[Bug 2062951] Re: Random flickering with Intel i915 (Gen9 GPUs in 6th-8th gen CPUs) on Linux 6.8

I filed a new bug for KBL and CML platforms, please check the test kernel there and help to verify if it works.
Thanks.
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2086587

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https://bugs.launchpad.net/bugs/2062951

Title:
Random flickering with Intel i915 (Gen9 GPUs in 6th-8th gen CPUs) on
Linux 6.8

Status in Linux:
New
Status in linux package in Ubuntu:
Fix Released
Status in linux-hwe-6.8 package in Ubuntu:
Invalid
Status in linux source package in Jammy:
Invalid
Status in linux-hwe-6.8 source package in Jammy:
Fix Released
Status in linux source package in Noble:
Fix Released
Status in linux-hwe-6.8 source package in Noble:
Invalid
Status in linux source package in Oracular:
Fix Released
Status in linux-hwe-6.8 source package in Oracular:
Invalid

Bug description:
SRU Justification

[Impact]
The flickering predominantly occurs when the mouse cursor is in the bottom quarter of the screen and stops when the cursor leaves this area.
Extensive testing across various kernel versions revealed that the issue became apparent from kernel version 6.8.0 onward, correlating with the mentioned configuration changes, CONFIG_INTEL_IOMMU_DEFAULT_ON and CONFIG_INTEL_IOMMU_SCALABLE_MODE_DEFAULT_ON.
The problem persists in the latest drm tip.

[Fix]
As an immediate workaround, adding 'intel_iommu=igfx_off' to the kernel command line has been found to effectively eliminate the flickering issue.
This patch proposes the same behavior as 'intel_iommu=igfx_off' and apply the rule to all Intel SKL GPU IDs.

[Test]
1. Boot up to desktop with 6.8.0-x kernels
2. Moving cursor around the desktop
3. There should be no flicker while cursor is moving

[Where problems could occur]
For virtualized environments using hardware-assisted virtualization for graphics (such as VFIO or mediated pass-through), disabling IOMMU for graphics could either disable these features or reduce their security and efficiency.
Usually, users pass the 'intel_iommu=on' by themselves when they are using related features. So, the impact of this change is minimal.

===========

After migrating Mantic 23.10 install to Linux 6.8.0, I am experiencing heavy flickering at random.
It happens after a moderate to heavy on screen activity like window switching, scrolling etc.
It never happens when playing video or when there is no interaction using keyboard/pointer.

It tend to happen when the mouse cursor is in the bottom quarter of the screen and stops immediately when the cursor leaves that screen region.
Around the same time the following appears in kernel log:
kernel: i915 0000:00:02.0: [drm] *ERROR* CPU pipe A FIFO underrun

Happens on both X11 and Wayland on both KDE plasma and Mate (though
less).

I first encountered the issue with Linux 6.8.0-11 and it still happens with Ubunutu's 6.8.0-28 and Mainline 6.8.7.
Kernels 6.6, 6.5, 6.1 and older did not have that issue.

Hardware is: Dell XPS 9350, Vendor Intel, Driver i915, Skylake GT2 [HD Graphics 520]
Software: Ubuntu 23.10, KDE Plasma 5.27.10 / Mate 1.26

Attaching kernel log for 6.8.0-28

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