drm/i915: Only allocate preempt context when required

If we remove some hardcoded assumptions about the preempt context having
a fixed id, reserved from use by normal user contexts, we may only
allocate the i915_gem_context when required. Then the subsequent
decisions on using preemption reduce to having the preempt context
available.

v2: Include an assert that we don't allocate the preempt context twice.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Acked-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207210544.26351-3-chris@chris-wilson.co.ukReviewed-by: Michel Thierry <michel.thierry@intel.com>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -449,12 +449,18 @@ destroy_kernel_context(struct i915_gem_context **ctxp)
 	i915_gem_context_free(ctx);
 }
 
+static bool needs_preempt_context(struct drm_i915_private *i915)
+{
+	return HAS_LOGICAL_RING_PREEMPTION(i915);
+}
+
 int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
 {
 	struct i915_gem_context *ctx;
-	int err;
 
+	/* Reassure ourselves we are only called once */
 	GEM_BUG_ON(dev_priv->kernel_context);
+	GEM_BUG_ON(dev_priv->preempt_context);
 
 	INIT_LIST_HEAD(&dev_priv->contexts.list);
 	INIT_WORK(&dev_priv->contexts.free_work, contexts_free_worker);
@@ -468,8 +474,7 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
 	ctx = i915_gem_context_create_kernel(dev_priv, I915_PRIORITY_MIN);
 	if (IS_ERR(ctx)) {
 		DRM_ERROR("Failed to create default global context\n");
-		err = PTR_ERR(ctx);
-		goto err;
+		return PTR_ERR(ctx);
 	}
 	/*
 	 * For easy recognisablity, we want the kernel context to be 0 and then
@@ -479,23 +484,18 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
 	dev_priv->kernel_context = ctx;
 
 	/* highest priority; preempting task */
-	ctx = i915_gem_context_create_kernel(dev_priv, INT_MAX);
-	if (IS_ERR(ctx)) {
-		DRM_ERROR("Failed to create default preempt context\n");
-		err = PTR_ERR(ctx);
-		goto err_kernel_context;
+	if (needs_preempt_context(dev_priv)) {
+		ctx = i915_gem_context_create_kernel(dev_priv, INT_MAX);
+		if (!IS_ERR(ctx))
+			dev_priv->preempt_context = ctx;
+		else
+			DRM_ERROR("Failed to create preempt context; disabling preemption\n");
 	}
-	dev_priv->preempt_context = ctx;
 
 	DRM_DEBUG_DRIVER("%s context support initialized\n",
 			 dev_priv->engine[RCS]->context_size ? "logical" :
 			 "fake");
 	return 0;
-
-err_kernel_context:
-	destroy_kernel_context(&dev_priv->kernel_context);
-err:
-	return err;
 }
 
 void i915_gem_contexts_lost(struct drm_i915_private *dev_priv)
@@ -521,7 +521,8 @@ void i915_gem_contexts_fini(struct drm_i915_private *i915)
 {
 	lockdep_assert_held(&i915->drm.struct_mutex);
 
-	destroy_kernel_context(&i915->preempt_context);
+	if (i915->preempt_context)
+		destroy_kernel_context(&i915->preempt_context);
 	destroy_kernel_context(&i915->kernel_context);
 
 	/* Must free all deferred contexts (via flush_workqueue) first */

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -631,7 +631,7 @@ int intel_engine_init_common(struct intel_engine_cs *engine)
 	 * Similarly the preempt context must always be available so that
 	 * we can interrupt the engine at any time.
 	 */
-	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915)) {
+	if (engine->i915->preempt_context) {
 		ring = engine->context_pin(engine,
 					   engine->i915->preempt_context);
 		if (IS_ERR(ring)) {
@@ -656,7 +656,7 @@ int intel_engine_init_common(struct intel_engine_cs *engine)
 err_breadcrumbs:
 	intel_engine_fini_breadcrumbs(engine);
 err_unpin_preempt:
-	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915))
+	if (engine->i915->preempt_context)
 		engine->context_unpin(engine, engine->i915->preempt_context);
 err_unpin_kernel:
 	engine->context_unpin(engine, engine->i915->kernel_context);
@@ -686,7 +686,7 @@ void intel_engine_cleanup_common(struct intel_engine_cs *engine)
 	if (engine->default_state)
 		i915_gem_object_put(engine->default_state);
 
-	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915))
+	if (engine->i915->preempt_context)
 		engine->context_unpin(engine, engine->i915->preempt_context);
 	engine->context_unpin(engine, engine->i915->kernel_context);
 }

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -688,7 +688,7 @@ static void guc_dequeue(struct intel_engine_cs *engine)
 		goto unlock;
 
 	if (port_isset(port)) {
-		if (HAS_LOGICAL_RING_PREEMPTION(engine->i915)) {
+		if (engine->i915->preempt_context) {
 			struct guc_preempt_work *preempt_work =
 				&engine->i915->guc.preempt_work[engine->id];
 
@@ -984,17 +984,19 @@ static int guc_clients_create(struct intel_guc *guc)
 	}
 	guc->execbuf_client = client;
 
-	client = guc_client_alloc(dev_priv,
-				  INTEL_INFO(dev_priv)->ring_mask,
-				  GUC_CLIENT_PRIORITY_KMD_HIGH,
-				  dev_priv->preempt_context);
-	if (IS_ERR(client)) {
-		DRM_ERROR("Failed to create GuC client for preemption!\n");
-		guc_client_free(guc->execbuf_client);
-		guc->execbuf_client = NULL;
-		return PTR_ERR(client);
+	if (dev_priv->preempt_context) {
+		client = guc_client_alloc(dev_priv,
+					  INTEL_INFO(dev_priv)->ring_mask,
+					  GUC_CLIENT_PRIORITY_KMD_HIGH,
+					  dev_priv->preempt_context);
+		if (IS_ERR(client)) {
+			DRM_ERROR("Failed to create GuC client for preemption!\n");
+			guc_client_free(guc->execbuf_client);
+			guc->execbuf_client = NULL;
+			return PTR_ERR(client);
+		}
+		guc->preempt_client = client;
 	}
-	guc->preempt_client = client;
 
 	return 0;
 }

drivers/gpu/drm/i915/intel_lrc.c

@@ -161,7 +161,6 @@
 #define EXECLISTS_REQUEST_SIZE 64 /* bytes */
 #define WA_TAIL_DWORDS 2
 #define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS)
-#define PREEMPT_ID 0x1
 
 static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
 					    struct intel_engine_cs *engine);
@@ -448,7 +447,8 @@ static void inject_preempt_context(struct intel_engine_cs *engine)
 		&engine->i915->preempt_context->engine[engine->id];
 	unsigned int n;
 
-	GEM_BUG_ON(engine->i915->preempt_context->hw_id != PREEMPT_ID);
+	GEM_BUG_ON(engine->execlists.preempt_complete_status !=
+		   upper_32_bits(ce->lrc_desc));
 	GEM_BUG_ON(!IS_ALIGNED(ce->ring->size, WA_TAIL_BYTES));
 
 	memset(ce->ring->vaddr + ce->ring->tail, 0, WA_TAIL_BYTES);
@@ -528,7 +528,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK))
 			goto unlock;
 
-		if (HAS_LOGICAL_RING_PREEMPTION(engine->i915) &&
+		if (engine->i915->preempt_context &&
 		    rb_entry(rb, struct i915_priolist, node)->priority >
 		    max(last->priotree.priority, 0)) {
 			/*
@@ -844,7 +844,7 @@ static void execlists_submission_tasklet(unsigned long data)
 			GEM_BUG_ON(status & GEN8_CTX_STATUS_IDLE_ACTIVE);
 
 			if (status & GEN8_CTX_STATUS_COMPLETE &&
-			    buf[2*head + 1] == PREEMPT_ID) {
+			    buf[2*head + 1] == execlists->preempt_complete_status) {
 				GEM_TRACE("%s preempt-idle\n", engine->name);
 
 				execlists_cancel_port_requests(execlists);
@@ -1967,7 +1967,7 @@ static void execlists_set_default_submission(struct intel_engine_cs *engine)
 	engine->i915->caps.scheduler =
 		I915_SCHEDULER_CAP_ENABLED |
 		I915_SCHEDULER_CAP_PRIORITY;
-	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915))
+	if (engine->i915->preempt_context)
 		engine->i915->caps.scheduler |= I915_SCHEDULER_CAP_PREEMPTION;
 }
 
@@ -2045,6 +2045,11 @@ static int logical_ring_init(struct intel_engine_cs *engine)
 	engine->execlists.elsp =
 		engine->i915->regs + i915_mmio_reg_offset(RING_ELSP(engine));
 
+	engine->execlists.preempt_complete_status = ~0u;
+	if (engine->i915->preempt_context)
+		engine->execlists.preempt_complete_status =
+			upper_32_bits(engine->i915->preempt_context->engine[engine->id].lrc_desc);
+
 	return 0;
 
 error:
@@ -2307,7 +2312,7 @@ populate_lr_context(struct i915_gem_context *ctx,
 	if (!engine->default_state)
 		regs[CTX_CONTEXT_CONTROL + 1] |=
 			_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT);
-	if (ctx->hw_id == PREEMPT_ID)
+	if (ctx == ctx->i915->preempt_context)
 		regs[CTX_CONTEXT_CONTROL + 1] |=
 			_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT |
 					   CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT);

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -279,6 +279,11 @@ struct intel_engine_execlists {
 	 * @csb_use_mmio: access csb through mmio, instead of hwsp
 	 */
 	bool csb_use_mmio;
+
+	/**
+	 * @preempt_complete_status: expected CSB upon completing preemption
+	 */
+	u32 preempt_complete_status;
 };
 
 #define INTEL_ENGINE_CS_MAX_NAME 8

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -243,16 +243,10 @@ struct drm_i915_private *mock_gem_device(void)
 	if (!i915->kernel_context)
 		goto err_engine;
 
-	i915->preempt_context = mock_context(i915, NULL);
-	if (!i915->preempt_context)
-		goto err_kernel_context;
-
 	WARN_ON(i915_gemfs_init(i915));
 
 	return i915;
 
-err_kernel_context:
-	i915_gem_context_put(i915->kernel_context);
 err_engine:
 	for_each_engine(engine, i915, id)
 		mock_engine_free(engine);