Public bug reported:
Kernel 7.0.0-28-generic broke screen brightness control on my ASUS X580VD
(Intel iGPU + NVIDIA Optimus, panel on i915/eDP). Confirmed via direct
bisection: works on 7.0.0-27-generic, broken on 7.0.0-28-generic with
everything else identical.
Root cause: the "Patchset for TUXEDO devices" (LP: #2152570) backported
into 7.0.0-28 includes commit 40d2f5820951 ("drm/i915/backlight: Remove
try_vesa_interface"). This removes the gate on
intel_dp_aux_supports_vesa_backlight() in
drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c, so the driver now
tries VESA/DPCD AUX backlight unconditionally whenever the panel's DPCD
capability bits look plausible - even when the VBT correctly declares
INTEL_BACKLIGHT_DISPLAY_DDI and the panel's real backlight control is
native PWM.
This exact regression was already found and fixed upstream:
commit 2914709c9141 ("Revert \"drm/i915/backlight: Remove
try_vesa_interface\""), merged 2026-05-17 by Suraj Kandpal (Intel),
reviewed by Ankit Nautiyal, already cherry-picked into a stable tree as
f30fddb44023.
Its commit message describes this bug precisely:
"Removing the try_vesa_interface gate caused a backlight regression on
panels whose VBT correctly reports INTEL_BACKLIGHT_DISPLAY_DDI and whose
PWM path is the actual backlight control, but whose DPCD optimistically
advertises DP_EDP_BACKLIGHT_AUX_ENABLE_CAP / _BRIGHTNESS_AUX_SET_CAP.
After the commit such panels silently bind to the VESA AUX backlight
funcs; AUX writes complete but the panel ignores them, leaving
brightness stuck (no-op backlight). Observed on at least KBL and TGL
eDP setups."
7.0.0-28's TUXEDO backport included the regression-causing commit but not
this later upstream revert. Requesting 2914709c9141 be cherry-picked into
the next resolute kernel SRU.
Confirmed locally: booting with i915.enable_dpcd_backlight=0 (forcing the
same native-PWM path the revert restores) fully fixes brightness. dmesg
with drm.debug=0x1e shows the driver binding to VESA AUX backlight by
default and native PCH PWM ("Using native PCH PWM for backlight control")
once forced off - AUX writes succeed but produce no physical change,
matching the revert's description exactly.
Panel DPCD (eDP): 02 b7 f6 00 00 (eDP 1.3, DP_EDP_BACKLIGHT_AUX_ENABLE_CAP
and DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP both set despite AUX backlight
not functioning on this hardware).
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
--
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/2162630
Title:
7.0.0-28 TUXEDO backlight backport missing upstream revert
2914709c9141 — breaks brightness on ASUS X580VD (and likely other
DISPLAY_DDI panels)
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2162630/+subscriptions
--
ubuntu-bugs mailing list
[email protected]
https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
