Re: Need hints trying to locate a compositing issue.

Hi Vlad,

24.09.2020 10:10, Vlad Zahorodnii:
[...]
>> of this pages then exhibits a vertically misplaced portion of the
>> picture, so part of the window looks "flashing" rapidly until I stop
>> moving the mouse. It feels a bit like maybe some memory corruption so
>> I'd like to try find and fix it.
>> The issue does not happen with Desktop Effects disabled or set to
>> XRender, therefore I suppose the RDP application is not to blame.
>> Obviously it is related to OpenGL and compositing and vertical block
>> movement. Now my suspect list is the following:
>
> Have you tried disabling buffer age support?
> https://community.kde.org/KWin/Environment_Variables#KWIN_USE_BUFFER_AGE
>
> My wild guess is that kwin's command stream is not synchronized with the
> X command stream.

Good shot! Indeed, KWIN_USE_BUFFER_AGE=0 with KWIN_TRIPLE_BUFFER=1 
apparently eliminated this visual defect. Additionally, switching intel 
x module from DRI3 to DRI2 does so too. At least I was not able to 
reproduce it anymore however I tried.
Now this rises some new questions:
- is this really a solution or rather hiding the problem under a carpet?
- what is a better choice: setting KWIN_* values or switching to DRI2?
- which part of code is to blame/bugreport/fix? Is it a problem of kwin 
or something else? I'd suppose some advanced buffer management in DRI3 
is there for a reason, so disabling it looks strange unless it is 
seriously broken?

Thank you!

Regards,
Nikolai

>
> Cheers,
> Vlad
>
>> 1. kernel 4.14.174 - buggy drm module?
>> 2. libdrm 2.4.67 - buggy library?
>> 3. Mesa 11.2.2 - buggy library?
>> 4. xorg 1.16.1 - buggy X?
>> 5. intel X module 48.20200205.fc33 (from Fedora core 33) - buggy intel
>> driver?
>> 6. kwin 4.11.20 - buggy compositor?
>>
>> So pretty large list, and I'm not sure how to better dig it further.
>> Any hints greatly appreciated. Maybe it is a known issue already?
>>
>> Thanks.
>>
>> Regards,
>> Nikolai
>
>