[Intel-gfx] [PATCH 8/9] drm/i915: Track the previous pinned context inside the request

[Chris Wilson]

As the contexts are accessed by the hardware until the switch is completed
to a new context, the hardware may still be writing to the context object
after the breadcrumb is visible. We must not unpin/unbind/prune that
object whilst still active and so we keep the previous context pinned until
the following request. If we move this tracking onto the request, we can
simplify the code and enable freeing of the request without the
struct_mutex in subsequent patches.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem_request.c |  8 ++++----
 drivers/gpu/drm/i915/i915_gem_request.h | 11 +++++++++++
 drivers/gpu/drm/i915/intel_lrc.c        | 12 +++++-------
 3 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index 33aacf1725dd..8d7c415f1896 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -643,12 +643,12 @@ void i915_gem_request_free(struct kref *req_ref)
 	if (req->file_priv)
 		i915_gem_request_remove_from_client(req);
 
-	if (ctx) {
+	if (req->pinned_context) {
 		if (i915.enable_execlists)
-			intel_lr_context_unpin(ctx, req->engine);
-
-		i915_gem_context_unreference(ctx);
+			intel_lr_context_unpin(req->pinned_context,
+					       req->engine);
 	}
 
+	i915_gem_context_unreference(ctx);
 	kmem_cache_free(to_i915(req)->requests, req);
 }
diff --git a/drivers/gpu/drm/i915/i915_gem_request.h b/drivers/gpu/drm/i915/i915_gem_request.h
index 69a4d4e2c97b..389813cbc19a 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.h
+++ b/drivers/gpu/drm/i915/i915_gem_request.h
@@ -85,6 +85,17 @@ struct drm_i915_gem_request {
 	struct intel_context *ctx;
 	struct intel_ringbuffer *ringbuf;
 
+	/**
+	 * Context related to the previous request.
+	 * As the contexts are accessed by the hardware until the switch is
+	 * completed to a new context, the hardware may still be writing
+	 * to the context object after the breadcrumb is visible. We must
+	 * not unpin/unbind/prune that object whilst still active and so
+	 * we keep the previous context pinned until the following (this)
+	 * request is retired.
+	 */
+	struct intel_context *pinned_context;
+
 	/** Batch buffer related to this request if any (used for
 	    error state dump only) */
 	struct drm_i915_gem_object *batch_obj;
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index b0d20af38574..0e55f206e592 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -708,6 +708,7 @@ int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request
 		request->ctx->engine[engine->id].initialised = true;
 	}
 
+	request->pinned_context = request->ctx;
 	return 0;
 }
 
@@ -782,12 +783,8 @@ intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
 	intel_logical_ring_emit(ringbuf, MI_NOOP);
 	intel_logical_ring_advance(ringbuf);
 
-	if (engine->last_context != request->ctx) {
-		if (engine->last_context)
-			intel_lr_context_unpin(engine->last_context, engine);
-		intel_lr_context_pin(request->ctx, engine);
-		engine->last_context = request->ctx;
-	}
+	request->pinned_context = engine->last_context;
+	engine->last_context = request->ctx;
 
 	if (dev_priv->guc.execbuf_client)
 		i915_guc_submit(dev_priv->guc.execbuf_client, request);
@@ -1009,7 +1006,8 @@ void intel_execlists_retire_requests(struct intel_engine_cs *engine)
 	spin_unlock_bh(&engine->execlist_lock);
 
 	list_for_each_entry_safe(req, tmp, &retired_list, execlist_link) {
-		intel_lr_context_unpin(req->ctx, engine);
+		if (req->pinned_context)
+			intel_lr_context_unpin(req->pinned_context, engine);
 
 		list_del(&req->execlist_link);
 		i915_gem_request_unreference(req);
-- 
2.8.0.rc3