D6011: let's continue in debug code instead of returning from XRandRConfig::applyKScreenConfig

[Christoph Lutz]

clutz added a comment.


  Thanks David for reviewing that. Oh, I already tried to find the underlying bug that causes the inconsistency. The debugging led me from libkscreen to xrandr to libxrandr (XRRGetOutputInfo) to the kernel. Here I noticed the following things:
  
  1. outputInfo = XRRGetOutputInfo(outputId) returns some outputInfo that dosn't contain the mode it should contain (concrete the ID 77). It was simply missing.
  2. crtcInfo = XRRGetCrtcInfo(outputInfo->crtc) But this crtcInfo-Objekt does reference the mode 77: crtcInfo->mode is 77
  3. XRRGetScreenResourcesCurrent() also thinks that mode 77 should be present
  4. Because mode 77 was not in outputInfo, XRandROutput::updateModes could not re-add this mode to the map m_modes and XRandROutput::currentMode() had to return NULL. Nothing wrong in updateModes!
  
  Because libxrandr in 1) also delegates the above mentioned requests directly to the CRTC-driver from the kernel, I came to the conclusion that the inconsistency behind this situation comes from the kernel. It was also hard to reproduce on side of the kernel since multiple requests returned different results (kind of sporadic).
  
  As mentioned above, I could sporadically reproduce the inconsistency with kernel 3.13.0 and as I tried a newer kernel the issue seemed to be vanished. This is the point where I decided to not dig deeper into such an old kernel version.
  
  So in conclusion for me the underlying bug seems to be a kernel bug and not a libkscreen-bug. Maybe in the meanwhile nobody will ever notice the above situation again. BUT in my concrete situation libkscreen seems to be able to recover things from this inconsistency and "continuing" instead of "returning" simply improves the behaviour of the complete system. So for me this is a clear improvement and I see no reason to not apply it. Do you?

REPOSITORY
  R110 KScreen Library

REVISION DETAIL
  https://phabricator.kde.org/D6011

To: clutz
Cc: davidedmundson, plasma-devel, ZrenBot, spstarr, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart, lukas
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