drm/i915: Assign every HW context a unique ID

The hardware tracks contexts and expects all live contexts (those active
on the hardware) to have a unique identifier. This is used by the
hardware to assign pagefaults and the like to a particular context.

v2: Reorder to make sure ctx->link is not left dangling if the
assignment of a hw_id fails (Mika).

v3: We have 21bits of context space, not 20.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-17-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2001,7 +2001,7 @@ static int i915_context_status(struct seq_file *m, void *unused)
 		    ctx->legacy_hw_ctx.rcs_state == NULL)
 			continue;
 
-		seq_puts(m, "HW context ");
+		seq_printf(m, "HW context %u ", ctx->hw_id);
 		describe_ctx(m, ctx);
 		if (ctx == dev_priv->kernel_context)
 			seq_printf(m, "(kernel context) ");

drivers/gpu/drm/i915/i915_drv.h

@@ -851,6 +851,9 @@ struct intel_context {
 	struct i915_ctx_hang_stats hang_stats;
 	struct i915_hw_ppgtt *ppgtt;
 
+	/* Unique identifier for this context, used by the hw for tracking */
+	unsigned hw_id;
+
 	/* Legacy ring buffer submission */
 	struct {
 		struct drm_i915_gem_object *rcs_state;
@@ -1838,6 +1841,13 @@ struct drm_i915_private {
 	DECLARE_HASHTABLE(mm_structs, 7);
 	struct mutex mm_lock;
 
+	/* The hw wants to have a stable context identifier for the lifetime
+	 * of the context (for OA, PASID, faults, etc). This is limited
+	 * in execlists to 21 bits.
+	 */
+	struct ida context_hw_ida;
+#define MAX_CONTEXT_HW_ID (1<<21) /* exclusive */
+
 	/* Kernel Modesetting */
 
 	struct drm_crtc *plane_to_crtc_mapping[I915_MAX_PIPES];

drivers/gpu/drm/i915/i915_gem_context.c

@@ -171,6 +171,8 @@ void i915_gem_context_free(struct kref *ctx_ref)
 	if (ctx->legacy_hw_ctx.rcs_state)
 		drm_gem_object_unreference(&ctx->legacy_hw_ctx.rcs_state->base);
 	list_del(&ctx->link);
+
+	ida_simple_remove(&ctx->i915->context_hw_ida, ctx->hw_id);
 	kfree(ctx);
 }
 
@@ -211,6 +213,28 @@ i915_gem_alloc_context_obj(struct drm_device *dev, size_t size)
 	return obj;
 }
 
+static int assign_hw_id(struct drm_i915_private *dev_priv, unsigned *out)
+{
+	int ret;
+
+	ret = ida_simple_get(&dev_priv->context_hw_ida,
+			     0, MAX_CONTEXT_HW_ID, GFP_KERNEL);
+	if (ret < 0) {
+		/* Contexts are only released when no longer active.
+		 * Flush any pending retires to hopefully release some
+		 * stale contexts and try again.
+		 */
+		i915_gem_retire_requests(dev_priv->dev);
+		ret = ida_simple_get(&dev_priv->context_hw_ida,
+				     0, MAX_CONTEXT_HW_ID, GFP_KERNEL);
+		if (ret < 0)
+			return ret;
+	}
+
+	*out = ret;
+	return 0;
+}
+
 static struct intel_context *
 __create_hw_context(struct drm_device *dev,
 		    struct drm_i915_file_private *file_priv)
@@ -223,6 +247,12 @@ __create_hw_context(struct drm_device *dev,
 	if (ctx == NULL)
 		return ERR_PTR(-ENOMEM);
 
+	ret = assign_hw_id(dev_priv, &ctx->hw_id);
+	if (ret) {
+		kfree(ctx);
+		return ERR_PTR(ret);
+	}
+
 	kref_init(&ctx->ref);
 	list_add_tail(&ctx->link, &dev_priv->context_list);
 	ctx->i915 = dev_priv;
@@ -366,6 +396,10 @@ int i915_gem_context_init(struct drm_device *dev)
 		}
 	}
 
+	/* Using the simple ida interface, the max is limited by sizeof(int) */
+	BUILD_BUG_ON(MAX_CONTEXT_HW_ID > INT_MAX);
+	ida_init(&dev_priv->context_hw_ida);
+
 	if (i915.enable_execlists) {
 		/* NB: intentionally left blank. We will allocate our own
 		 * backing objects as we need them, thank you very much */
@@ -429,6 +463,8 @@ void i915_gem_context_fini(struct drm_device *dev)
 
 	i915_gem_context_unreference(dctx);
 	dev_priv->kernel_context = NULL;
+
+	ida_destroy(&dev_priv->context_hw_ida);
 }
 
 static int context_idr_cleanup(int id, void *p, void *data)