[BUG] VC4 DRM waiting for flip down makes UI freeze a while with kernel 5.15

[Jian-Hong Pan]

Hi,

I tested Linux mainline kernel 5.15 (aarch64) with enabled VC4 on RPi 4B. I
notice UI freezes a while (about 10 seconds) some times.

The kernel shows the error message during the time:

[drm:drm_crtc_commit_wait] *ERROR* flip_done timed out
[drm:drm_atomic_helper_wait_for_flip_done] *ERROR* [CRTC:68:crtc-3]
flip_done timed out
[drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CRTC:68:crtc-3]
commit wait timed out
[drm:drm_crtc_commit_wait] *ERROR* flip_done timed out
vc4-drm gpu: [drm] *ERROR* Timed out waiting for commit

Here is the full log:
https://github.com/lategoodbye/rpi-zero/files/7518076/dmesg-5.15.log

It is easy to reproduce this issue by invoking GL related things, for
example "es2gears".

After detail test, I found it is related to these commits:
* f3c420fe19f8 ("drm/vc4: kms: Convert to atomic helpers")
* 82faa3276012 ("drm/vc4: kms: Remove async modeset semaphore")

This issue cannot be reproduced after I revert the commits.

Jian-Hong Pan
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