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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Oct 27, 2022 at 12:37 PM Hans de Goede <hdegoede@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?