[PATCH v5 02/31] drm/i915: Don't register backlight when another backlight should be used (v2)

[Hans de Goede]

Hi,

On 10/27/22 11:52, Matthew Garrett wrote:
> On Thu, Oct 27, 2022 at 11:39:38AM +0200, Hans de Goede wrote:
> 
>> The *only* behavior which actually is new in 6.1 is the native GPU
>> drivers now doing the equivalent of:
>>
>> 	if (acpi_video_get_backlight_type() != acpi_backlight_native)
>> 		return;
>>
>> In their backlight register paths (i), which is causing the native
>> backlight to disappear on your custom laptop setup and on Chromebooks
>> (with the Chromebooks case being already solved I hope.).
> 
> It's causing the backlight control to vanish on any machine that isn't 
> ((acpi_video || vendor interface) || !acpi). Most machines that fall 
> into that are either weird or Chromebooks or old, but there are machines 
> that fall into that.

I acknowledge that their are machines that fall into this category,
but I expect / hope there to be so few of them that we can just DMI
quirk our way out if this.

I believe the old group to be small because:

1. Generally speaking the "native" control method is usually not
present on the really old (pre ACPI video spec) mobile GPUs.

2. On most old laptops I would still expect there to be a vendor
interface too, and if both get registered standard desktop environments
will prefer the vendor one, so then we need a native DMI quirk to
disable the vendor interface anyways and we already have a bunch of
those, so some laptops in this group are already covered by DMI quirks.

And a fix for the Chromebook case is already in Linus' tree, which
just leaves the weird case, of which there will hopefully be only
a few.

I do share your worry that this might break some machines, but
the only way to really find out is to get this code out there
I'm afraid.

I have just written a blog post asking for people to check if
their laptop might be affected; and to report various details
to me of their laptop is affected:

https://hansdegoede.dreamwidth.org/26548.html

Lets wait and see how this goes. If I get (too) many reports then
I will send a revert of the addition of the:

 	if (acpi_video_get_backlight_type() != acpi_backlight_native)
 		return;

check to the i915 / radeon / amd / nouveau drivers.

(And if I only get a couple of reports I will probably just submit
DMI quirks for the affected models).

Regards,

Hans