ACK: [SRU][N][PATCH 0/1] Random flickering with Intel i915 (CoffeeLake and WhiskeyLake) on Linux 6.8

[Benjamin Wheeler]

Acked-by: Benjamin Wheeler <benjamin.wheeler at canonical.com>

On Mon, Jan 12, 2026 at 10:34 PM AceLan Kao <acelan.kao at canonical.com> wrote:
>
> From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>
>
> BugLink: https://bugs.launchpad.net/bugs/2136958
>
> [Impact]
> Systems with Intel CoffeeLake (CFL) and WhiskeyLake (WHL) integrated
> graphics experience screen flickering issues when running Ubuntu kernel
> 6.8.0-x.
>
> The issue occurs because:
> 1. Ubuntu kernel configuration enables CONFIG_INTEL_IOMMU_DEFAULT_ON and
>    CONFIG_INTEL_IOMMU_SCALABLE_MODE_DEFAULT_ON by default (commit 77e530c1a864c)
> 2. These Intel integrated graphics generations have compatibility issues
>    with DMAR (DMA Remapping) enabled
> 3. The interaction between IOMMU scalable mode and the i915 graphics
>    driver causes intermittent display flickering
>
> This is part of a broader issue affecting multiple Intel integrated
> graphics generations (Skylake, KabyLake, CoffeeLake, WhiskeyLake, and
> CometLake) as documented in the upstream bug report. Previous patches have
> already addressed SKL, KBL, and CML; this patch completes the series by
> adding CFL and WHL.
>
> Affected hardware: Systems with Intel CoffeeLake or WhiskeyLake
> integrated graphics (device IDs: 0x3E90-0x3E9C, 0x3EA0-0x3EA9)
>
> [Fix]
> Single UBUNTU SAUCE patch adds CFL and WHL device IDs to the IOMMU quirk list:
>
> UBUNTU: SAUCE: iommu/intel: disable DMAR for CFL and WHL integrated gfx
> - Adds 16 CoffeeLake device IDs (CFL_S, CFL_H, CFL_U variants)
> - Adds 5 WhiskeyLake device IDs (WHL_U variants)
> - Follows the same pattern as previous SKL/KBL/CML quirk patches
> - Device IDs sourced from include/drm/i915_pciids.h for completeness
>
> This is an Ubuntu-specific SAUCE patch as upstream has not yet provided a
> proper fix. The quirk approach is used temporarily until a proper upstream
> solution is available.
>
> Previous related commits in this series:
> - fe35c48a46619 "UBUNTU: SAUCE: iommu/intel: disable DMAR for SKL integrated gfx"
> - 69a3b8e5f6ebd "UBUNTU: SAUCE: iommu/intel: disable DMAR for KBL and CML integrated gfx"
>
> [Test Plan]
> Test on hardware with CoffeeLake or WhiskeyLake integrated graphics:
>
> 1. Identify if the system has affected hardware:
>    ```
>    lspci -nn | grep VGA
>    ```
>    Look for Intel graphics with device IDs in the 0x3E90-0x3EA9 range.
>
> 2. Reproduce the flickering (before the fix):
>    a. Boot with kernel 6.8.0-x without the patch
>    b. Move windows around or trigger compositor effects
>    c. Observe intermittent screen flickering or tearing
>
> 3. Verify the fix:
>    a. Boot with kernel containing the patch
>    b. Check that IOMMU is bypassed for the graphics device:
>       ```
>       sudo dmesg | grep -i "Disabling IOMMU for graphics on this chipset"
>       ```
>    c. Should see: "Disabling IOMMU for graphics on this chipset"
>    d. Verify no flickering occurs during video playback and window animations
>    e. Confirm graphics performance remains normal
>
> [Where problems could occur]
> The changes affect the Intel IOMMU driver initialization for CoffeeLake
> and WhiskeyLake integrated graphics only.
>
> Potential issues:
>
> 1. **Security reduction**: Disabling DMAR for integrated graphics removes
> IOMMU protection for GPU DMA operations. However, integrated graphics are
> generally trusted components, and the security impact is minimal compared
> to the usability impact of screen flickering.
>
> 2. **Hardware-specific behavior**: Some CFL/WHL systems might not exhibit
> the flickering issue, and for those systems, this patch unnecessarily
> disables IOMMU for graphics. However, the functional impact is negligible,
> and disabling DMAR has no negative performance implications.
>
> 3. **Interaction with other IOMMU features**: The quirk only affects the
> integrated graphics device. Other IOMMU functionality (ATS, interrupt
> remapping, etc.) remains unaffected.
>
> The impact is limited to the IOMMU configuration for CoffeeLake and
> WhiskeyLake integrated graphics. All other system components continue to
> benefit from IOMMU protection.
>
> [Other Info]
> This is an Ubuntu SAUCE patch (not upstream) that works around the issue
> until a proper upstream fix is available.
>
> Upstream tracking bug: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11504
>
> The issue was introduced in Ubuntu 6.8 kernel by commit:
> 77e530c1a864c "UBUNTU: [Config] enable Intel DMA remapping by default"
>
> This patch completes a series of quirks for affected Intel graphics
> generations:
> 1. Skylake (SKL) - fe35c48a46619
> 2. KabyLake (KBL) and CometLake (CML) - 69a3b8e5f6ebd
> 3. CoffeeLake (CFL) and WhiskeyLake (WHL) - this patch
>
> All affected device IDs were verified against the upstream i915 driver's
> device ID definitions in include/drm/i915_pciids.h.
>
> Chia-Lin Kao (AceLan) (1):
>   UBUNTU: SAUCE: iommu/intel: disable DMAR for CFL and WHL integrated
>     gfx
>
>  drivers/iommu/intel/iommu.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> --
> 2.51.0
>
>
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