[Intel-gfx] [PATCH 11/12] drm/i915: annotate intel_atomic_commit_fence_wait

[Daniel Vetter]

In i915 we also have i915_sw_fence, as some kind of super fence. Since
those include dma-fences (it's the main use really, aside from
chaining them for the scheduler) they're relevant for any lockdep
cycles involving dma_fence.

But i915_sw_fence is also really tough to properly annonate:
- Most of the use (all of it in the scheduler) is entirely driven by
  callbacks. That creates dependency chains, but for all practical
  purposes we could treat that as part of the hw magic that executes
  requests for us. Eventually a dma_fence_signal() comes out of that,
  which we do annotate.

- For dma-fences we only annotate cpu waits, and don't follow any of
  the chains going through the hardware. Since dma_fence are supposed
  to be ordered an always complete that should be good enough, as long
  as we don't accidentally deadlock on the cpu side. So treating
  i915_sw_fence as part of the hw magic shouldn't reduce our deadlock
  coverage, as long as i915_sw_fence itself doesn't deadlock. And
  there's lots of debug checks for that already.

- There's one exception: intel_atomic_commit_fence_wait() is the only
  cpu wait on a i915_sw_fence we have. That one we should annotate,
  and annotate should be able to teach lockdep about our gpu reset vs
  modeset locking inversion. But in reality this is a dma-fence wait,
  and it using i915_sw_fence is really just an implementation detail.
  All other kms drivers use drm_atomic_helper_wait_for_fences()
  instead.

- There is a i915_sw_fence_wait for normal cpu waits (the one in the
  modeset code is open-coded because it also waits for gpu reset
  wakups), but that's only used for selftests.

So the simplest solution for annotating i915_sw_fence is therefore to
annotate intel_atomic_commit_fence() as a dma_fence wait.

Cc: Jani Nikula <jani.nikula at linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
Cc: intel-gfx at lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 0aabc4b9854a..a2eee76da46b 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12728,6 +12728,7 @@ static void intel_atomic_commit_fence_wait(struct intel_atomic_state *intel_stat
 	struct wait_queue_entry wait_fence, wait_reset;
 	struct drm_i915_private *dev_priv = to_i915(intel_state->base.dev);
 
+	dma_fence_wait_acquire();
 	init_wait_entry(&wait_fence, 0);
 	init_wait_entry(&wait_reset, 0);
 	for (;;) {
@@ -12745,6 +12746,7 @@ static void intel_atomic_commit_fence_wait(struct intel_atomic_state *intel_stat
 	}
 	finish_wait(&intel_state->commit_ready.wait, &wait_fence);
 	finish_wait(&dev_priv->gpu_error.wait_queue, &wait_reset);
+	dma_fence_wait_release();
 }
 
 static void intel_atomic_cleanup_work(struct work_struct *work)
-- 
2.19.1