drm/i915: Record the position of the start of the request

Not only does it make for good documentation and debugging aide, but it is
also vital for when we want to unwind requests - such as when throwing away
an incomplete request.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1470414607-32453-2-git-send-email-arun.siluvery@linux.intel.comReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -557,6 +557,7 @@ struct drm_i915_error_state {
 		struct drm_i915_error_request {
 			long jiffies;
 			u32 seqno;
+			u32 head;
 			u32 tail;
 		} *requests;
 

drivers/gpu/drm/i915/i915_gem_request.c

@@ -426,6 +426,13 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
 	if (ret)
 		goto err_ctx;
 
+	/* Record the position of the start of the request so that
+	 * should we detect the updated seqno part-way through the
+	 * GPU processing the request, we never over-estimate the
+	 * position of the head.
+	 */
+	req->head = req->ring->tail;
+
 	return req;
 
 err_ctx:
@@ -500,8 +507,6 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 
 	trace_i915_gem_request_add(request);
 
-	request->head = request_start;
-
 	/* Seal the request and mark it as pending execution. Note that
 	 * we may inspect this state, without holding any locks, during
 	 * hangcheck. Hence we apply the barrier to ensure that we do not
@@ -514,10 +519,10 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 	list_add_tail(&request->link, &engine->request_list);
 	list_add_tail(&request->ring_link, &ring->request_list);
 
-	/* Record the position of the start of the request so that
+	/* Record the position of the start of the breadcrumb so that
 	 * should we detect the updated seqno part-way through the
 	 * GPU processing the request, we never over-estimate the
-	 * position of the head.
+	 * position of the ring's HEAD.
 	 */
 	request->postfix = ring->tail;
 

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -455,9 +455,10 @@ int i915_error_state_to_str(struct drm_i915_error_state_buf *m,
 				   dev_priv->engine[i].name,
 				   ee->num_requests);
 			for (j = 0; j < ee->num_requests; j++) {
-				err_printf(m, "  seqno 0x%08x, emitted %ld, tail 0x%08x\n",
+				err_printf(m, "  seqno 0x%08x, emitted %ld, head 0x%08x, tail 0x%08x\n",
 					   ee->requests[j].seqno,
 					   ee->requests[j].jiffies,
+					   ee->requests[j].head,
 					   ee->requests[j].tail);
 			}
 		}
@@ -1205,7 +1206,8 @@ static void i915_gem_record_rings(struct drm_i915_private *dev_priv,
 			erq = &ee->requests[count++];
 			erq->seqno = request->fence.seqno;
 			erq->jiffies = request->emitted_jiffies;
-			erq->tail = request->postfix;
+			erq->head = request->head;
+			erq->tail = request->tail;
 		}
 	}
 }