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* [PATCH] drm/amdgpu/dc: Stop dma_resv_lock inversion in commit_tail
@ 2020-07-27 21:30 Daniel Vetter
  2020-07-28  6:56 ` Christian König
  0 siblings, 1 reply; 3+ messages in thread
From: Daniel Vetter @ 2020-07-27 21:30 UTC (permalink / raw)
  To: DRI Development
  Cc: Daniel Vetter, linux-media, linaro-mm-sig, linux-rdma, amd-gfx,
	intel-gfx, Chris Wilson, Maarten Lankhorst, Christian König,
	Daniel Vetter

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.

v2: Also take into account tmz_surface boolean, plus just delete the
old code.

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>
---
DC-folks, I think this split out patch from my series here

https://lore.kernel.org/dri-devel/20200707201229.472834-1-daniel.vetter@ffwll.ch/

should be ready for review/merging. I fixed it up a bit so that it's not
just a gross hack :-)

Cheers, Daniel


---
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

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 21ec64fe5527..a20b62b1f2ef 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -6959,20 +6959,13 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 			DRM_ERROR("Waiting for fences timed out!");
 
 		/*
-		 * TODO This might fail and hence better not used, wait
-		 * explicitly on fences instead
-		 * and in general should be called for
-		 * blocking commit to as per framework helpers
+		 * We cannot reserve buffers here, which means the normal flag
+		 * access functions don't work. Paper over this with READ_ONCE,
+		 * but maybe the flags are invariant enough that not even that
+		 * would be needed.
 		 */
-		r = amdgpu_bo_reserve(abo, true);
-		if (unlikely(r != 0))
-			DRM_ERROR("failed to reserve buffer before flip\n");
-
-		amdgpu_bo_get_tiling_flags(abo, &tiling_flags);
-
-		tmz_surface = amdgpu_bo_encrypted(abo);
-
-		amdgpu_bo_unreserve(abo);
+		tiling_flags = READ_ONCE(abo->tiling_flags);
+		tmz_surface = READ_ONCE(abo->flags) & AMDGPU_GEM_CREATE_ENCRYPTED;
 
 		fill_dc_plane_info_and_addr(
 			dm->adev, new_plane_state, tiling_flags,
-- 
2.27.0


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2020-07-27 21:30 [PATCH] drm/amdgpu/dc: Stop dma_resv_lock inversion in commit_tail Daniel Vetter
2020-07-28  6:56 ` Christian König
2020-07-29 22:10   ` Alex Deucher

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