/*
 * SPDX-License-Identifier: MIT
 *
 * Copyright © 2019 Intel Corporation
 */

#include <linux/debugobjects.h>

#include "gt/intel_context.h"
#include "gt/intel_engine_heartbeat.h"
#include "gt/intel_engine_pm.h"
#include "gt/intel_ring.h"

#include "i915_drv.h"
#include "i915_active.h"
#include "i915_globals.h"

/*
 * Active refs memory management
 *
 * To be more economical with memory, we reap all the i915_active trees as
 * they idle (when we know the active requests are inactive) and allocate the
 * nodes from a local slab cache to hopefully reduce the fragmentation.
 */
static struct i915_global_active {
        struct i915_global base;
        struct kmem_cache *slab_cache;
} global;

struct active_node {
        struct rb_node node;
        struct i915_active_fence base;
        struct i915_active *ref;
        u64 timeline;
};

#define fetch_node(x) rb_entry(READ_ONCE(x), typeof(struct active_node), node)

static inline struct active_node *
node_from_active(struct i915_active_fence *active)
{
        return container_of(active, struct active_node, base);
}

#define take_preallocated_barriers(x) llist_del_all(&(x)->preallocated_barriers)

static inline bool is_barrier(const struct i915_active_fence *active)
{
        return IS_ERR(rcu_access_pointer(active->fence));
}

static inline struct llist_node *barrier_to_ll(struct active_node *node)
{
        GEM_BUG_ON(!is_barrier(&node->base));
        return (struct llist_node *)&node->base.cb.node;
}

static inline struct intel_engine_cs *
__barrier_to_engine(struct active_node *node)
{
        return (struct intel_engine_cs *)READ_ONCE(node->base.cb.node.prev);
}

static inline struct intel_engine_cs *
barrier_to_engine(struct active_node *node)
{
        GEM_BUG_ON(!is_barrier(&node->base));
        return __barrier_to_engine(node);
}

static inline struct active_node *barrier_from_ll(struct llist_node *x)
{
        return container_of((struct list_head *)x,
                            struct active_node, base.cb.node);
}

#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM) && IS_ENABLED(CONFIG_DEBUG_OBJECTS)

static void *active_debug_hint(void *addr)
{
        struct i915_active *ref = addr;

        return (void *)ref->active ?: (void *)ref->retire ?: (void *)ref;
}

static const struct debug_obj_descr active_debug_desc = {
        .name = "i915_active",
        .debug_hint = active_debug_hint,
};

static void debug_active_init(struct i915_active *ref)
{
        debug_object_init(ref, &active_debug_desc);
}

static void debug_active_activate(struct i915_active *ref)
{
        lockdep_assert_held(&ref->tree_lock);
        if (!atomic_read(&ref->count)) /* before the first inc */
                debug_object_activate(ref, &active_debug_desc);
}

static void debug_active_deactivate(struct i915_active *ref)
{
        lockdep_assert_held(&ref->tree_lock);
        if (!atomic_read(&ref->count)) /* after the last dec */
                debug_object_deactivate(ref, &active_debug_desc);
}

static void debug_active_fini(struct i915_active *ref)
{
        debug_object_free(ref, &active_debug_desc);
}

static void debug_active_assert(struct i915_active *ref)
{
        debug_object_assert_init(ref, &active_debug_desc);
}

#else

static inline void debug_active_init(struct i915_active *ref) { }
static inline void debug_active_activate(struct i915_active *ref) { }
static inline void debug_active_deactivate(struct i915_active *ref) { }
static inline void debug_active_fini(struct i915_active *ref) { }
static inline void debug_active_assert(struct i915_active *ref) { }

#endif

static void
__active_retire(struct i915_active *ref)
{
        struct rb_root root = RB_ROOT;
        struct active_node *it, *n;
        unsigned long flags;

        GEM_BUG_ON(i915_active_is_idle(ref));

        /* return the unused nodes to our slabcache -- flushing the allocator */
        if (!atomic_dec_and_lock_irqsave(&ref->count, &ref->tree_lock, flags))
                return;

        GEM_BUG_ON(rcu_access_pointer(ref->excl.fence));
        debug_active_deactivate(ref);

        /* Even if we have not used the cache, we may still have a barrier */
        if (!ref->cache)
                ref->cache = fetch_node(ref->tree.rb_node);

        /* Keep the MRU cached node for reuse */
        if (ref->cache) {
                /* Discard all other nodes in the tree */
                rb_erase(&ref->cache->node, &ref->tree);
                root = ref->tree;

                /* Rebuild the tree with only the cached node */
                rb_link_node(&ref->cache->node, NULL, &ref->tree.rb_node);
                rb_insert_color(&ref->cache->node, &ref->tree);
                GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);

                /* Make the cached node available for reuse with any timeline */
                ref->cache->timeline = 0; /* needs cmpxchg(u64) */
        }

        spin_unlock_irqrestore(&ref->tree_lock, flags);

        /* After the final retire, the entire struct may be freed */
        if (ref->retire)
                ref->retire(ref);

        /* ... except if you wait on it, you must manage your own references! */
        wake_up_var(ref);

        /* Finally free the discarded timeline tree  */
        rbtree_postorder_for_each_entry_safe(it, n, &root, node) {
                GEM_BUG_ON(i915_active_fence_isset(&it->base));
                kmem_cache_free(global.slab_cache, it);
        }
}

static void
active_work(struct work_struct *wrk)
{
        struct i915_active *ref = container_of(wrk, typeof(*ref), work);

        GEM_BUG_ON(!atomic_read(&ref->count));
        if (atomic_add_unless(&ref->count, -1, 1))
                return;

        __active_retire(ref);
}

static void
active_retire(struct i915_active *ref)
{
        GEM_BUG_ON(!atomic_read(&ref->count));
        if (atomic_add_unless(&ref->count, -1, 1))
                return;

        if (ref->flags & I915_ACTIVE_RETIRE_SLEEPS) {
                queue_work(system_unbound_wq, &ref->work);
                return;
        }

        __active_retire(ref);
}

static inline struct dma_fence **
__active_fence_slot(struct i915_active_fence *active)
{
        return (struct dma_fence ** __force)&active->fence;
}

static inline bool
active_fence_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
{
        struct i915_active_fence *active =
                container_of(cb, typeof(*active), cb);

        return cmpxchg(__active_fence_slot(active), fence, NULL) == fence;
}

static void
node_retire(struct dma_fence *fence, struct dma_fence_cb *cb)
{
        if (active_fence_cb(fence, cb))
                active_retire(container_of(cb, struct active_node, base.cb)->ref);
}

static void
excl_retire(struct dma_fence *fence, struct dma_fence_cb *cb)
{
        if (active_fence_cb(fence, cb))
                active_retire(container_of(cb, struct i915_active, excl.cb));
}

static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
{
        struct active_node *it;

        GEM_BUG_ON(idx == 0); /* 0 is the unordered timeline, rsvd for cache */

        /*
         * We track the most recently used timeline to skip a rbtree search
         * for the common case, under typical loads we never need the rbtree
         * at all. We can reuse the last slot if it is empty, that is
         * after the previous activity has been retired, or if it matches the
         * current timeline.
         */
        it = READ_ONCE(ref->cache);
        if (it) {
                u64 cached = READ_ONCE(it->timeline);

                /* Once claimed, this slot will only belong to this idx */
                if (cached == idx)
                        return it;

                /*
                 * An unclaimed cache [.timeline=0] can only be claimed once.
                 *
                 * If the value is already non-zero, some other thread has
                 * claimed the cache and we know that is does not match our
                 * idx. If, and only if, the timeline is currently zero is it
                 * worth competing to claim it atomically for ourselves (for
                 * only the winner of that race will cmpxchg return the old
                 * value of 0).
                 */
                if (!cached && !cmpxchg64(&it->timeline, 0, idx))
                        return it;
        }

        BUILD_BUG_ON(offsetof(typeof(*it), node));

        /* While active, the tree can only be built; not destroyed */
        GEM_BUG_ON(i915_active_is_idle(ref));

        it = fetch_node(ref->tree.rb_node);
        while (it) {
                if (it->timeline < idx) {
                        it = fetch_node(it->node.rb_right);
                } else if (it->timeline > idx) {
                        it = fetch_node(it->node.rb_left);
                } else {
                        WRITE_ONCE(ref->cache, it);
                        break;
                }
        }

        /* NB: If the tree rotated beneath us, we may miss our target. */
        return it;
}

static struct i915_active_fence *
active_instance(struct i915_active *ref, u64 idx)
{
        struct active_node *node, *prealloc;
        struct rb_node **p, *parent;

        node = __active_lookup(ref, idx);
        if (likely(node))
                return &node->base;

        /* Preallocate a replacement, just in case */
        prealloc = kmem_cache_alloc(global.slab_cache, GFP_KERNEL);
        if (!prealloc)
                return NULL;

        spin_lock_irq(&ref->tree_lock);
        GEM_BUG_ON(i915_active_is_idle(ref));

        parent = NULL;
        p = &ref->tree.rb_node;
        while (*p) {
                parent = *p;

                node = rb_entry(parent, struct active_node, node);
                if (node->timeline == idx) {
                        kmem_cache_free(global.slab_cache, prealloc);
                        goto out;
                }

                if (node->timeline < idx)
                        p = &parent->rb_right;
                else
                        p = &parent->rb_left;
        }

        node = prealloc;
        __i915_active_fence_init(&node->base, NULL, node_retire);
        node->ref = ref;
        node->timeline = idx;

        rb_link_node(&node->node, parent, p);
        rb_insert_color(&node->node, &ref->tree);

out:
        WRITE_ONCE(ref->cache, node);
        spin_unlock_irq(&ref->tree_lock);

        return &node->base;
}

void __i915_active_init(struct i915_active *ref,
                        int (*active)(struct i915_active *ref),
                        void (*retire)(struct i915_active *ref),
                        struct lock_class_key *mkey,
                        struct lock_class_key *wkey)
{
        unsigned long bits;

        debug_active_init(ref);

        ref->flags = 0;
        ref->active = active;
        ref->retire = ptr_unpack_bits(retire, &bits, 2);
        if (bits & I915_ACTIVE_MAY_SLEEP)
                ref->flags |= I915_ACTIVE_RETIRE_SLEEPS;

        spin_lock_init(&ref->tree_lock);
        ref->tree = RB_ROOT;
        ref->cache = NULL;

        init_llist_head(&ref->preallocated_barriers);
        atomic_set(&ref->count, 0);
        __mutex_init(&ref->mutex, "i915_active", mkey);
        __i915_active_fence_init(&ref->excl, NULL, excl_retire);
        INIT_WORK(&ref->work, active_work);
#if IS_ENABLED(CONFIG_LOCKDEP)
        lockdep_init_map(&ref->work.lockdep_map, "i915_active.work", wkey, 0);
#endif
}

static bool ____active_del_barrier(struct i915_active *ref,
                                   struct active_node *node,
                                   struct intel_engine_cs *engine)

{
        struct llist_node *head = NULL, *tail = NULL;
        struct llist_node *pos, *next;

        GEM_BUG_ON(node->timeline != engine->kernel_context->timeline->fence_context);

        /*
         * Rebuild the llist excluding our node. We may perform this
         * outside of the kernel_context timeline mutex and so someone
         * else may be manipulating the engine->barrier_tasks, in
         * which case either we or they will be upset :)
         *
         * A second __active_del_barrier() will report failure to claim
         * the active_node and the caller will just shrug and know not to
         * claim ownership of its node.
         *
         * A concurrent i915_request_add_active_barriers() will miss adding
         * any of the tasks, but we will try again on the next -- and since
         * we are actively using the barrier, we know that there will be
         * at least another opportunity when we idle.
         */
        llist_for_each_safe(pos, next, llist_del_all(&engine->barrier_tasks)) {
                if (node == barrier_from_ll(pos)) {
                        node = NULL;
                        continue;
                }

                pos->next = head;
                head = pos;
                if (!tail)
                        tail = pos;
        }
        if (head)
                llist_add_batch(head, tail, &engine->barrier_tasks);

        return !node;
}

static bool
__active_del_barrier(struct i915_active *ref, struct active_node *node)
{
        return ____active_del_barrier(ref, node, barrier_to_engine(node));
}

static bool
replace_barrier(struct i915_active *ref, struct i915_active_fence *active)
{
        if (!is_barrier(active)) /* proto-node used by our idle barrier? */
                return false;

        /*
         * This request is on the kernel_context timeline, and so
         * we can use it to substitute for the pending idle-barrer
         * request that we want to emit on the kernel_context.
         */
        __active_del_barrier(ref, node_from_active(active));
        return true;
}

int i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence)
{
        struct i915_active_fence *active;
        int err;

        /* Prevent reaping in case we malloc/wait while building the tree */
        err = i915_active_acquire(ref);
        if (err)
                return err;

        active = active_instance(ref, idx);
        if (!active) {
                err = -ENOMEM;
                goto out;
        }

        if (replace_barrier(ref, active)) {
                RCU_INIT_POINTER(active->fence, NULL);
                atomic_dec(&ref->count);
        }
        if (!__i915_active_fence_set(active, fence))
                __i915_active_acquire(ref);

out:
        i915_active_release(ref);
        return err;
}

static struct dma_fence *
__i915_active_set_fence(struct i915_active *ref,
                        struct i915_active_fence *active,
                        struct dma_fence *fence)
{
        struct dma_fence *prev;

        if (replace_barrier(ref, active)) {
                RCU_INIT_POINTER(active->fence, fence);
                return NULL;
        }

        rcu_read_lock();
        prev = __i915_active_fence_set(active, fence);
        if (prev)
                prev = dma_fence_get_rcu(prev);
        else
                __i915_active_acquire(ref);
        rcu_read_unlock();

        return prev;
}

static struct i915_active_fence *
__active_fence(struct i915_active *ref, u64 idx)
{
        struct active_node *it;

        it = __active_lookup(ref, idx);
        if (unlikely(!it)) { /* Contention with parallel tree builders! */
                spin_lock_irq(&ref->tree_lock);
                it = __active_lookup(ref, idx);
                spin_unlock_irq(&ref->tree_lock);
        }
        GEM_BUG_ON(!it); /* slot must be preallocated */

        return &it->base;
}

struct dma_fence *
__i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence)
{
        /* Only valid while active, see i915_active_acquire_for_context() */
        return __i915_active_set_fence(ref, __active_fence(ref, idx), fence);
}

struct dma_fence *
i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
{
        /* We expect the caller to manage the exclusive timeline ordering */
        return __i915_active_set_fence(ref, &ref->excl, f);
}

bool i915_active_acquire_if_busy(struct i915_active *ref)
{
        debug_active_assert(ref);
        return atomic_add_unless(&ref->count, 1, 0);
}

static void __i915_active_activate(struct i915_active *ref)
{
        spin_lock_irq(&ref->tree_lock); /* __active_retire() */
        if (!atomic_fetch_inc(&ref->count))
                debug_active_activate(ref);
        spin_unlock_irq(&ref->tree_lock);
}

int i915_active_acquire(struct i915_active *ref)
{
        int err;

        if (i915_active_acquire_if_busy(ref))
                return 0;

        if (!ref->active) {
                __i915_active_activate(ref);
                return 0;
        }

        err = mutex_lock_interruptible(&ref->mutex);
        if (err)
                return err;

        if (likely(!i915_active_acquire_if_busy(ref))) {
                err = ref->active(ref);
                if (!err)
                        __i915_active_activate(ref);
        }

        mutex_unlock(&ref->mutex);

        return err;
}

int i915_active_acquire_for_context(struct i915_active *ref, u64 idx)
{
        struct i915_active_fence *active;
        int err;

        err = i915_active_acquire(ref);
        if (err)
                return err;

        active = active_instance(ref, idx);
        if (!active) {
                i915_active_release(ref);
                return -ENOMEM;
        }

        return 0; /* return with active ref */
}

void i915_active_release(struct i915_active *ref)
{
        debug_active_assert(ref);
        active_retire(ref);
}

static void enable_signaling(struct i915_active_fence *active)
{
        struct dma_fence *fence;

        if (unlikely(is_barrier(active)))
                return;

        fence = i915_active_fence_get(active);
        if (!fence)
                return;

        dma_fence_enable_sw_signaling(fence);
        dma_fence_put(fence);
}

static int flush_barrier(struct active_node *it)
{
        struct intel_engine_cs *engine;

        if (likely(!is_barrier(&it->base)))
                return 0;

        engine = __barrier_to_engine(it);
        smp_rmb(); /* serialise with add_active_barriers */
        if (!is_barrier(&it->base))
                return 0;

        return intel_engine_flush_barriers(engine);
}

static int flush_lazy_signals(struct i915_active *ref)
{
        struct active_node *it, *n;
        int err = 0;

        enable_signaling(&ref->excl);
        rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
                err = flush_barrier(it); /* unconnected idle barrier? */
                if (err)
                        break;

                enable_signaling(&it->base);
        }

        return err;
}

int __i915_active_wait(struct i915_active *ref, int state)
{
        might_sleep();

        /* Any fence added after the wait begins will not be auto-signaled */
        if (i915_active_acquire_if_busy(ref)) {
                int err;

                err = flush_lazy_signals(ref);
                i915_active_release(ref);
                if (err)
                        return err;

                if (___wait_var_event(ref, i915_active_is_idle(ref),
                                      state, 0, 0, schedule()))
                        return -EINTR;
        }

        /*
         * After the wait is complete, the caller may free the active.
         * We have to flush any concurrent retirement before returning.
         */
        flush_work(&ref->work);
        return 0;
}

static int __await_active(struct i915_active_fence *active,
                          int (*fn)(void *arg, struct dma_fence *fence),
                          void *arg)
{
        struct dma_fence *fence;

        if (is_barrier(active)) /* XXX flush the barrier? */
                return 0;

        fence = i915_active_fence_get(active);
        if (fence) {
                int err;

                err = fn(arg, fence);
                dma_fence_put(fence);
                if (err < 0)
                        return err;
        }

        return 0;
}

struct wait_barrier {
        struct wait_queue_entry base;
        struct i915_active *ref;
};

static int
barrier_wake(wait_queue_entry_t *wq, unsigned int mode, int flags, void *key)
{
        struct wait_barrier *wb = container_of(wq, typeof(*wb), base);

        if (i915_active_is_idle(wb->ref)) {
                list_del(&wq->entry);
                i915_sw_fence_complete(wq->private);
                kfree(wq);
        }

        return 0;
}

static int __await_barrier(struct i915_active *ref, struct i915_sw_fence *fence)
{
        struct wait_barrier *wb;

        wb = kmalloc(sizeof(*wb), GFP_KERNEL);
        if (unlikely(!wb))
                return -ENOMEM;

        GEM_BUG_ON(i915_active_is_idle(ref));
        if (!i915_sw_fence_await(fence)) {
                kfree(wb);
                return -EINVAL;
        }

        wb->base.flags = 0;
        wb->base.func = barrier_wake;
        wb->base.private = fence;
        wb->ref = ref;

        add_wait_queue(__var_waitqueue(ref), &wb->base);
        return 0;
}

static int await_active(struct i915_active *ref,
                        unsigned int flags,
                        int (*fn)(void *arg, struct dma_fence *fence),
                        void *arg, struct i915_sw_fence *barrier)
{
        int err = 0;

        if (!i915_active_acquire_if_busy(ref))
                return 0;

        if (flags & I915_ACTIVE_AWAIT_EXCL &&
            rcu_access_pointer(ref->excl.fence)) {
                err = __await_active(&ref->excl, fn, arg);
                if (err)
                        goto out;
        }

        if (flags & I915_ACTIVE_AWAIT_ACTIVE) {
                struct active_node *it, *n;

                rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
                        err = __await_active(&it->base, fn, arg);
                        if (err)
                                goto out;
                }
        }

        if (flags & I915_ACTIVE_AWAIT_BARRIER) {
                err = flush_lazy_signals(ref);
                if (err)
                        goto out;

                err = __await_barrier(ref, barrier);
                if (err)
                        goto out;
        }

out:
        i915_active_release(ref);
        return err;
}

static int rq_await_fence(void *arg, struct dma_fence *fence)
{
        return i915_request_await_dma_fence(arg, fence);
}

int i915_request_await_active(struct i915_request *rq,
                              struct i915_active *ref,
                              unsigned int flags)
{
        return await_active(ref, flags, rq_await_fence, rq, &rq->submit);
}

static int sw_await_fence(void *arg, struct dma_fence *fence)
{
        return i915_sw_fence_await_dma_fence(arg, fence, 0,
                                             GFP_NOWAIT | __GFP_NOWARN);
}

int i915_sw_fence_await_active(struct i915_sw_fence *fence,
                               struct i915_active *ref,
                               unsigned int flags)
{
        return await_active(ref, flags, sw_await_fence, fence, fence);
}

void i915_active_fini(struct i915_active *ref)
{
        debug_active_fini(ref);
        GEM_BUG_ON(atomic_read(&ref->count));
        GEM_BUG_ON(work_pending(&ref->work));
        mutex_destroy(&ref->mutex);

        if (ref->cache)
                kmem_cache_free(global.slab_cache, ref->cache);
}

static inline bool is_idle_barrier(struct active_node *node, u64 idx)
{
        return node->timeline == idx && !i915_active_fence_isset(&node->base);
}

static struct active_node *reuse_idle_barrier(struct i915_active *ref, u64 idx)
{
        struct rb_node *prev, *p;

        if (RB_EMPTY_ROOT(&ref->tree))
                return NULL;

        GEM_BUG_ON(i915_active_is_idle(ref));

        /*
         * Try to reuse any existing barrier nodes already allocated for this
         * i915_active, due to overlapping active phases there is likely a
         * node kept alive (as we reuse before parking). We prefer to reuse
         * completely idle barriers (less hassle in manipulating the llists),
         * but otherwise any will do.
         */
        if (ref->cache && is_idle_barrier(ref->cache, idx)) {
                p = &ref->cache->node;
                goto match;
        }

        prev = NULL;
        p = ref->tree.rb_node;
        while (p) {
                struct active_node *node =
                        rb_entry(p, struct active_node, node);

                if (is_idle_barrier(node, idx))
                        goto match;

                prev = p;
                if (node->timeline < idx)
                        p = READ_ONCE(p->rb_right);
                else
                        p = READ_ONCE(p->rb_left);
        }

        /*
         * No quick match, but we did find the leftmost rb_node for the
         * kernel_context. Walk the rb_tree in-order to see if there were
         * any idle-barriers on this timeline that we missed, or just use
         * the first pending barrier.
         */
        for (p = prev; p; p = rb_next(p)) {
                struct active_node *node =
                        rb_entry(p, struct active_node, node);
                struct intel_engine_cs *engine;

                if (node->timeline > idx)
                        break;

                if (node->timeline < idx)
                        continue;

                if (is_idle_barrier(node, idx))
                        goto match;

                /*
                 * The list of pending barriers is protected by the
                 * kernel_context timeline, which notably we do not hold
                 * here. i915_request_add_active_barriers() may consume
                 * the barrier before we claim it, so we have to check
                 * for success.
                 */
                engine = __barrier_to_engine(node);
                smp_rmb(); /* serialise with add_active_barriers */
                if (is_barrier(&node->base) &&
                    ____active_del_barrier(ref, node, engine))
                        goto match;
        }

        return NULL;

match:
        spin_lock_irq(&ref->tree_lock);
        rb_erase(p, &ref->tree); /* Hide from waits and sibling allocations */
        if (p == &ref->cache->node)
                WRITE_ONCE(ref->cache, NULL);
        spin_unlock_irq(&ref->tree_lock);

        return rb_entry(p, struct active_node, node);
}

int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
                                            struct intel_engine_cs *engine)
{
        intel_engine_mask_t tmp, mask = engine->mask;
        struct llist_node *first = NULL, *last = NULL;
        struct intel_gt *gt = engine->gt;

        GEM_BUG_ON(i915_active_is_idle(ref));

        /* Wait until the previous preallocation is completed */
        while (!llist_empty(&ref->preallocated_barriers))
                cond_resched();

        /*
         * Preallocate a node for each physical engine supporting the target
         * engine (remember virtual engines have more than one sibling).
         * We can then use the preallocated nodes in
         * i915_active_acquire_barrier()
         */
        GEM_BUG_ON(!mask);
        for_each_engine_masked(engine, gt, mask, tmp) {
                u64 idx = engine->kernel_context->timeline->fence_context;
                struct llist_node *prev = first;
                struct active_node *node;

                rcu_read_lock();
                node = reuse_idle_barrier(ref, idx);
                rcu_read_unlock();
                if (!node) {
                        node = kmem_cache_alloc(global.slab_cache, GFP_KERNEL);
                        if (!node)
                                goto unwind;

                        RCU_INIT_POINTER(node->base.fence, NULL);
                        node->base.cb.func = node_retire;
                        node->timeline = idx;
                        node->ref = ref;
                }

                if (!i915_active_fence_isset(&node->base)) {
                        /*
                         * Mark this as being *our* unconnected proto-node.
                         *
                         * Since this node is not in any list, and we have
                         * decoupled it from the rbtree, we can reuse the
                         * request to indicate this is an idle-barrier node
                         * and then we can use the rb_node and list pointers
                         * for our tracking of the pending barrier.
                         */
                        RCU_INIT_POINTER(node->base.fence, ERR_PTR(-EAGAIN));
                        node->base.cb.node.prev = (void *)engine;
                        __i915_active_acquire(ref);
                }
                GEM_BUG_ON(rcu_access_pointer(node->base.fence) != ERR_PTR(-EAGAIN));

                GEM_BUG_ON(barrier_to_engine(node) != engine);
                first = barrier_to_ll(node);
                first->next = prev;
                if (!last)
                        last = first;
                intel_engine_pm_get(engine);
        }

        GEM_BUG_ON(!llist_empty(&ref->preallocated_barriers));
        llist_add_batch(first, last, &ref->preallocated_barriers);

        return 0;

unwind:
        while (first) {
                struct active_node *node = barrier_from_ll(first);

                first = first->next;

                atomic_dec(&ref->count);
                intel_engine_pm_put(barrier_to_engine(node));

                kmem_cache_free(global.slab_cache, node);
        }
        return -ENOMEM;
}

void i915_active_acquire_barrier(struct i915_active *ref)
{
        struct llist_node *pos, *next;
        unsigned long flags;

        GEM_BUG_ON(i915_active_is_idle(ref));

        /*
         * Transfer the list of preallocated barriers into the
         * i915_active rbtree, but only as proto-nodes. They will be
         * populated by i915_request_add_active_barriers() to point to the
         * request that will eventually release them.
         */
        llist_for_each_safe(pos, next, take_preallocated_barriers(ref)) {
                struct active_node *node = barrier_from_ll(pos);
                struct intel_engine_cs *engine = barrier_to_engine(node);
                struct rb_node **p, *parent;

                spin_lock_irqsave_nested(&ref->tree_lock, flags,
                                         SINGLE_DEPTH_NESTING);
                parent = NULL;
                p = &ref->tree.rb_node;
                while (*p) {
                        struct active_node *it;

                        parent = *p;

                        it = rb_entry(parent, struct active_node, node);
                        if (it->timeline < node->timeline)
                                p = &parent->rb_right;
                        else
                                p = &parent->rb_left;
                }
                rb_link_node(&node->node, parent, p);

                rb_insert_color(&node->node, &ref->tree);
                spin_unlock_irqrestore(&ref->tree_lock, flags);

                GEM_BUG_ON(!intel_engine_pm_is_awake(engine));
                llist_add(barrier_to_ll(node), &engine->barrier_tasks);
                intel_engine_pm_put_delay(engine, 1);
        }
}

static struct dma_fence **ll_to_fence_slot(struct llist_node *node)
{
        return __active_fence_slot(&barrier_from_ll(node)->base);
}

void i915_request_add_active_barriers(struct i915_request *rq)
{
        struct intel_engine_cs *engine = rq->engine;
        struct llist_node *node, *next;
        unsigned long flags;

        GEM_BUG_ON(!intel_context_is_barrier(rq->context));
        GEM_BUG_ON(intel_engine_is_virtual(engine));
        GEM_BUG_ON(i915_request_timeline(rq) != engine->kernel_context->timeline);

        node = llist_del_all(&engine->barrier_tasks);
        if (!node)
                return;
        /*
         * Attach the list of proto-fences to the in-flight request such
         * that the parent i915_active will be released when this request
         * is retired.
         */
        spin_lock_irqsave(&rq->lock, flags);
        llist_for_each_safe(node, next, node) {
                /* serialise with reuse_idle_barrier */
                smp_store_mb(*ll_to_fence_slot(node), &rq->fence);
                list_add_tail((struct list_head *)node, &rq->fence.cb_list);
        }
        spin_unlock_irqrestore(&rq->lock, flags);
}

/*
 * __i915_active_fence_set: Update the last active fence along its timeline
 * @active: the active tracker
 * @fence: the new fence (under construction)
 *
 * Records the new @fence as the last active fence along its timeline in
 * this active tracker, moving the tracking callbacks from the previous
 * fence onto this one. Returns the previous fence (if not already completed),
 * which the caller must ensure is executed before the new fence. To ensure
 * that the order of fences within the timeline of the i915_active_fence is
 * understood, it should be locked by the caller.
 */
struct dma_fence *
__i915_active_fence_set(struct i915_active_fence *active,
                        struct dma_fence *fence)
{
        struct dma_fence *prev;
        unsigned long flags;

        if (fence == rcu_access_pointer(active->fence))
                return fence;

        GEM_BUG_ON(test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags));

        /*
         * Consider that we have two threads arriving (A and B), with
         * C already resident as the active->fence.
         *
         * A does the xchg first, and so it sees C or NULL depending
         * on the timing of the interrupt handler. If it is NULL, the
         * previous fence must have been signaled and we know that
         * we are first on the timeline. If it is still present,
         * we acquire the lock on that fence and serialise with the interrupt
         * handler, in the process removing it from any future interrupt
         * callback. A will then wait on C before executing (if present).
         *
         * As B is second, it sees A as the previous fence and so waits for
         * it to complete its transition and takes over the occupancy for
         * itself -- remembering that it needs to wait on A before executing.
         *
         * Note the strong ordering of the timeline also provides consistent
         * nesting rules for the fence->lock; the inner lock is always the
         * older lock.
         */
        spin_lock_irqsave(fence->lock, flags);
        prev = xchg(__active_fence_slot(active), fence);
        if (prev) {
                GEM_BUG_ON(prev == fence);
                spin_lock_nested(prev->lock, SINGLE_DEPTH_NESTING);
                __list_del_entry(&active->cb.node);
                spin_unlock(prev->lock); /* serialise with prev->cb_list */
        }
        list_add_tail(&active->cb.node, &fence->cb_list);
        spin_unlock_irqrestore(fence->lock, flags);

        return prev;
}

int i915_active_fence_set(struct i915_active_fence *active,
                          struct i915_request *rq)
{
        struct dma_fence *fence;
        int err = 0;

        /* Must maintain timeline ordering wrt previous active requests */
        rcu_read_lock();
        fence = __i915_active_fence_set(active, &rq->fence);
        if (fence) /* but the previous fence may not belong to that timeline! */
                fence = dma_fence_get_rcu(fence);
        rcu_read_unlock();
        if (fence) {
                err = i915_request_await_dma_fence(rq, fence);
                dma_fence_put(fence);
        }

        return err;
}

void i915_active_noop(struct dma_fence *fence, struct dma_fence_cb *cb)
{
        active_fence_cb(fence, cb);
}

struct auto_active {
        struct i915_active base;
        struct kref ref;
};

struct i915_active *i915_active_get(struct i915_active *ref)
{
        struct auto_active *aa = container_of(ref, typeof(*aa), base);

        kref_get(&aa->ref);
        return &aa->base;
}

static void auto_release(struct kref *ref)
{
        struct auto_active *aa = container_of(ref, typeof(*aa), ref);

        i915_active_fini(&aa->base);
        kfree(aa);
}

void i915_active_put(struct i915_active *ref)
{
        struct auto_active *aa = container_of(ref, typeof(*aa), base);

        kref_put(&aa->ref, auto_release);
}

static int auto_active(struct i915_active *ref)
{
        i915_active_get(ref);
        return 0;
}

__i915_active_call static void
auto_retire(struct i915_active *ref)
{
        i915_active_put(ref);
}

struct i915_active *i915_active_create(void)
{
        struct auto_active *aa;

        aa = kmalloc(sizeof(*aa), GFP_KERNEL);
        if (!aa)
                return NULL;

        kref_init(&aa->ref);
        i915_active_init(&aa->base, auto_active, auto_retire);

        return &aa->base;
}

#if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
#include "selftests/i915_active.c"
#endif

static void i915_global_active_shrink(void)
{
        kmem_cache_shrink(global.slab_cache);
}

static void i915_global_active_exit(void)
{
        kmem_cache_destroy(global.slab_cache);
}

static struct i915_global_active global = { {
        .shrink = i915_global_active_shrink,
        .exit = i915_global_active_exit,
} };

int __init i915_global_active_init(void)
{
        global.slab_cache = KMEM_CACHE(active_node, SLAB_HWCACHE_ALIGN);
        if (!global.slab_cache)
                return -ENOMEM;

        i915_global_register(&global.base);
        return 0;
}