Public bug reported:
# [i915] eDP backlight max_brightness drops from 7500 to 1023 between
kernel 6.8 and 7.0, screen stays dark even at maximum brightness (Dell
Latitude 7490)
## Summary
On a Dell Latitude 7490 (Skylake-generation VBT), the internal eDP panel
becomes very dark when running kernel 7.0.0-14-generic (Xubuntu 26.04),
even at the maximum backlight value. The same hardware, same BIOS
version, and byte-identical VBT works correctly under kernel 6.8.0 and
5.15.0. The root cause appears to be a regression in how
`intel_backlight`'s `max_brightness` is computed: it drops from 7500
(correct, working) to 1023 (broken) between these kernel versions,
meaning the "maximum" brightness value under the broken kernel
corresponds to only a small fraction of the panel's actual full-scale
PWM duty cycle.
## Environment
- **Model:** Dell Latitude 7490
- **BIOS version:** 1.41.0 (identical on both working and broken test units)
- **VBT:** `$VBT SKYLAKE`, BDB version 212, build 1000, dated 01/21/2015 —
confirmed **byte-identical** between a working unit (kernel 5.15) and the
broken unit (kernel 7.0), extracted via `/sys/kernel/debug/dri/*/i915_vbt`
- **LFP Backlight block (VBT block 43), decoded fields:**
- `type = 2` (Display DDI / native PWM, not DPCD/AUX)
- `active_low_pwm = 0`
- `pwm_freq_hz = 200`
- `min_brightness = 0`
- **Broken:** Xubuntu 26.04, kernel `7.0.0-14-generic`
- **Working (same hardware/BIOS):** kernel `6.8.0-060800-generic` (Ubuntu
mainline build) and, on a second identical Latitude 7490 unit, Xubuntu 22.04
with kernel `5.15.0-186-generic`
## Steps to Reproduce
1. Boot Dell Latitude 7490 with Xubuntu 26.04, kernel 7.0.0-14-generic.
2. Observe the internal display is very dark immediately from boot (i.e.,
before any user session/compositor starts).
3. Check backlight state:
```
cat /sys/class/backlight/intel_backlight/max_brightness
cat /sys/class/backlight/intel_backlight/brightness
```
4. Boot the same machine with kernel 6.8.0-060800-generic (installed in
parallel via Ubuntu mainline builds) and repeat step 3.
## Expected Result
`max_brightness` should reflect the panel's real full-scale PWM range,
and setting `brightness` to `max_brightness` should produce full/normal
screen brightness — as seen on kernel 6.8.0 and 5.15.0.
## Actual Result
| Kernel | max_brightness | brightness at max | Screen appearance |
|---|---|---|---|
| 5.15.0-186-generic | 7500 | 7500 | Normal / bright |
| 6.8.0-060800-generic | 7500 | 7500 | Normal / bright |
| 7.0.0-14-generic | 1023 | 1023 | Very dark, even at max value |
Writing arbitrary values to
`/sys/class/backlight/intel_backlight/brightness` under the broken
kernel does change the screen's brightness (e.g., writing 300 makes it
visibly darker than writing 1023), confirming the sysfs interface is
functionally connected to the hardware — it is simply capped at a much
lower duty-cycle ceiling than the panel actually supports.
## Additional Diagnostics (broken kernel, 7.0.0-14-generic)
- `cat /sys/class/backlight/intel_backlight/type` → `raw`
- `acpi_backlight=video` boot parameter: `acpi_video0` backlight device appears
(`max_brightness=66`), but writing to it also produces no visible brightness
change.
- `i915.enable_dpcd_backlight=1` and `=3` boot parameters: no change.
- `i915.enable_psr=0 i915.enable_dc=0` boot parameters (to rule out
PSR/DC-state PWM miscalculation): no change.
- `nomodeset` boot parameter: screen boots at full/normal brightness and stays
there (firmware/UEFI-set backlight level, untouched because i915 never takes
over the panel) — this was the first strong indicator that i915's own
panel/backlight initialization on this kernel is the culprit, not a downstream
consumer (Xfce power manager, compositor, etc.).
- Also observed in `kern.log` on the 7.0 kernel (possibly related, possibly a
separate regression from the same ACPI-handling change):
```
ACPI Warning: SystemMemory range 0x00000000FE028000-0x00000000FE0281FF
conflicts with OpRegion 0x00000000FE028000-0x00000000FE028207
(\_SB.PCI0.GEXP.BAR0) (20251212/utaddress-204)
ACPI: OSL: Resource conflict; ACPI support missing from driver?
intel-lpss INT3446:00: probe with driver intel-lpss failed with error -16
```
## Notes
- Ruled out: BIOS/firmware VBT differences (confirmed byte-identical between
working and broken systems), xfce4-power-manager or other userspace brightness
helpers (issue reproduces on a bare TTY with no X11/compositor running),
gamma/compositor dimming (`xrandr --verbose` shows `Gamma: 1.0:1.0:1.0`),
DPCD/AUX vs native PWM backlight type mismatch (VBT explicitly specifies type 2
/ native DDI PWM, and forcing DPCD via `enable_dpcd_backlight` made no
difference).
- This strongly points to a regression in the i915 PWM/backlight range
calculation code introduced somewhere between kernel 6.8 and 7.0 (exact commit
not yet bisected).
## Suggested Next Steps for Investigation
- Bisect between v6.8 and v7.0 mainline kernels to identify the offending
commit in `drivers/gpu/drm/i915/display/intel_backlight.c` (or related PWM
period/precision calculation code).
- Compare computed PWM period/precision-bit values between the two kernel
versions for this specific panel (Skylake-era native PWM, VBT type 2, 200 Hz).
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/2161713
Title:
[i915] eDP backlight max_brightness drops from 7500 to 1023 between
kernel 6.8 and 7.0, screen stays dark even at maximum brightness (Dell
Latitude 7490)
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