[Bug 2161244] [NEW] Regression in linux 7.0.0-28: Intel Tiger Lake internal display backlight disabled unless i915.enable_dpcd_backlight=0

Public bug reported: [Impact] The internal display backlight is unusable after upgrading from linux- image-7.0.0-27-generic to linux-image-7.0.0-28-generic on a Dell Latitude 5320 laptop. The issue affects the internal panel only. External monitors continue to work. [Steps to reproduce] 1. Boot Ubuntu 26.04 with kernel 7.0.0-28-generic. 2. Do not add any kernel parameter. 3. After i915 loads, the internal display backlight turns almost completely off. 4. The image is still visible with a flashlight, but the backlight is effectively disabled. 5. Brightness controls appear to work, but have no visible effect. [Expected behaviour] The internal display backlight should work normally, as it does with kernel 7.0.0-27-generic. [Regression] Working: - linux-image-7.0.0-27-generic Broken: - linux-image-7.0.0-28-generic [Workaround] Adding the kernel parameter: i915.enable_dpcd_backlight=0 restores normal backlight operation on kernel 7.0.0-28. [Hardware] Dell Latitude 5320 Intel Tiger Lake integrated graphics PCI device: Intel Corporation TigerLake-LP GT2 [Iris Xe Graphics] (device ID 9a49) [Additional information] Without the workaround, brightnessctl reports: Device 'intel_backlight' of class 'backlight': Current brightness: 1023 (100%) Max brightness: 1023 However, the physical backlight is effectively off. The issue started immediately after the kernel update to 7.0.0-28. ** Affects: linux (Ubuntu) Importance: Undecided Status: New -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb https://bugs.launchpad.net/bugs/2161244 Title: Regression in linux 7.0.0-28: Intel Tiger Lake internal display backlight disabled unless i915.enable_dpcd_backlight=0 Status in linux package in Ubuntu: New Bug description: [Impact] The internal display backlight is unusable after upgrading from linux- image-7.0.0-27-generic to linux-image-7.0.0-28-generic on a Dell Latitude 5320 laptop. The issue affects the internal panel only. External monitors continue to work. [Steps to reproduce] 1. Boot Ubuntu 26.04 with kernel 7.0.0-28-generic. 2. Do not add any kernel parameter. 3. After i915 loads, the internal display backlight turns almost completely off. 4. The image is still visible with a flashlight, but the backlight is effectively disabled. 5. Brightness controls appear to work, but have no visible effect. [Expected behaviour] The internal display backlight should work normally, as it does with kernel 7.0.0-27-generic. [Regression] Working: - linux-image-7.0.0-27-generic Broken: - linux-image-7.0.0-28-generic [Workaround] Adding the kernel parameter: i915.enable_dpcd_backlight=0 restores normal backlight operation on kernel 7.0.0-28. [Hardware] Dell Latitude 5320 Intel Tiger Lake integrated graphics PCI device: Intel Corporation TigerLake-LP GT2 [Iris Xe Graphics] (device ID 9a49) [Additional information] Without the workaround, brightnessctl reports: Device 'intel_backlight' of class 'backlight': Current brightness: 1023 (100%) Max brightness: 1023 However, the physical backlight is effectively off. The issue started immediately after the kernel update to 7.0.0-28. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2161244/+subscriptions