From: Shirish S <shirish.s@xxxxxxx>
In amdgpu_dm_commit_tail(), wait until flip_done() is signaled before
we signal hw_done().
[Why]
This is to temporary address a paging error that occurs when a
nonblocking commit contends with another commit, particularly in a
mirrored multi-display configuration. 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 worker |
|
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@xxxxxxx>
Signed-off-by: Leo Li <sunpeng.li@xxxxxxx>
---
Hi Shirish,
Sorry for the late reply. I took some time to dig at this issue, and found
that a proper fix will need more thought. I'm OK with this fix for now, until
we can address it with a proper fix.
I've updated the commit message and comments below to reflect the underlying
issue.
Thanks,
Leo
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 774db34..bbfffaf 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -4755,12 +4755,18 @@ 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);
/*
--
2.7.4
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