drm/amdgpu: Register ACPI video backlight when skipping amdgpu backlight registration

Typically the acpi_video driver will initialize before amdgpu, which
used to cause /sys/class/backlight/acpi_video0 to get registered and then
amdgpu would register its own amdgpu_bl# 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 amdgpu skips
registering its own backlight device because of either the firmware_flags
or the acpi_video_get_backlight_type() return value. This ensures that
if the acpi_video backlight device should be used, it will be available
before the amdgpu drm_device gets registered with userspace.
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/gpu/drm/amd/amdgpu/atombios_encoders.c

@@ -184,11 +184,11 @@ void amdgpu_atombios_encoder_init_backlight(struct amdgpu_encoder *amdgpu_encode
 		return;
 
 	if (!(adev->mode_info.firmware_flags & ATOM_BIOS_INFO_BL_CONTROLLED_BY_GPU))
-		return;
+		goto register_acpi_backlight;
 
 	if (!acpi_video_backlight_use_native()) {
 		drm_info(dev, "Skipping amdgpu atom DIG backlight registration\n");
-		return;
+		goto register_acpi_backlight;
 	}
 
 	pdata = kmalloc(sizeof(struct amdgpu_backlight_privdata), GFP_KERNEL);
@@ -225,6 +225,11 @@ void amdgpu_atombios_encoder_init_backlight(struct amdgpu_encoder *amdgpu_encode
 error:
 	kfree(pdata);
 	return;
+
+register_acpi_backlight:
+	/* Try registering an ACPI video backlight device instead. */
+	acpi_video_register_backlight();
+	return;
 }
 
 void

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4059,6 +4059,8 @@ amdgpu_dm_register_backlight_device(struct amdgpu_display_manager *dm)
 
 	if (!acpi_video_backlight_use_native()) {
 		drm_info(adev_to_drm(dm->adev), "Skipping amdgpu DM backlight registration\n");
+		/* Try registering an ACPI video backlight device instead. */
+		acpi_video_register_backlight();
 		return;
 	}