[PATCH 4.9 14/33] drm/radeon: Fix backlight control on iMac 12,1

[Greg Kroah-Hartman]

From: Nicholas Bishop <nicholasbishop@xxxxxxxxxx>

commit 364438fd629f7611a84c8e6d7de91659300f1502 upstream.

The iMac 12,1 does not use the gmux driver for backlight, so the radeon
backlight device is needed to set the brightness.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1838
Signed-off-by: Nicholas Bishop <nicholasbishop@xxxxxxxxxx>
Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/radeon/atombios_encoders.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/radeon/atombios_encoders.c
+++ b/drivers/gpu/drm/radeon/atombios_encoders.c
@@ -192,7 +192,8 @@ void radeon_atom_backlight_init(struct r
 	 * so don't register a backlight device
 	 */
 	if ((rdev->pdev->subsystem_vendor == PCI_VENDOR_ID_APPLE) &&
-	    (rdev->pdev->device == 0x6741))
+	    (rdev->pdev->device == 0x6741) &&
+	    !dmi_match(DMI_PRODUCT_NAME, "iMac12,1"))
 		return;
 
 	if (!radeon_encoder->enc_priv)