[PATCH] i915: fix backlight configuration issue

[Arnd Bergmann]

The i915 driver can use the backlight subsystem as an option, and usually
selects it when CONFIG_ACPI is set. However it is possible to configure
a kernel with modular backlight classdev support and a built-in i915
driver, which leads to a linker error:

drivers/gpu/drm/i915/display/intel_panel.o: In function `intel_backlight_device_register':
intel_panel.c:(.text+0x2f58): undefined reference to `backlight_device_register'
drivers/gpu/drm/i915/display/intel_panel.o: In function `intel_backlight_device_unregister':
intel_panel.c:(.text+0x2fe4): undefined reference to `backlight_device_unregister'

Add another Kconfig option to ensure the driver only tries to use
the backlight support when it can in fact be linked that way. The
new option is on by default to keep the existing behavior.

This is roughly what other drivers like nouveau do as well.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
I've had this one lying around for a long time, it is still needed
but I am not sure which solution is best here. This version is
probably the least invasive, but it does not solve the bigger
problem around too many 'select' statements in drm
---
 drivers/gpu/drm/i915/Kconfig               | 11 ++++++++++-
 drivers/gpu/drm/i915/display/intel_panel.c |  4 ++--
 drivers/gpu/drm/i915/display/intel_panel.h |  6 +++---
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
index ba9595960bbe..81d956040d18 100644
--- a/drivers/gpu/drm/i915/Kconfig
+++ b/drivers/gpu/drm/i915/Kconfig
@@ -16,7 +16,7 @@ config DRM_I915
 	select IRQ_WORK
 	# i915 depends on ACPI_VIDEO when ACPI is enabled
 	# but for select to work, need to select ACPI_VIDEO's dependencies, ick
-	select BACKLIGHT_CLASS_DEVICE if ACPI
+	select DRM_I915_BACKLIGHT if ACPI
 	select INPUT if ACPI
 	select ACPI_VIDEO if ACPI
 	select ACPI_BUTTON if ACPI
@@ -68,6 +68,15 @@ config DRM_I915_FORCE_PROBE
 
 	  Use "*" to force probe the driver for all known devices.
 
+config DRM_I915_BACKLIGHT
+	tristate "Control backlight support"
+	depends on DRM_I915
+	default DRM_I915
+	select BACKLIGHT_CLASS_DEVICE
+	help
+          Say Y here if you want to control the backlight of your display
+          (e.g. a laptop panel).
+
 config DRM_I915_CAPTURE_ERROR
 	bool "Enable capturing GPU state following a hang"
 	depends on DRM_I915
diff --git a/drivers/gpu/drm/i915/display/intel_panel.c b/drivers/gpu/drm/i915/display/intel_panel.c
index 7b3ec6eb3382..e2fe7a50dcbf 100644
--- a/drivers/gpu/drm/i915/display/intel_panel.c
+++ b/drivers/gpu/drm/i915/display/intel_panel.c
@@ -1203,7 +1203,7 @@ void intel_panel_enable_backlight(const struct intel_crtc_state *crtc_state,
 	mutex_unlock(&dev_priv->backlight_lock);
 }
 
-#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
+#if IS_ENABLED(CONFIG_DRM_I915_BACKLIGHT)
 static u32 intel_panel_get_backlight(struct intel_connector *connector)
 {
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
@@ -1370,7 +1370,7 @@ void intel_backlight_device_unregister(struct intel_connector *connector)
 		panel->backlight.device = NULL;
 	}
 }
-#endif /* CONFIG_BACKLIGHT_CLASS_DEVICE */
+#endif /* CONFIG_DRM_I915_BACKLIGHT */
 
 /*
  * CNP: PWM clock frequency is 19.2 MHz or 24 MHz.
diff --git a/drivers/gpu/drm/i915/display/intel_panel.h b/drivers/gpu/drm/i915/display/intel_panel.h
index cedeea443336..e6e81268b7ed 100644
--- a/drivers/gpu/drm/i915/display/intel_panel.h
+++ b/drivers/gpu/drm/i915/display/intel_panel.h
@@ -49,10 +49,10 @@ intel_panel_edid_fixed_mode(struct intel_connector *connector);
 struct drm_display_mode *
 intel_panel_vbt_fixed_mode(struct intel_connector *connector);
 
-#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
+#if IS_ENABLED(CONFIG_DRM_I915_BACKLIGHT)
 int intel_backlight_device_register(struct intel_connector *connector);
 void intel_backlight_device_unregister(struct intel_connector *connector);
-#else /* CONFIG_BACKLIGHT_CLASS_DEVICE */
+#else /* CONFIG_DRM_I915_BACKLIGHT */
 static inline int intel_backlight_device_register(struct intel_connector *connector)
 {
 	return 0;
@@ -60,6 +60,6 @@ static inline int intel_backlight_device_register(struct intel_connector *connec
 static inline void intel_backlight_device_unregister(struct intel_connector *connector)
 {
 }
-#endif /* CONFIG_BACKLIGHT_CLASS_DEVICE */
+#endif /* CONFIG_DRM_I915_BACKLIGHT */
 
 #endif /* __INTEL_PANEL_H__ */
-- 
2.20.0