pointer(b->first_signal)) {
                wake_up_process(b->signaler);
                busy = true;
        }

        spin_unlock_irq(&b->rb_lock);

        return busy;
}

#if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
#include "selftests/intel_breadcrumbs.c"
#endif