drm/i915: context switch implementation

Implement the context switch code as well as the interfaces to do the
context switch. This patch also doesn't match 1:1 with the RFC patches.
The main difference is that from Daniel's responses the last context
object is now stored instead of the last context. This aids in allows us
to free the context data structure, and context object independently.

There is room for optimization: this code will pin the context object
until the next context is active. The optimal way to do it is to
actually pin the object, move it to the active list, do the context
switch, and then unpin it. This allows the eviction code to actually
evict the context object if needed.

The context switch code is missing workarounds, they will be implemented
in future patches.

v2: actually do obj->dirty=1 in switch (daniel)
Modified comment around above
Remove flags to context switch (daniel)
Move mi_set_context code to i915_gem_context.c (daniel)
Remove seqno , use lazy request instead (daniel)

v3: use i915_gem_request_next_seqno instead of
      outstanding_lazy_request (Daniel)
remove id's from trace events (Daniel)
Put the context BO in the instruction domain (Daniel)
Don't unref the BO is context switch fails (Chris)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>

drivers/gpu/drm/i915/i915_drv.h

@@ -313,6 +313,7 @@ struct i915_hw_ppgtt {
 #define DEFAULT_CONTEXT_ID 0
 struct i915_hw_context {
 	int id;
+	bool is_initialized;
 	struct drm_i915_file_private *file_priv;
 	struct intel_ring_buffer *ring;
 	struct drm_i915_gem_object *obj;
@@ -1382,6 +1383,8 @@ void i915_gem_context_init(struct drm_device *dev);
 void i915_gem_context_fini(struct drm_device *dev);
 void i915_gem_context_open(struct drm_device *dev, struct drm_file *file);
 void i915_gem_context_close(struct drm_device *dev, struct drm_file *file);
+int i915_switch_context(struct intel_ring_buffer *ring,
+			struct drm_file *file, int to_id);
 
 /* i915_gem_gtt.c */
 int __must_check i915_gem_init_aliasing_ppgtt(struct drm_device *dev);

drivers/gpu/drm/i915/i915_gem_context.c

@@ -190,6 +190,11 @@ create_hw_context(struct drm_device *dev,
 	return ret;
 }
 
+static inline bool is_default_context(struct i915_hw_context *ctx)
+{
+	return (ctx == ctx->ring->default_context);
+}
+
 /**
  * The default context needs to exist per ring that uses contexts. It stores the
  * context state of the GPU for applications that don't utilize HW contexts, as
@@ -306,8 +311,147 @@ void i915_gem_context_close(struct drm_device *dev, struct drm_file *file)
 	mutex_unlock(&dev->struct_mutex);
 }
 
-static __used struct i915_hw_context *
+static struct i915_hw_context *
 i915_gem_context_get(struct drm_i915_file_private *file_priv, u32 id)
 {
 	return (struct i915_hw_context *)idr_find(&file_priv->context_idr, id);
 }
+
+static inline int
+mi_set_context(struct intel_ring_buffer *ring,
+	       struct i915_hw_context *new_context,
+	       u32 hw_flags)
+{
+	int ret;
+
+	ret = intel_ring_begin(ring, 4);
+	if (ret)
+		return ret;
+
+	intel_ring_emit(ring, MI_NOOP);
+	intel_ring_emit(ring, MI_SET_CONTEXT);
+	intel_ring_emit(ring, new_context->obj->gtt_offset |
+			MI_MM_SPACE_GTT |
+			MI_SAVE_EXT_STATE_EN |
+			MI_RESTORE_EXT_STATE_EN |
+			hw_flags);
+	/* w/a: MI_SET_CONTEXT must always be followed by MI_NOOP */
+	intel_ring_emit(ring, MI_NOOP);
+
+	intel_ring_advance(ring);
+
+	return ret;
+}
+
+static int do_switch(struct drm_i915_gem_object *from_obj,
+		     struct i915_hw_context *to,
+		     u32 seqno)
+{
+	struct intel_ring_buffer *ring = NULL;
+	u32 hw_flags = 0;
+	int ret;
+
+	BUG_ON(to == NULL);
+	BUG_ON(from_obj != NULL && from_obj->pin_count == 0);
+
+	ret = i915_gem_object_pin(to->obj, CONTEXT_ALIGN, false);
+	if (ret)
+		return ret;
+
+	if (!to->is_initialized || is_default_context(to))
+		hw_flags |= MI_RESTORE_INHIBIT;
+	else if (WARN_ON_ONCE(from_obj == to->obj)) /* not yet expected */
+		hw_flags |= MI_FORCE_RESTORE;
+
+	ring = to->ring;
+	ret = mi_set_context(ring, to, hw_flags);
+	if (ret) {
+		i915_gem_object_unpin(to->obj);
+		return ret;
+	}
+
+	/* The backing object for the context is done after switching to the
+	 * *next* context. Therefore we cannot retire the previous context until
+	 * the next context has already started running. In fact, the below code
+	 * is a bit suboptimal because the retiring can occur simply after the
+	 * MI_SET_CONTEXT instead of when the next seqno has completed.
+	 */
+	if (from_obj != NULL) {
+		from_obj->base.read_domains = I915_GEM_DOMAIN_INSTRUCTION;
+		i915_gem_object_move_to_active(from_obj, ring, seqno);
+		/* As long as MI_SET_CONTEXT is serializing, ie. it flushes the
+		 * whole damn pipeline, we don't need to explicitly mark the
+		 * object dirty. The only exception is that the context must be
+		 * correct in case the object gets swapped out. Ideally we'd be
+		 * able to defer doing this until we know the object would be
+		 * swapped, but there is no way to do that yet.
+		 */
+		from_obj->dirty = 1;
+		BUG_ON(from_obj->ring != to->ring);
+		i915_gem_object_unpin(from_obj);
+	}
+
+	ring->last_context_obj = to->obj;
+	to->is_initialized = true;
+
+	return 0;
+}
+
+/**
+ * i915_switch_context() - perform a GPU context switch.
+ * @ring: ring for which we'll execute the context switch
+ * @file_priv: file_priv associated with the context, may be NULL
+ * @id: context id number
+ * @seqno: sequence number by which the new context will be switched to
+ * @flags:
+ *
+ * The context life cycle is simple. The context refcount is incremented and
+ * decremented by 1 and create and destroy. If the context is in use by the GPU,
+ * it will have a refoucnt > 1. This allows us to destroy the context abstract
+ * object while letting the normal object tracking destroy the backing BO.
+ */
+int i915_switch_context(struct intel_ring_buffer *ring,
+			struct drm_file *file,
+			int to_id)
+{
+	struct drm_i915_private *dev_priv = ring->dev->dev_private;
+	struct drm_i915_file_private *file_priv = NULL;
+	struct i915_hw_context *to;
+	struct drm_i915_gem_object *from_obj = ring->last_context_obj;
+	int ret;
+
+	if (dev_priv->hw_contexts_disabled)
+		return 0;
+
+	if (ring != &dev_priv->ring[RCS])
+		return 0;
+
+	if (file)
+		file_priv = file->driver_priv;
+
+	if (to_id == DEFAULT_CONTEXT_ID) {
+		to = ring->default_context;
+	} else {
+		to = i915_gem_context_get(file_priv, to_id);
+		if (to == NULL)
+			return -EINVAL;
+	}
+
+	if (from_obj == to->obj)
+		return 0;
+
+	ret = do_switch(from_obj, to, i915_gem_next_request_seqno(to->ring));
+	if (ret)
+		return ret;
+
+	/* Just to make the code a little cleaner we take the object reference
+	 * after the switch was successful. It would be more intuitive to ref
+	 * the 'to' object before the switch but we know the refcount must be >0
+	 * if context_get() succeeded, and we hold struct mutex. So it's safe to
+	 * do this here/now
+	 */
+	drm_gem_object_reference(&to->obj->base);
+	if (from_obj != NULL)
+		drm_gem_object_unreference(&from_obj->base);
+	return ret;
+}

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -117,6 +117,7 @@ struct  intel_ring_buffer {
 	wait_queue_head_t irq_queue;
 
 	struct i915_hw_context *default_context;
+	struct drm_i915_gem_object *last_context_obj;
 
 	void *private;
 };