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 "i915_drv.h"

static void intel_breadcrumbs_fake_irq(unsigned long data)
{
	struct intel_engine_cs *engine = (struct intel_engine_cs *)data;

	/*
	 * The timer persists in case we cannot enable interrupts,
	 * or if we have previously seen seqno/interrupt incoherency
	 * ("missed interrupt" syndrome). Here the worker will wake up
	 * every jiffie in order to kick the oldest waiter to do the
	 * coherent seqno check.
	 */
	rcu_read_lock();
	if (intel_engine_wakeup(engine))
		mod_timer(&engine->breadcrumbs.fake_irq, jiffies + 1);
	rcu_read_unlock();
}

static void irq_enable(struct intel_engine_cs *engine)
{
	WARN_ON(!engine->irq_get(engine));
}

static void irq_disable(struct intel_engine_cs *engine)
{
	engine->irq_put(engine);
}

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 irq_posted = false;

	assert_spin_locked(&b->lock);
	if (b->rpm_wakelock)
		return false;

	/* Since we are waiting on a request, the GPU should be busy
	 * and should have its own rpm reference. For completeness,
	 * record an rpm reference for ourselves to cover the
	 * interrupt we unmask.
	 */
	intel_runtime_pm_get_noresume(i915);
	b->rpm_wakelock = true;

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

	if (!b->irq_enabled ||
	    test_bit(engine->id, &i915->gpu_error.missed_irq_rings))
		mod_timer(&b->fake_irq, jiffies + 1);

	return irq_posted;
}

static void __intel_breadcrumbs_disable_irq(struct intel_breadcrumbs *b)
{
	struct intel_engine_cs *engine =
		container_of(b, struct intel_engine_cs, breadcrumbs);

	assert_spin_locked(&b->lock);
	if (!b->rpm_wakelock)
		return;

	if (b->irq_enabled) {
		irq_disable(engine);
		b->irq_enabled = false;
	}

	intel_runtime_pm_put(engine->i915);
	b->rpm_wakelock = false;
}

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

static inline void __intel_breadcrumbs_finish(struct intel_breadcrumbs *b,
					      struct intel_wait *wait)
{
	assert_spin_locked(&b->lock);

	/* This request is completed, so remove it from the tree, mark it as
	 * complete, and *then* wake up the associated task.
	 */
	rb_erase(&wait->node, &b->waiters);
	RB_CLEAR_NODE(&wait->node);

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

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;
	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.
	 */
	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);
	GEM_BUG_ON(!first && !b->tasklet);

	if (completed) {
		struct rb_node *next = rb_next(completed);

		GEM_BUG_ON(!next && !first);
		if (next && next != &wait->node) {
			GEM_BUG_ON(first);
			b->first_wait = to_wait(next);
			smp_store_mb(b->tasklet, b->first_wait->tsk);
			/* As there is a delay between reading the current
			 * seqno, processing the completed tasks and selecting
			 * the next waiter, we may have missed the interrupt
			 * and so need for the next bottom-half to wakeup.
			 *
			 * Also as we enable the IRQ, we may miss the
			 * interrupt for that seqno, so we have to wake up
			 * the next bottom-half in order to do a coherent check
			 * in case the seqno passed.
			 */
			__intel_breadcrumbs_enable_irq(b);
			wake_up_process(to_wait(next)->tsk);
		}

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

	if (first) {
		GEM_BUG_ON(rb_first(&b->waiters) != &wait->node);
		b->first_wait = wait;
		smp_store_mb(b->tasklet, wait->tsk);
		first = __intel_breadcrumbs_enable_irq(b);
	}
	GEM_BUG_ON(!b->tasklet);
	GEM_BUG_ON(!b->first_wait);
	GEM_BUG_ON(rb_first(&b->waiters) != &b->first_wait->node);

	return first;
}

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

	spin_lock(&b->lock);
	first = __intel_engine_add_wait(engine, wait);
	spin_unlock(&b->lock);

	return first;
}

void intel_engine_enable_fake_irq(struct intel_engine_cs *engine)
{
	mod_timer(&engine->breadcrumbs.fake_irq, jiffies + 1);
}

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

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))
		return;

	spin_lock(&b->lock);

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

	if (b->first_wait == wait) {
		struct rb_node *next;
		const int priority = wait->tsk->prio;

		GEM_BUG_ON(b->tasklet != wait->tsk);

		/* 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;
			}
		}

		if (next) {
			/* In our haste, we may have completed the first waiter
			 * before we enabled the interrupt. Do so now as we
			 * have a second waiter for a future seqno. Afterwards,
			 * we have to wake up that waiter in case we missed
			 * the interrupt, or if we have to handle an
			 * exception rather than a seqno completion.
			 */
			b->first_wait = to_wait(next);
			smp_store_mb(b->tasklet, b->first_wait->tsk);
			if (b->first_wait->seqno != wait->seqno)
				__intel_breadcrumbs_enable_irq(b);
			wake_up_process(b->tasklet);
		} else {
			b->first_wait = NULL;
			WRITE_ONCE(b->tasklet, NULL);
			__intel_breadcrumbs_disable_irq(b);
		}
	} else {
		GEM_BUG_ON(rb_first(&b->waiters) == &wait->node);
	}

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

out_unlock:
	GEM_BUG_ON(b->first_wait == wait);
	GEM_BUG_ON(rb_first(&b->waiters) !=
		   (b->first_wait ? &b->first_wait->node : NULL));
	GEM_BUG_ON(!b->tasklet ^ RB_EMPTY_ROOT(&b->waiters));
	spin_unlock(&b->lock);
}

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

	spin_lock_init(&b->lock);
	setup_timer(&b->fake_irq,
		    intel_breadcrumbs_fake_irq,
		    (unsigned long)engine);

	return 0;
}

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

	del_timer_sync(&b->fake_irq);
}

unsigned int intel_kick_waiters(struct drm_i915_private *i915)
{
	struct intel_engine_cs *engine;
	unsigned int mask = 0;

	/* To avoid the task_struct disappearing beneath us as we wake up
	 * the process, we must first inspect the task_struct->state under the
	 * RCU lock, i.e. as we call wake_up_process() we must be holding the
	 * rcu_read_lock().
	 */
	rcu_read_lock();
	for_each_engine(engine, i915)
		if (unlikely(intel_engine_wakeup(engine)))
			mask |= intel_engine_flag(engine);
	rcu_read_unlock();

	return mask;
}