drm/nouveau: Register ACPI video backlight when nv_backlight registration fails (v2)

Typically the acpi_video driver will initialize before nouveau, which
used to cause /sys/class/backlight/acpi_video0 to get registered and then
nouveau would register its own nv_backlight device later. After which
the drivers/acpi/video_detect.c code unregistered the acpi_video0 device
to avoid there being 2 backlight devices.

This means that userspace used to briefly see 2 devices and the
disappearing of acpi_video0 after a brief time confuses the systemd
backlight level save/restore code, see e.g.:
https://bbs.archlinux.org/viewtopic.php?id=269920

To fix this the ACPI video code has been modified to make backlight class
device registration a separate step, relying on the drm/kms driver to
ask for the acpi_video backlight registration after it is done setting up
its native backlight device.

Add a call to the new acpi_video_register_backlight() when native backlight
device registration has failed / was skipped to ensure that there is a
backlight device available before the drm_device gets registered with
userspace.

Changes in v2:
- Add nouveau_acpi_video_register_backlight() wrapper to avoid unresolved
  symbol errors on non X86

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/gpu/drm/nouveau/nouveau_acpi.c b/drivers/gpu/drm/nouveau/nouveau_acpi.c
index 1592c9cd7750..8cf096f841a9 100644
--- a/drivers/gpu/drm/nouveau/nouveau_acpi.c
+++ b/drivers/gpu/drm/nouveau/nouveau_acpi.c
@@ -391,3 +391,8 @@ bool nouveau_acpi_video_backlight_use_native(void)
 {
 	return acpi_video_backlight_use_native();
 }
+
+void nouveau_acpi_video_register_backlight(void)
+{
+	acpi_video_register_backlight();
+}