Ubuntu
linux package

  1. Oracular (24.10)
  2. Bug #2062951
  3. Comment #51

Comment 51 for bug 2062951

Revision history for this message
Jeffrey Knockel (jeff250) wrote : Re: Random flickering with Intel i915 (Gen9 GPUs) on Linux 6.8 (Ubuntu 24.04)

> Thank you for doing your research into the issue but I am confused by
> your results.
>
> Are you claiming that removing mentioned Ubuntu specific commit resolves
> the issue?

Specifically it's the first commit where I experience the bug, so I would assume that reverting it will resolve the issue. The commit enables the following compile time kernel config options:

CONFIG_INTEL_IOMMU_DEFAULT_ON=y
CONFIG_INTEL_IOMMU_SCALABLE_MODE_DEFAULT_ON=y

These enable the Intel IOMMU by default (although it can still be disabled by the linux command line at runtime via Grub).

For completeness here is the corresponding commit in the noble kernel:

https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/noble/commit/?id=77e530c1a864c601b96622db03bc1f38e51155f1

> What about the fact that the issue also affect upstream/mainline
> kernels?

I don't know how the kernel config values for the mainline builds that I linked to were derived, but in the build that I tried (built July 25 2024) the above config options were also enabled. I don't know if this is because the upstream kernel has now enabled these config options by default or if the builds that I linked to still use custom Ubuntu config options.

> Are you trying to say that the issue is just related to the conditions
> described in the commit and intel_iommu=igfx_off?

Specifying intel_iommu= on the linux command line is a way of overriding at runtime whatever is specified by the CONFIG_INTEL_IOMMU_DEFAULT_ON config option when the kernel is compiled.

Specifying intel_iommu=off would be the way to completely undo the config changed by the problematic commit, but since the bug that we are experiencing only (to my knowledge) affects graphics, I figured that intel_iommu=igfx_off would be more conservative workaround, as it only disables Intel's integrated graphics from going through the IOMMU but otherwise keeps the IOMMU enabled.

It's not exactly clear to me what the benefits of having graphics going through the Intel IOMMU are, but if Intel's IOMMU was only enabled recently in Ubuntu kernels then I just assume that I won't miss whatever those benefits are.