drm/i915: Stop automatically retiring requests after a GPU hang

Following a GPU hang, we break out of the request loop in order to
unlock the struct_mutex for use by the GPU reset. However, if we retire
all the requests at that moment, we cannot identify the guilty request
after performing the reset.

v2: Not automatically retiring requests forces us to recheck for
available ringspace.

Fixes: f4457ae7 ("drm/i915: Prevent leaking of -EIO from i915_wait_request()")
Testcase: igt/gem_reset_stats/ban-*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Tested-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463137042-9669-4-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -1462,7 +1462,10 @@ i915_wait_request(struct drm_i915_gem_request *req)
 	if (ret)
 		return ret;
 
-	__i915_gem_request_retire__upto(req);
+	/* If the GPU hung, we want to keep the requests to find the guilty. */
+	if (req->reset_counter == i915_reset_counter(&dev_priv->gpu_error))
+		__i915_gem_request_retire__upto(req);
+
 	return 0;
 }
 
@@ -1519,7 +1522,8 @@ i915_gem_object_retire_request(struct drm_i915_gem_object *obj,
 	else if (obj->last_write_req == req)
 		i915_gem_object_retire__write(obj);
 
-	__i915_gem_request_retire__upto(req);
+	if (req->reset_counter == i915_reset_counter(&req->i915->gpu_error))
+		__i915_gem_request_retire__upto(req);
 }
 
 /* A nonblocking variant of the above wait. This is a highly dangerous routine

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2450,6 +2450,8 @@ int intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
 			return ret;
 
 		intel_ring_update_space(ringbuf);
+		if (unlikely(ringbuf->space < wait_bytes))
+			return -EAGAIN;
 	}
 
 	if (unlikely(need_wrap)) {