drm/radeon: add a module parameter for backlight control (v2)

Add a module parameter to disable the radeon GPU backlight
controller to override the automatic detection.  Some
laptops seems to indicate that they use the integrated
controller, but appear to actually use an external
controller.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=81382

v2: fix module parameter description
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon.h

@@ -106,6 +106,7 @@ extern int radeon_vm_block_size;
 extern int radeon_deep_color;
 extern int radeon_use_pflipirq;
 extern int radeon_bapm;
+extern int radeon_backlight;
 
 /*
  * Copy from radeon_drv.h so we don't have to include both and have conflicting

drivers/gpu/drm/radeon/radeon_drv.c

@@ -181,6 +181,7 @@ int radeon_vm_block_size = -1;
 int radeon_deep_color = 0;
 int radeon_use_pflipirq = 2;
 int radeon_bapm = -1;
+int radeon_backlight = -1;
 
 MODULE_PARM_DESC(no_wb, "Disable AGP writeback for scratch registers");
 module_param_named(no_wb, radeon_no_wb, int, 0444);
@@ -263,6 +264,9 @@ module_param_named(use_pflipirq, radeon_use_pflipirq, int, 0444);
 MODULE_PARM_DESC(bapm, "BAPM support (1 = enable, 0 = disable, -1 = auto)");
 module_param_named(bapm, radeon_bapm, int, 0444);
 
+MODULE_PARM_DESC(backlight, "backlight support (1 = enable, 0 = disable, -1 = auto)");
+module_param_named(backlight, radeon_backlight, int, 0444);
+
 static struct pci_device_id pciidlist[] = {
 	radeon_PCI_IDS
 };

drivers/gpu/drm/radeon/radeon_encoders.c

@@ -158,10 +158,36 @@ radeon_get_encoder_enum(struct drm_device *dev, uint32_t supported_device, uint8
 	return ret;
 }
 
+static void radeon_encoder_add_backlight(struct radeon_encoder *radeon_encoder,
+					 struct drm_connector *connector)
+{
+	struct drm_device *dev = radeon_encoder->base.dev;
+	struct radeon_device *rdev = dev->dev_private;
+	bool use_bl = false;
+
+	if (!(radeon_encoder->devices & (ATOM_DEVICE_LCD_SUPPORT)))
+		return;
+
+	if (radeon_backlight == 0) {
+		return;
+	} else if (radeon_backlight == 1) {
+		use_bl = true;
+	} else if (radeon_backlight == -1) {
+		use_bl = true;
+	}
+
+	if (use_bl) {
+		if (rdev->is_atom_bios)
+			radeon_atom_backlight_init(radeon_encoder, connector);
+		else
+			radeon_legacy_backlight_init(radeon_encoder, connector);
+		rdev->mode_info.bl_encoder = radeon_encoder;
+	}
+}
+
 void
 radeon_link_encoder_connector(struct drm_device *dev)
 {
-	struct radeon_device *rdev = dev->dev_private;
 	struct drm_connector *connector;
 	struct radeon_connector *radeon_connector;
 	struct drm_encoder *encoder;
@@ -174,13 +200,8 @@ radeon_link_encoder_connector(struct drm_device *dev)
 			radeon_encoder = to_radeon_encoder(encoder);
 			if (radeon_encoder->devices & radeon_connector->devices) {
 				drm_mode_connector_attach_encoder(connector, encoder);
-				if (radeon_encoder->devices & (ATOM_DEVICE_LCD_SUPPORT)) {
-					if (rdev->is_atom_bios)
-						radeon_atom_backlight_init(radeon_encoder, connector);
-					else
-						radeon_legacy_backlight_init(radeon_encoder, connector);
-					rdev->mode_info.bl_encoder = radeon_encoder;
-				}
+				if (radeon_encoder->devices & (ATOM_DEVICE_LCD_SUPPORT))
+					radeon_encoder_add_backlight(radeon_encoder, connector);
 			}
 		}
 	}