drm/i915: Delay semaphore submission until the start of the signaler

Currently we submit the semaphore busywait as soon as the signaler is
submitted to HW. However, we may submit the signaler as the tail of a
batch of requests, and even not as the first context in the HW list,
i.e. the busywait may start spinning far in advance of the signaler even
starting.

If we wait until the request before the signaler is completed before
submitting the busywait, we prevent the busywait from starting too
early, if the signaler is not first in submission port.

To handle the case where the signaler is at the start of the second (or
later) submission port, we will need to delay the execution callback
until we know the context is promoted to port0. A challenge for later.

Fixes: e88619646971 ("drm/i915: Use HW semaphores for inter-engine synchronisation on gen8+")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190501114541.10077-9-chris@chris-wilson.co.uk
(cherry picked from commit 0d90ccb70211cbf55140e91bd39db684aa4c16e9)
[Joonas: edited Fixes: tag into single line.]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index b836721d3b1304f226e3bd8a3b9a9d65901725be..20c7c77b3768e1bd27d5929c95a7d458045b759f 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -798,6 +798,21 @@ err_unreserve:
        return ERR_PTR(ret);
 }
 
+static int
+i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
+{
+       if (list_is_first(&signal->ring_link, &signal->ring->request_list))
+               return 0;
+
+       signal = list_prev_entry(signal, ring_link);
+       if (i915_timeline_sync_is_later(rq->timeline, &signal->fence))
+               return 0;
+
+       return i915_sw_fence_await_dma_fence(&rq->submit,
+                                            &signal->fence, 0,
+                                            I915_FENCE_GFP);
+}
+
 static int
 emit_semaphore_wait(struct i915_request *to,
                    struct i915_request *from,
@@ -816,6 +831,10 @@ emit_semaphore_wait(struct i915_request *to,
                                                     &from->fence, 0,
                                                     I915_FENCE_GFP);
 
+       err = i915_request_await_start(to, from);
+       if (err < 0)
+               return err;
+
        err = i915_sw_fence_await_dma_fence(&to->semaphore,
                                            &from->fence, 0,
                                            I915_FENCE_GFP);