drm/i915/selftests: Exercise timeslice rewinding

Originally, I did not expect having to rewind a context upon
timeslicing: the point was to replace the executing context with a
non-executing one! However, given a second context that depends on
requests from the first, we may have to split the requests along the
first context to execute the second, causing us to partially replay the
first context and so have to rewind its RING_TAIL.

References: 5ba32c7b ("drm/i915/execlists: Always force a context reload when rewinding RING_TAIL")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200213140150.3639027-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/selftest_lrc.c

@@ -76,8 +76,11 @@ static int wait_for_submit(struct intel_engine_cs *engine,
 	do {
 		cond_resched();
 		intel_engine_flush_submission(engine);
-		if (i915_request_is_active(rq))
+		if (i915_request_is_active(rq) &&
+		    !READ_ONCE(engine->execlists.pending[0])) {
+			tasklet_unlock_wait(&engine->execlists.tasklet);
 			return 0;
+		}
 	} while (time_before(jiffies, timeout));
 
 	return -ETIME;
@@ -857,6 +860,215 @@ static int live_timeslice_preempt(void *arg)
 	return err;
 }
 
+static struct i915_request *
+create_rewinder(struct intel_context *ce,
+		struct i915_request *wait,
+		void *slot, int idx)
+{
+	const u32 offset =
+		i915_ggtt_offset(ce->engine->status_page.vma) +
+		offset_in_page(slot);
+	struct i915_request *rq;
+	u32 *cs;
+	int err;
+
+	rq = intel_context_create_request(ce);
+	if (IS_ERR(rq))
+		return rq;
+
+	if (wait) {
+		err = i915_request_await_dma_fence(rq, &wait->fence);
+		if (err)
+			goto err;
+	}
+
+	cs = intel_ring_begin(rq, 10);
+	if (IS_ERR(cs)) {
+		err = PTR_ERR(cs);
+		goto err;
+	}
+
+	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
+	*cs++ = MI_NOOP;
+
+	*cs++ = MI_SEMAPHORE_WAIT |
+		MI_SEMAPHORE_GLOBAL_GTT |
+		MI_SEMAPHORE_POLL |
+		MI_SEMAPHORE_SAD_NEQ_SDD;
+	*cs++ = 0;
+	*cs++ = offset;
+	*cs++ = 0;
+
+	*cs++ = MI_STORE_REGISTER_MEM_GEN8 | MI_USE_GGTT;
+	*cs++ = i915_mmio_reg_offset(RING_TIMESTAMP(rq->engine->mmio_base));
+	*cs++ = offset + idx * sizeof(u32);
+	*cs++ = 0;
+
+	intel_ring_advance(rq, cs);
+
+	rq->sched.attr.priority = I915_PRIORITY_MASK;
+	err = 0;
+err:
+	i915_request_get(rq);
+	i915_request_add(rq);
+	if (err) {
+		i915_request_put(rq);
+		return ERR_PTR(err);
+	}
+
+	return rq;
+}
+
+static int live_timeslice_rewind(void *arg)
+{
+	struct intel_gt *gt = arg;
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+
+	/*
+	 * The usual presumption on timeslice expiration is that we replace
+	 * the active context with another. However, given a chain of
+	 * dependencies we may end up with replacing the context with itself,
+	 * but only a few of those requests, forcing us to rewind the
+	 * RING_TAIL of the original request.
+	 */
+	if (!IS_ACTIVE(CONFIG_DRM_I915_TIMESLICE_DURATION))
+		return 0;
+
+	for_each_engine(engine, gt, id) {
+		enum { A1, A2, B1 };
+		enum { X = 1, Y, Z };
+		struct i915_request *rq[3] = {};
+		struct intel_context *ce;
+		unsigned long heartbeat;
+		unsigned long timeslice;
+		int i, err = 0;
+		u32 *slot;
+
+		if (!intel_engine_has_timeslices(engine))
+			continue;
+
+		/*
+		 * A:rq1 -- semaphore wait, timestamp X
+		 * A:rq2 -- write timestamp Y
+		 *
+		 * B:rq1 [await A:rq1] -- write timestamp Z
+		 *
+		 * Force timeslice, release semaphore.
+		 *
+		 * Expect execution/evaluation order XZY
+		 */
+
+		engine_heartbeat_disable(engine, &heartbeat);
+		timeslice = xchg(&engine->props.timeslice_duration_ms, 1);
+
+		slot = memset32(engine->status_page.addr + 1000, 0, 4);
+
+		ce = intel_context_create(engine);
+		if (IS_ERR(ce)) {
+			err = PTR_ERR(ce);
+			goto err;
+		}
+
+		rq[0] = create_rewinder(ce, NULL, slot, 1);
+		if (IS_ERR(rq[0])) {
+			intel_context_put(ce);
+			goto err;
+		}
+
+		rq[1] = create_rewinder(ce, NULL, slot, 2);
+		intel_context_put(ce);
+		if (IS_ERR(rq[1]))
+			goto err;
+
+		err = wait_for_submit(engine, rq[1], HZ / 2);
+		if (err) {
+			pr_err("%s: failed to submit first context\n",
+			       engine->name);
+			goto err;
+		}
+
+		ce = intel_context_create(engine);
+		if (IS_ERR(ce)) {
+			err = PTR_ERR(ce);
+			goto err;
+		}
+
+		rq[2] = create_rewinder(ce, rq[0], slot, 3);
+		intel_context_put(ce);
+		if (IS_ERR(rq[2]))
+			goto err;
+
+		err = wait_for_submit(engine, rq[2], HZ / 2);
+		if (err) {
+			pr_err("%s: failed to submit second context\n",
+			       engine->name);
+			goto err;
+		}
+		GEM_BUG_ON(!timer_pending(&engine->execlists.timer));
+
+		/* ELSP[] = { { A:rq1, A:rq2 }, { B:rq1 } } */
+		GEM_BUG_ON(!i915_request_is_active(rq[A1]));
+		GEM_BUG_ON(!i915_request_is_active(rq[A2]));
+		GEM_BUG_ON(!i915_request_is_active(rq[B1]));
+
+		/* Wait for the timeslice to kick in */
+		del_timer(&engine->execlists.timer);
+		tasklet_hi_schedule(&engine->execlists.tasklet);
+		intel_engine_flush_submission(engine);
+
+		/* -> ELSP[] = { { A:rq1 }, { B:rq1 } } */
+		GEM_BUG_ON(!i915_request_is_active(rq[A1]));
+		GEM_BUG_ON(!i915_request_is_active(rq[B1]));
+		GEM_BUG_ON(i915_request_is_active(rq[A2]));
+
+		/* Release the hounds! */
+		slot[0] = 1;
+		wmb(); /* "pairs" with GPU; paranoid kick of internal CPU$ */
+
+		for (i = 1; i <= 3; i++) {
+			unsigned long timeout = jiffies + HZ / 2;
+
+			while (!READ_ONCE(slot[i]) &&
+			       time_before(jiffies, timeout))
+				;
+
+			if (!time_before(jiffies, timeout)) {
+				pr_err("%s: rq[%d] timed out\n",
+				       engine->name, i - 1);
+				err = -ETIME;
+				goto err;
+			}
+
+			pr_debug("%s: slot[%d]:%x\n", engine->name, i, slot[i]);
+		}
+
+		/* XZY: XZ < XY */
+		if (slot[Z] - slot[X] >= slot[Y] - slot[X]) {
+			pr_err("%s: timeslicing did not run context B [%u] before A [%u]!\n",
+			       engine->name,
+			       slot[Z] - slot[X],
+			       slot[Y] - slot[X]);
+			err = -EINVAL;
+		}
+
+err:
+		memset32(&slot[0], -1, 4);
+		wmb();
+
+		engine->props.timeslice_duration_ms = timeslice;
+		engine_heartbeat_enable(engine, heartbeat);
+		for (i = 0; i < 3; i++)
+			i915_request_put(rq[i]);
+		if (igt_flush_test(gt->i915))
+			err = -EIO;
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 static struct i915_request *nop_request(struct intel_engine_cs *engine)
 {
 	struct i915_request *rq;
@@ -3711,6 +3923,7 @@ int intel_execlists_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(live_hold_reset),
 		SUBTEST(live_error_interrupt),
 		SUBTEST(live_timeslice_preempt),
+		SUBTEST(live_timeslice_rewind),
 		SUBTEST(live_timeslice_queue),
 		SUBTEST(live_busywait_preempt),
 		SUBTEST(live_preempt),