[RFC 12/17] drm/amdgpu/dc: Stop dma_resv_lock inversion in commit_tail

[Daniel Vetter <daniel.vetter@ffwll.ch>]

Trying to grab dma_resv_lock while in commit_tail before we've done
all the code that leads to the eventual signalling of the vblank event
(which can be a dma_fence) is deadlock-y. Don't do that.

Here the solution is easy because just grabbing locks to read
something races anyway. We don't need to bother, READ_ONCE is
equivalent. And avoids the locking issue.

Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: linux-rdma@vger.kernel.org
Cc: amd-gfx@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 9bfaa4cad483..28e1af9f823c 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -6699,7 +6699,11 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 		 * explicitly on fences instead
 		 * and in general should be called for
 		 * blocking commit to as per framework helpers
+		 *
+		 * Yes, this deadlocks, since you're calling dma_resv_lock in a
+		 * path that leads to a dma_fence_signal(). Don't do that.
 		 */
+#if 0
 		r = amdgpu_bo_reserve(abo, true);
 		if (unlikely(r != 0))
 			DRM_ERROR("failed to reserve buffer before flip\n");
@@ -6709,6 +6713,12 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 		tmz_surface = amdgpu_bo_encrypted(abo);
 
 		amdgpu_bo_unreserve(abo);
+#endif
+		/*
+		 * this races anyway, so READ_ONCE isn't any better or worse
+		 * than the stuff above. Except the stuff above can deadlock.
+		 */
+		tiling_flags = READ_ONCE(abo->tiling_flags);
 
 		fill_dc_plane_info_and_addr(
 			dm->adev, new_plane_state, tiling_flags,
-- 
2.26.2

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