[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 14:09, Rafael J. Wysocki wrote:
> On Thu, Oct 27, 2022 at 12:37 PM Hans de Goede <hdegoede at redhat.com> wrote:
>>
>> 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).
> 
> Sounds reasonable to me, FWIW.
> 
> And IIUC the check above can be overridden by passing
> acpi_backlight=native in the kernel command line, right?

Right, that can be used as a quick workaround, but we really do
want this to work OOTB everywhere.

Regards,

Hans