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Subject[PATCH AUTOSEL 4.18 082/100] drm/amd/display: Signal hw_done() after waiting for flip_done()
Date
From: Shirish S <shirish.s@amd.com>

[ Upstream commit 987bf116445db5d63a5c2ed94c4479687d9c9973 ]

In amdgpu_dm_commit_tail(), wait until flip_done() is signaled before
we signal hw_done().

[Why]

This is to temporarily address a paging error that occurs when a
nonblocking commit contends with another commit, particularly in a
mirrored display configuration where at least 2 CRTCs are updated.
The error occurs in drm_atomic_helper_wait_for_flip_done(), when we
attempt to access the contents of new_crtc_state->commit.

Here's the sequence for a mirrored 2 display setup (irrelevant steps
left out for clarity):

**THREAD 1** | **THREAD 2**
|
Initialize atomic state for flip |
|
Queue worker |
...

| Do work for flip
|
| Signal hw_done() on CRTC 1
| Signal hw_done() on CRTC 2
|
| Wait for flip_done() on CRTC 1

<---- **PREEMPTED BY THREAD 1**

Initialize atomic state for cursor |
update (1) |
|
Do cursor update work on both CRTCs |
|
Clear atomic state (2) |
**DONE** |
...
|
| Wait for flip_done() on CRTC 2
| *ERROR*
|

The issue starts with (1). When the atomic state is initialized, the
current CRTC states are duplicated to be the new_crtc_states, and
referenced to be the old_crtc_states. (The new_crtc_states are to be
filled with update data.)

Some things to note:

* Due to the mirrored configuration, the cursor updates on both CRTCs.

* At this point, the pflip IRQ has already been handled, and flip_done
signaled on all CRTCs. The cursor commit can therefore continue.

* The old_crtc_states used by the cursor update are the **same states**
as the new_crtc_states used by the flip worker.

At (2), the old_crtc_state is freed (*), and the cursor commit
completes. We then context switch back to the flip worker, where we
attempt to access the new_crtc_state->commit object. This is
problematic, as this state has already been freed.

(*) Technically, 'state->crtcs[i].state' is freed, which was made to
reference old_crtc_state in drm_atomic_helper_swap_state()

[How]

By moving hw_done() after wait_for_flip_done(), we're guaranteed that
the new_crtc_state (from the flip worker's perspective) still exists.
This is because any other commit will be blocked, waiting for the
hw_done() signal.

Note that both the i915 and imx drivers have this sequence flipped
already, masking this problem.

Signed-off-by: Shirish S <shirish.s@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 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 e484d0a94bdc..5b9cc3aeaa55 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -4494,12 +4494,18 @@ static void amdgpu_dm_atomic_commit_tail(struct drm_atomic_state *state)
}
spin_unlock_irqrestore(&adev->ddev->event_lock, flags);

- /* Signal HW programming completion */
- drm_atomic_helper_commit_hw_done(state);

if (wait_for_vblank)
drm_atomic_helper_wait_for_flip_done(dev, state);

+ /*
+ * FIXME:
+ * Delay hw_done() until flip_done() is signaled. This is to block
+ * another commit from freeing the CRTC state while we're still
+ * waiting on flip_done.
+ */
+ drm_atomic_helper_commit_hw_done(state);
+
drm_atomic_helper_cleanup_planes(dev, state);

/* Finally, drop a runtime PM reference for each newly disabled CRTC,
--
2.17.1
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