ACPI: video: Stop trying to use vendor backlight control on laptops from after ~2012

There have been 2 separate reports now about a non working
"dell_backlight" device getting registered under /sys/class/backlight
1 report for a Raptor Lake based Dell and 1 report for a Meteor Lake
(development) platform.

On hw from the last 10 years dell-laptop will not register "dell_backlight"
because acpi_video_get_backlight_type() will return acpi_backlight_video
there if called before the GPU/kms driver loads. So it does not matter if
the GPU driver's native backlight is registered after dell-laptop loads.

But it seems that on the latest generation laptops the ACPI tables
no longer contain acpi_video backlight control support which causes
acpi_video_get_backlight_type() to return acpi_backlight_vendor causing
"dell_backlight" to get registered if the dell-laptop module is loaded
before the GPU/kms driver.

Vendor specific backlight control like the "dell_backlight" device is
only necessary on quite old hw (from before acpi_video backlight control
was introduced). Work around "dell_backlight" registering on very new
hw (where acpi_video backlight control seems to be no more) by making
acpi_video_get_backlight_type() return acpi_backlight_none instead
of acpi_backlight_vendor as final fallback when the ACPI tables have
support for Windows 8 or later (laptops from after ~2012).

Suggested-by: Matthew Garrett <mjg59@srcf.ucam.org>
Reported-by: AceLan Kao <acelan.kao@canonical.com>
Closes: https://lore.kernel.org/platform-driver-x86/20230607034331.576623-1-acelan.kao@canonical.com/
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c
index b87783c..eb014c0 100644 (file)
--- a/drivers/acpi/video_detect.c
+++ b/drivers/acpi/video_detect.c
@@ -844,6 +844,27 @@ enum acpi_backlight_type __acpi_video_get_backlight_type(bool native, bool *auto
        if (native_available)
                return acpi_backlight_native;
 
+       /*
+        * The vendor specific BIOS interfaces are only necessary for
+        * laptops from before ~2008.
+        *
+        * For laptops from ~2008 till ~2023 this point is never reached
+        * because on those (video_caps & ACPI_VIDEO_BACKLIGHT) above is true.
+        *
+        * Laptops from after ~2023 no longer support ACPI_VIDEO_BACKLIGHT,
+        * if this point is reached on those, this likely means that
+        * the GPU kms driver which sets native_available has not loaded yet.
+        *
+        * Returning acpi_backlight_vendor in this case is known to sometimes
+        * cause a non working vendor specific /sys/class/backlight device to
+        * get registered.
+        *
+        * Return acpi_backlight_none on laptops with ACPI tables written
+        * for Windows 8 (laptops from after ~2012) to avoid this problem.
+        */
+       if (acpi_osi_is_win8())
+               return acpi_backlight_none;
+
        /* No ACPI video/native (old hw), use vendor specific fw methods. */
        return acpi_backlight_vendor;
 }