drm/amdgpu/dc: Stop dma_resv_lock inversion in commit_tail

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>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -7081,20 +7081,13 @@ 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,