Public bug reported: linux-modules-7.0.0-28-generic includes changes to backlight control in i915 module. This was supposed to be addressing LP: #2152570 making backlight work on some older laptops. The changes break backlight control on a Samsung 940X laptop with i5-3317 CPU.
- drm/i915/backlight: Remove try_vesa_interface https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=40d2f5820951dee818d05c14677277048bd85f9f The previous version of intel_dp_aux_init_backlight_funcs in drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c had a display->params.enable_dpcd_backlight case of INTEL_DP_AUX_BACKLIGHT_AUTO which would only try VESA interface for a panel->vbt.backlight.type of INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE or "if (intel_dp->edp_dpcd[0] >= DP_EDP_15)" The new version of intel_dp_aux_init_backlight_funcs tries a VESA interface for all values of panel->vbt.backlight.type including INTEL_BACKLIGHT_PMIC or INTEL_BACKLIGHT_LPSS or INTEL_BACKLIGHT_DSI_DCS or INTEL_BACKLIGHT_PANEL_DRIVER_INTERFACE or INTEL_BACKLIGHT_DISPLAY_DDI even when "(intel_dp->edp_dpcd[0] < DP_EDP_15)". That is overriding funcs that are set in drivers/gpu/drm/i915/display/intel_backlight.c by intel_backlight_init_funcs before it calls intel_dp_aux_init_backlight_funcs. Looking at the output of drm_dbg_kms() calls in i915 driver using kernel boot parameter drm.debug=0x06 shows that the VESA control is used. I added a call to log vbt.backlight.type from intel_bios.c parse_lfp_backlight(). i915 0000:00:02.0: [drm:parse_lfp_backlight [i915]] vbt.backlight.type: INTEL_BACKLIGHT_DISPLAY_DDI That confirmed that vbt.backlight.type is INTEL_BACKLIGHT_DISPLAY_DDI on this laptop. The dmesg output from intel_edp_init_dpcd shows that intel_dp->edp_dpcd[0] is DP_EDP_13, (0x02). i915 0000:00:02.0: [drm:intel_edp_init_dpcd [i915]] eDP DPCD: 02 b7 f6 00 00 Whatever extra tests added in the new check_if_vesa_backlight_possible() function does not spare this laptop. The VESA control is pushed in replacing the working functions previously set by intel_backlight_init_funcs(). ProblemType: Bug DistroRelease: Ubuntu 26.04 Package: linux-image-7.0.0-28-generic 7.0.0-28.28 ProcVersionSignature: Ubuntu 7.0.0-28.28-generic 7.0.12 Uname: Linux 7.0.0-28-generic x86_64 ApportVersion: 2.34.0-0ubuntu2 Architecture: amd64 AudioDevicesInUse: USER PID ACCESS COMMAND /dev/snd/controlC1: mike 2707 F.... wireplumber /dev/snd/controlC0: mike 2707 F.... wireplumber /dev/snd/seq: mike 2697 F.... pipewire CasperMD5CheckResult: pass CurrentDesktop: ubuntu:GNOME Date: Mon Jul 20 12:43:48 2026 InstallationDate: Installed on 2025-09-10 (313 days ago) InstallationMedia: Ubuntu 24.04.3 LTS "Noble Numbat" - Release amd64 (20250805.1) MachineType: SAMSUNG ELECTRONICS CO., LTD. 940X3G/930X3G ProcEnviron: LANG=en_US.UTF-8 PATH=(custom, no user) SHELL=/bin/bash TERM=xterm-256color XDG_RUNTIME_DIR=<set> ProcFB: 0 i915drmfb ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-28-generic root=UUID=ff2b50fd-2ec0-4e8d-8be8-8d109a2614e9 ro quiet splash crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M drm.debug=0x06 SourcePackage: linux UpgradeStatus: Upgraded to resolute on 2026-04-24 (87 days ago) dmi.bios.date: 08/19/2014 dmi.bios.release: 4.6 dmi.bios.vendor: American Megatrends Inc. dmi.bios.version: P05ACJ.128.140819.dg dmi.board.asset.tag: No Asset Tag dmi.board.name: NP940X3G-K04US dmi.board.vendor: SAMSUNG ELECTRONICS CO., LTD. dmi.board.version: SAMSUNG_SW_REVISION_1234567890ABCD dmi.chassis.asset.tag: No Asset Tag dmi.chassis.type: 9 dmi.chassis.vendor: SAMSUNG ELECTRONICS CO., LTD. dmi.chassis.version: N/A dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrP05ACJ.128.140819.dg:bd08/19/2014:br4.6:svnSAMSUNGELECTRONICSCO.,LTD.:pn940X3G/930X3G:pvrP05ACJ:rvnSAMSUNGELECTRONICSCO.,LTD.:rnNP940X3G-K04US:rvrSAMSUNG_SW_REVISION_1234567890ABCD:cvnSAMSUNGELECTRONICSCO.,LTD.:ct9:cvrN/A:skuSAMSUNGSENSSeries:pfaSAMSUNGSENS: dmi.product.family: SAMSUNG SENS dmi.product.name: 940X3G/930X3G dmi.product.sku: SAMSUNG SENS Series dmi.product.version: P05ACJ dmi.sys.vendor: SAMSUNG ELECTRONICS CO., LTD. ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Tags: amd64 apport-bug resolute wayland-session -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2161359 Title: linux-modules-7.0.0-28-generic breaks backlight control on some laptops To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2161359/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
