[Bug 221430] New: nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

[Bug 221430] New: nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221430

            Bug ID: 221430
           Summary: nvidia_wmi_ec_backlight: brightness writes silently
                    dropped on HP OMEN 16 (2025)
           Product: Drivers
           Version: 2.5
    Kernel Version: 6.19.14
          Hardware: Intel
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Platform_x86
          Assignee: drivers_platform_x86@kernel-bugs.osdl.org
          Reporter: floquimaurice@gmail.com
        Regression: No

Created attachment 309992
  --> https://bugzilla.kernel.org/attachment.cgi?id=309992&action=edit
ACPI tables (DSDT + all SSDTs, raw .dat and disassebled .dsl)

Reporting a backlight issue on a recent HP OMEN 16 with hybrid Optimus.

# System

- HP OMEN by HP Gaming Laptop 16 (2024/2025)
- Intel Meteor Lake + NVIDIA RTX 5070 Mobile, NVIDIA driver 595.x
- Fedora 44, kernel 6.19.14-300.fc44.x86_64
- Hybrid graphics mode (BIOS)

# Symptom

nvidia_wmi_ec_backlight loads, sysfs is created, writes to brightness are
accepted, but the panel does not respond and actual_brightness returns
"No data available".

  $ echo 50 | sudo tee /sys/class/backlight/nvidia_wmi_ec_backlight/brightness
  $ cat /sys/class/backlight/nvidia_wmi_ec_backlight/actual_brightness
  cat: actual_brightness: No data available

The AE_AML_OPERAND_VALUE errors visible in dmesg come from hp_bioscfg
against \_SB.WMID.* and are unrelated to the backlight path.

# What the firmware does

The NVIDIA-bound WMAA is at \_SB.PC00.AWMI.WMAA, GUID
603E9613-EF25-4338-A3D0-C46177516DB7. Its write path declares LEDB as the
new brightness value from Arg2[4], but never reads it:

    ElseIf ((MODF == One))
    {
        Sleep (0x64)
        Local1 = \_SB.PC00.GFX0.CBL1     /* reads existing value */
        Local3 = (Local1 * 0x02)
        If ((Local3 == Zero)) { Local3 += 0x02 }
        \_SB.PC00.LPCB.ECPW = Local3
        Return (Zero)
    }

The firmware propagates \_SB.PC00.GFX0.CBL1 (which the Windows display
driver populates before invoking WMI) to the EC PWM register, instead of
using the value the OS driver passed in. Under Linux nothing populates
CBL1, so the WMI call returns success but is effectively a no-op.

# Attachments

- dmesg-clean.txt
- dmidecode-clean.txt
- acpi-tables-omen16.tar.gz (DSDT + all SSDTs, raw and disassembled)
- lspci-full.txt

Happy to test patches or provide additional traces.

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221430

--- Comment #1 from floquimaurice@gmail.com ---
Created attachment 309993
  --> https://bugzilla.kernel.org/attachment.cgi?id=309993&action=edit
Full boot dmesg (username/hostname redacted)

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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https://bugzilla.kernel.org/show_bug.cgi?id=221430

--- Comment #2 from floquimaurice@gmail.com ---
Created attachment 309994
  --> https://bugzilla.kernel.org/attachment.cgi?id=309994&action=edit
dmidecode output (serial/UUID/asset redacted)

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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--- Comment #3 from floquimaurice@gmail.com ---
Created attachment 309995
  --> https://bugzilla.kernel.org/attachment.cgi?id=309995&action=edit
lspci -nnvv full output

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221430

Hans de Goede (hansg@kernel.org) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hansg@kernel.org

--- Comment #4 from Hans de Goede (hansg@kernel.org) ---
Hi, thank you for your bug report.

Problems like this with nvidia_wmi_ec backlight control are a known issue. I've
had a plan to properly fix this for a while now, but I don't have the time to
implement this.

I've taken the time to write this down, see:
https://lore.kernel.org/dri-devel/f10c039b-e9ef-4aae-8291-8f1e71074586@oss.qualcomm.com/

In the mean time as a workaround can you try adding "acpi_backlight=native" to
your kernel commandline I suspect/hope that that will help.

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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--- Comment #5 from floquimaurice@gmail.com ---
Hi Hans,

Thanks for the lore link. The backlight-aggregate plan to mimic Windows'
behavior makes perfect sense!

Regarding the workaround: I tested acpi_backlight=native, but unfortunately it
fails on this OMEN. The sysfs entries populate, but the panel ignores the
changes (likely because the EC is still waiting for the unpopulated CBL1
variable).

Since you are short on time, let me know if I can help draft/test for this
specific model in the meantime. 

Thanks again!

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221430

--- Comment #6 from Hans de Goede (hansg@kernel.org) ---
Hmm, can you try adding the following to your kernel commandline:

acpi_backlight=native nvidia_wmi_ec_backlight.force=1

That should give you both nvidia_wmi_ec and native (possibly multiple native)
entries under /sys/class/backlight

and then try echoing values in the 0 - max_brightness range to all the
/sys/class/backlight/*/brightness

attributes and see if that does something? Note the max_brightness variable can
differ quite a bit between different backlight interfaces and can be found by
doing:

cat /sys/class/backlight/<foo>/max_brightness

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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https://bugzilla.kernel.org/show_bug.cgi?id=221430

--- Comment #7 from floquimaurice@gmail.com ---
Hi Hans,

I added acpi_backlight=native nvidia_wmi_ec_backlight.force=1 (and
nvidia.NVreg_EnableBacklightHandler=0) to my kernel command line.

Here are the folders that appeared in /sys/class/backlight/:

    intel_backlight (max_brightness: 192000)

    nvidia_0 (max_brightness: 100)

    nvidia_wmi_ec_backlight (max_brightness: 100)

I tried echoing different values into the brightness file for each of these
three folders. The commands were accepted without any errors, but the physical
screen brightness did not change at all.

I am just reporting the facts here. Let me know if you need me to run any other
specific commands or tests!

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221430

--- Comment #8 from Hans de Goede (hansg@kernel.org) ---
(In reply to floquimaurice from comment #7)
> Hi Hans,
> 
> I added acpi_backlight=native nvidia_wmi_ec_backlight.force=1 (and
> nvidia.NVreg_EnableBacklightHandler=0) to my kernel command line.
> 
> Here are the folders that appeared in /sys/class/backlight/:
> 
>     intel_backlight (max_brightness: 192000)
> 
>     nvidia_0 (max_brightness: 100)
> 
>     nvidia_wmi_ec_backlight (max_brightness: 100)
> 
> I tried echoing different values into the brightness file for each of these
> three folders. The commands were accepted without any errors, but the
> physical screen brightness did not change at all.
> 
> I am just reporting the facts here. Let me know if you need me to run any
> other specific commands or tests!

Ok, so this is not a classic "which backlight driver should we use" problem.
But rather some issue with one of the backlight drivers.

The 192000 max-brightness for the intel_backlight looks suspicious.

Assuming that the LCD panel is indeed being driven by the Intel GPU and not
muxed to the nvidia GPU then I would start with filing a bug against the i915
driver:

https://drm.pages.freedesktop.org/intel-docs/how-to-file-i915-bugs.html

What value did you select in the BIOS for the hybrid-GPU config ? If you
selected discrete only instead of one of the hybrid options then this more
likely is an nvidia driver issue...

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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--- Comment #9 from floquimaurice@gmail.com ---
Hi Hans,

To answer your question: The BIOS is set to Hybrid Graphics (Optimus). The eDP
panel is driven directly by the Intel GPU.

You are right about the anomalous 192000 max_brightness on intel_backlight. I
have gathered the full dmesg (drm.debug=0xe) and the i915_vbt dump.

I will open a bug report with the Intel DRM team on GitLab and link this thread
there.

Thanks for your help and for pointing me in the right direction!

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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--- Comment #10 from floquimaurice@gmail.com ---
Created attachment 310098
  --> https://bugzilla.kernel.org/attachment.cgi?id=310098&action=edit
dmesg debug

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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--- Comment #11 from floquimaurice@gmail.com ---
Created attachment 310099
  --> https://bugzilla.kernel.org/attachment.cgi?id=310099&action=edit
i915 VBT dump

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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floquimaurice@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
 Attachment #310098|dmesg debug                 |dmesg with drm.debug=0xe
        description|                            |

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[Bug 221430] nvidia_wmi_ec_backlight: brightness writes silently dropped on HP OMEN 16 (2025)

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--- Comment #12 from floquimaurice@gmail.com ---
Hi,

I opened a ticket with the Intel DRM team, and they analyzed the drm.debug=0xe
logs.

https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16065

They confirmed that the VBT correctly points to PCH PWM, and the i915 driver is
doing its job perfectly. The logs show the driver successfully calculating and
sending the PWM updates to the registers (set backlight level = 98259).

However, the physical panel ignores it. The Intel developers concluded this is
not an i915 issue. The hardware is deliberately ignoring the PCH PWM registers,
which perfectly confirms your initial theory: the HP Embedded Controller is
waiting for the \_SB.PC00.GFX0.CBL1 ACPI trigger to release the backlight
control.

Now that Intel has officially bounced the ball back to the ACPI/firmware side,
how can we proceed?

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