drm/i915: context basic create & destroy

Invent an abstraction for a hw context which is passed around through
the core functions. The main bit a hw context holds is the buffer object
which backs the context. The rest of the members are just helper
functions. Specifically the ring member, which could likely go away if
we decide to never implement whatever other hw context support exists.

Of note here is the introduction of the 64k alignment constraint for the
BO. If contexts become heavily used, we should consider tweaking this
down to 4k. Until the contexts are merged and tested a bit though, I
think 64k is a nice start (based on docs).

Since we don't yet switch contexts, there is really not much complexity
here. Creation/destruction works pretty much as one would expect. An idr
is used to generate the context id numbers which are unique per file
descriptor.

v2: add DRM_DEBUG_DRIVERS to distinguish ENOMEM failures (ben)
convert a BUG_ON to WARN_ON, default destruction is still fatal (ben)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>

drivers/gpu/drm/i915/i915_drv.h

@@ -308,6 +308,16 @@ struct i915_hw_ppgtt {
 	dma_addr_t scratch_page_dma_addr;
 };
 
+
+/* This must match up with the value previously used for execbuf2.rsvd1. */
+#define DEFAULT_CONTEXT_ID 0
+struct i915_hw_context {
+	int id;
+	struct drm_i915_file_private *file_priv;
+	struct intel_ring_buffer *ring;
+	struct drm_i915_gem_object *obj;
+};
+
 enum no_fbc_reason {
 	FBC_NO_OUTPUT, /* no outputs enabled to compress */
 	FBC_STOLEN_TOO_SMALL, /* not enough space to hold compressed buffers */
@@ -1032,6 +1042,7 @@ struct drm_i915_file_private {
 		struct spinlock lock;
 		struct list_head request_list;
 	} mm;
+	struct idr context_idr;
 };
 
 #define INTEL_INFO(dev)	(((struct drm_i915_private *) (dev)->dev_private)->info)

drivers/gpu/drm/i915/i915_gem_context.c

@@ -89,6 +89,15 @@
 #include "i915_drm.h"
 #include "i915_drv.h"
 
+/* This is a HW constraint. The value below is the largest known requirement
+ * I've seen in a spec to date, and that was a workaround for a non-shipping
+ * part. It should be safe to decrease this, but it's more future proof as is.
+ */
+#define CONTEXT_ALIGN (64<<10)
+
+static struct i915_hw_context *
+i915_gem_context_get(struct drm_i915_file_private *file_priv, u32 id);
+
 static int get_context_size(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
@@ -111,6 +120,76 @@ static int get_context_size(struct drm_device *dev)
 	return ret;
 }
 
+static void do_destroy(struct i915_hw_context *ctx)
+{
+	struct drm_device *dev = ctx->obj->base.dev;
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	if (ctx->file_priv)
+		idr_remove(&ctx->file_priv->context_idr, ctx->id);
+	else
+		BUG_ON(ctx != dev_priv->ring[RCS].default_context);
+
+	drm_gem_object_unreference(&ctx->obj->base);
+	kfree(ctx);
+}
+
+static int
+create_hw_context(struct drm_device *dev,
+		  struct drm_i915_file_private *file_priv,
+		  struct i915_hw_context **ctx_out)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+	int ret, id;
+
+	*ctx_out = kzalloc(sizeof(struct drm_i915_file_private), GFP_KERNEL);
+	if (*ctx_out == NULL)
+		return -ENOMEM;
+
+	(*ctx_out)->obj = i915_gem_alloc_object(dev,
+						dev_priv->hw_context_size);
+	if ((*ctx_out)->obj == NULL) {
+		kfree(*ctx_out);
+		DRM_DEBUG_DRIVER("Context object allocated failed\n");
+		return -ENOMEM;
+	}
+
+	/* The ring associated with the context object is handled by the normal
+	 * object tracking code. We give an initial ring value simple to pass an
+	 * assertion in the context switch code.
+	 */
+	(*ctx_out)->ring = &dev_priv->ring[RCS];
+
+	/* Default context will never have a file_priv */
+	if (file_priv == NULL)
+		return 0;
+
+	(*ctx_out)->file_priv = file_priv;
+
+again:
+	if (idr_pre_get(&file_priv->context_idr, GFP_KERNEL) == 0) {
+		ret = -ENOMEM;
+		DRM_DEBUG_DRIVER("idr allocation failed\n");
+		goto err_out;
+	}
+
+	ret = idr_get_new_above(&file_priv->context_idr, *ctx_out,
+				DEFAULT_CONTEXT_ID + 1, &id);
+	if (ret == 0)
+		(*ctx_out)->id = id;
+
+	if (ret == -EAGAIN)
+		goto again;
+	else if (ret)
+		goto err_out;
+
+	return 0;
+
+err_out:
+	do_destroy(*ctx_out);
+	return ret;
+}
+
 /**
  * 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
@@ -118,7 +197,30 @@ static int get_context_size(struct drm_device *dev)
  */
 static int create_default_context(struct drm_i915_private *dev_priv)
 {
-	return 0;
+	struct i915_hw_context *ctx;
+	int ret;
+
+	BUG_ON(!mutex_is_locked(&dev_priv->dev->struct_mutex));
+
+	ret = create_hw_context(dev_priv->dev, NULL,
+				&dev_priv->ring[RCS].default_context);
+	if (ret)
+		return ret;
+
+	/* We may need to do things with the shrinker which require us to
+	 * immediately switch back to the default context. This can cause a
+	 * problem as pinning the default context also requires GTT space which
+	 * may not be available. To avoid this we always pin the
+	 * default context.
+	 */
+	ctx = dev_priv->ring[RCS].default_context;
+	ret = i915_gem_object_pin(ctx->obj, CONTEXT_ALIGN, false);
+	if (ret) {
+		do_destroy(ctx);
+		return ret;
+	}
+
+	return ret;
 }
 
 void i915_gem_context_init(struct drm_device *dev)
@@ -130,7 +232,8 @@ void i915_gem_context_init(struct drm_device *dev)
 		return;
 
 	/* If called from reset, or thaw... we've been here already */
-	if (dev_priv->hw_contexts_disabled)
+	if (dev_priv->hw_contexts_disabled ||
+	    dev_priv->ring[RCS].default_context)
 		return;
 
 	ctx_size = get_context_size(dev);
@@ -156,20 +259,55 @@ void i915_gem_context_fini(struct drm_device *dev)
 
 	if (dev_priv->hw_contexts_disabled)
 		return;
+
+	i915_gem_object_unpin(dev_priv->ring[RCS].default_context->obj);
+
+	do_destroy(dev_priv->ring[RCS].default_context);
 }
 
 void i915_gem_context_open(struct drm_device *dev, struct drm_file *file)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	struct drm_i915_file_private *file_priv = file->driver_priv;
 
 	if (dev_priv->hw_contexts_disabled)
 		return;
+
+	idr_init(&file_priv->context_idr);
+}
+
+static int context_idr_cleanup(int id, void *p, void *data)
+{
+	struct drm_file *file = (struct drm_file *)data;
+	struct drm_i915_file_private *file_priv = file->driver_priv;
+	struct i915_hw_context *ctx;
+
+	BUG_ON(id == DEFAULT_CONTEXT_ID);
+	ctx = i915_gem_context_get(file_priv, id);
+	if (WARN_ON(ctx == NULL))
+		return -ENXIO;
+
+	do_destroy(ctx);
+
+	return 0;
 }
 
 void i915_gem_context_close(struct drm_device *dev, struct drm_file *file)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	struct drm_i915_file_private *file_priv = file->driver_priv;
 
 	if (dev_priv->hw_contexts_disabled)
 		return;
+
+	mutex_lock(&dev->struct_mutex);
+	idr_for_each(&file_priv->context_idr, context_idr_cleanup, file);
+	idr_destroy(&file_priv->context_idr);
+	mutex_unlock(&dev->struct_mutex);
+}
+
+static __used 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);
 }

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -116,6 +116,8 @@ struct  intel_ring_buffer {
 
 	wait_queue_head_t irq_queue;
 
+	struct i915_hw_context *default_context;
+
 	void *private;
 };