drm/nouveau: Don't register backlight when another backlight should be used (v2)

Before this commit when we want userspace to use the acpi_video backlight
device we register both the GPU's native backlight device and acpi_video's
firmware acpi_video# backlight device. This relies on userspace preferring
firmware type backlight devices over native ones.

Registering 2 backlight devices for a single display really is
undesirable, don't register the GPU's native backlight device when
another backlight device should be used.

Changes in v2:
- Add nouveau_acpi_video_backlight_use_native() 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>

drivers/gpu/drm/nouveau/nouveau_acpi.c

@@ -386,3 +386,8 @@ nouveau_acpi_edid(struct drm_device *dev, struct drm_connector *connector)
 
 	return kmemdup(edid, EDID_LENGTH, GFP_KERNEL);
 }
+
+bool nouveau_acpi_video_backlight_use_native(void)
+{
+	return acpi_video_backlight_use_native();
+}

drivers/gpu/drm/nouveau/nouveau_acpi.h

@@ -11,6 +11,7 @@ void nouveau_register_dsm_handler(void);
 void nouveau_unregister_dsm_handler(void);
 void nouveau_switcheroo_optimus_dsm(void);
 void *nouveau_acpi_edid(struct drm_device *, struct drm_connector *);
+bool nouveau_acpi_video_backlight_use_native(void);
 #else
 static inline bool nouveau_is_optimus(void) { return false; };
 static inline bool nouveau_is_v1_dsm(void) { return false; };
@@ -18,6 +19,7 @@ static inline void nouveau_register_dsm_handler(void) {}
 static inline void nouveau_unregister_dsm_handler(void) {}
 static inline void nouveau_switcheroo_optimus_dsm(void) {}
 static inline void *nouveau_acpi_edid(struct drm_device *dev, struct drm_connector *connector) { return NULL; }
+static inline bool nouveau_acpi_video_backlight_use_native(void) { return true; }
 #endif
 
 #endif

drivers/gpu/drm/nouveau/nouveau_backlight.c

@@ -38,6 +38,7 @@
 #include "nouveau_reg.h"
 #include "nouveau_encoder.h"
 #include "nouveau_connector.h"
+#include "nouveau_acpi.h"
 
 static struct ida bl_ida;
 #define BL_NAME_SIZE 15 // 12 for name + 2 for digits + 1 for '\0'
@@ -405,6 +406,11 @@ nouveau_backlight_init(struct drm_connector *connector)
 		goto fail_alloc;
 	}
 
+	if (!nouveau_acpi_video_backlight_use_native()) {
+		NV_INFO(drm, "Skipping nv_backlight registration\n");
+		goto fail_alloc;
+	}
+
 	if (!nouveau_get_backlight_name(backlight_name, bl)) {
 		NV_ERROR(drm, "Failed to retrieve a unique name for the backlight interface\n");
 		goto fail_alloc;