A misplaced error check can cause this failure scenario, and does so reliably as tested on Ubuntu 21.04 with KDE Plasma 5 desktop within the first few seconds of login session startup, rendering VRR under modesetting-ddx unusable:
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Some X11 client application changes some window property.
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ms_change_property() is called as part of the property change handling call chain (client->requestVector[X_ChangeProperty]). It removes itself temporarily from the call chain - or so it thinks, hooking up saved_change_property instead.
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ret = saved_change_property(client) is called and fails temporarily for some non-critical reason.
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The misplaced error check returns early (error abort), without first restoring ms_change_property() as initial X_ChangeProperty handler in the call chain again.
-> Now ms_change_property() has removed itself permanently from the property handler call chain for the remainder of the X session and VRR property changes on windows are no longer handled, ie. VRR no longer gets enabled/disabled in response to window VRR property changes.
Place the error check at the proper place, just as it is correctly done by amdgpu-ddx, and in modesetting-ddx ms_delete_property() function.
Verified to fix VRR handling with an AMD gpu under KDE desktop session.
Please consider merging before branching the server 1.21 branch.
Signed-off-by: Mario Kleiner mario.kleiner.de@gmail.com