Plan to fix nvidia_wmi_ec backlight issues, help wanted

[Hans de Goede <johannes.goede@oss.qualcomm.com>]

Hi All,

A while ago Nvidia introduced a new Nvidia specific firmware method
for controlling the backlight on laptops with runtime switchable
Nvidia hybrid graphics.

This is supported through the nvidia-wmi-ec-backlight driver, but has
never worked 100%.

The new WMI firmware interface has a method to ask the firmware if
the backlight is controlled though the EC and this the new WMI interface
should be used; or if it is directly controlled by the GPU and the GPU
driver's native backlight support should be used.

There are several bugs in the implementation of this on various laptops:

1. The method to get the backlight control source sometimes does not
report the correct value.

2. On some laptop models which method (native-gpu vs nvidia-wmi) works
changes at runtime, e.g. when plugging in a charger.

Known affected laptop models/bug reports about this:
- Acer Predator PH315-55: needs acpi_backlight=native
- Dell G15 5515 with RTX 3050: *needs* acpi_backlight=native
- Dell G15 5515 with RTX 3060: *breaks* with acpi_backlight=native
- Lenovo Legion Slim 7 2021
- https://bugzilla.kernel.org/show_bug.cgi?id=217026
- https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/work_items/784
- https://gitlab.freedesktop.org/drm/amd/-/issues/4512
- https://bugzilla.suse.com/show_bug.cgi?id=1217750

It turns out that under Windows both a WMI backlight driver and a GPU
native backlight driver get installed and Windows simply always calls
both when the backlight changes working around these kind of firmware
bugs.

When this first came up, about 2 years ago, I came up with the below
plan to fix this. Nvidia was supposed to work in implementing this,
but so far an implementation of this plan has not happened.

Note I do not have time to work on this myself. I'm posting this here
in the hope that either Nvidia will pick this up after all; or that
someone else can make this happen.

I'm more then happy to help answering any questions which may come up
while implementing this; and to review the resulting patches.


The plan
========

1. Modify the core backlight code to allow registering a backlight-device
for in kernel use only, disabling the registering of a class device under
/sys/class/backlight .

2. Add a new backlight-aggregate.c backlight driver, which exports a
devm_backlight_aggregate_register_or_add() function. Drivers can call
this passing in an existing backlight-device (with its sysfs interface
disabled). This either creates a singleton /sys/class/backlight/aggregate
device, or adds the passed in backlight to the existing singleton instance
if it already exists.

This new driver always exposes a range of 0-255 to userspace. This acts
as a proxie for any backlight-devices registered to be part of
the aggregate, forwarding any brightness set calls to all backlights,
scaled to their actual range. For reads this will report the last set
brightness value (starting with the value of the first registered
backlight-device).

3. Add a new nvidia_wmi_ec_and_native type to drivers/acpi/video_detect.c
for both DMI quirks and commandline handling.

4. Modify acpi_video_backlight_use_native() to also return true if
the __acpi_video_get_backlight_type() call there returns the new
acpi_backlight_nvidia_wmi_ec_and_native.

5. Add a new devm_backlight_device_register_native() helper which
calls __acpi_video_get_backlight_type(true, NULL) and if that returns
the new nvidia_wmi_ec_and_native type then registers the passed in
backlight with the flag to not register a sysfs class interface and
then on success calls the new devm_backlight_aggregate_register_or_add()
with the just registered backlight device; and if the returned type
instead is acpi_backlight_native register the passed in backlight
normally through devm_backlight_device_register()

5. Modify the i915 and amdgpu drivers to use the new
devm_backlight_device_register_native() helper. Since this new helper
checks the backlight-type itself, drop acpi_video_backlight_use_native()
checks *if* the registration is the only thing guarded by that check.

6. drivers/platform/x86/nvidia-wmi-ec-backlight.c to not return
early from probe when acpi_video_get_backlight_type() returns
the new nvidia_wmi_ec_and_native type and for that type make it
registers its backlight with the flag to not register a sysfs class
interface and then on success calls the new
devm_backlight_aggregate_register_or_add().

Regards,

Hans