drm/dp: Move panel DP AUX backlight support to drm_dp_helper

We were getting a depmod error:
depmod: ERROR: Cycle detected: drm_kms_helper -> drm -> drm_kms_helper

It looks like the rule is that drm_kms_helper can call into drm, but
drm can't call into drm_kms_helper. That means we've got to move the
DP AUX backlight support into drm_dp_helper.

NOTE: as part of this, I didn't try to do any renames of the main
registration function. Even though it's in the drm_dp_helper, it still
feels very parallel to drm_panel_of_backlight().

Fixes: 10f7b40e4f30 ("drm/panel: add basic DP AUX backlight support")
Reported-by: Ville Syrjälä <[e-mail]>
Reported-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Douglas Anderson <[e-mail]>
Reviewed-by: Rajeev Nandan <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712075933.v2.1.I23eb4cc5a680341e7b3e791632a635566fa5806a@changeid

drivers/gpu/drm/drm_dp_helper.c

···
 #include <drm/drm_print.h>
 #include <drm/drm_vblank.h>
 #include <drm/drm_dp_mst_helper.h>
+#include <drm/drm_panel.h>
 
 #include "drm_crtc_helper_internal.h"
+
+struct dp_aux_backlight {
+	struct backlight_device *base;
+	struct drm_dp_aux *aux;
+	struct drm_edp_backlight_info info;
+	bool enabled;
+};
 
 /**
  * DOC: dp helpers
···
 	return 0;
 }
 EXPORT_SYMBOL(drm_edp_backlight_init);
+
+#if IS_BUILTIN(CONFIG_BACKLIGHT_CLASS_DEVICE) || \
+	(IS_MODULE(CONFIG_DRM_KMS_HELPER) && IS_MODULE(CONFIG_BACKLIGHT_CLASS_DEVICE))
+
+static int dp_aux_backlight_update_status(struct backlight_device *bd)
+{
+	struct dp_aux_backlight *bl = bl_get_data(bd);
+	u16 brightness = backlight_get_brightness(bd);
+	int ret = 0;
+
+	if (!backlight_is_blank(bd)) {
+		if (!bl->enabled) {
+			drm_edp_backlight_enable(bl->aux, &bl->info, brightness);
+			bl->enabled = true;
+			return 0;
+		}
+		ret = drm_edp_backlight_set_level(bl->aux, &bl->info, brightness);
+	} else {
+		if (bl->enabled) {
+			drm_edp_backlight_disable(bl->aux, &bl->info);
+			bl->enabled = false;
+		}
+	}
+
+	return ret;
+}
+
+static const struct backlight_ops dp_aux_bl_ops = {
+	.update_status = dp_aux_backlight_update_status,
+};
+
+/**
+ * drm_panel_dp_aux_backlight - create and use DP AUX backlight
+ * @panel: DRM panel
+ * @aux: The DP AUX channel to use
+ *
+ * Use this function to create and handle backlight if your panel
+ * supports backlight control over DP AUX channel using DPCD
+ * registers as per VESA's standard backlight control interface.
+ *
+ * When the panel is enabled backlight will be enabled after a
+ * successful call to &drm_panel_funcs.enable()
+ *
+ * When the panel is disabled backlight will be disabled before the
+ * call to &drm_panel_funcs.disable().
+ *
+ * A typical implementation for a panel driver supporting backlight
+ * control over DP AUX will call this function at probe time.
+ * Backlight will then be handled transparently without requiring
+ * any intervention from the driver.
+ *
+ * drm_panel_dp_aux_backlight() must be called after the call to drm_panel_init().
+ *
+ * Return: 0 on success or a negative error code on failure.
+ */
+int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux)
+{
+	struct dp_aux_backlight *bl;
+	struct backlight_properties props = { 0 };
+	u16 current_level;
+	u8 current_mode;
+	u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE];
+	int ret;
+
+	if (!panel || !panel->dev || !aux)
+		return -EINVAL;
+
+	ret = drm_dp_dpcd_read(aux, DP_EDP_DPCD_REV, edp_dpcd,
+			       EDP_DISPLAY_CTL_CAP_SIZE);
+	if (ret < 0)
+		return ret;
+
+	if (!drm_edp_backlight_supported(edp_dpcd)) {
+		DRM_DEV_INFO(panel->dev, "DP AUX backlight is not supported\n");
+		return 0;
+	}
+
+	bl = devm_kzalloc(panel->dev, sizeof(*bl), GFP_KERNEL);
+	if (!bl)
+		return -ENOMEM;
+
+	bl->aux = aux;
+
+	ret = drm_edp_backlight_init(aux, &bl->info, 0, edp_dpcd,
+				     &current_level, &current_mode);
+	if (ret < 0)
+		return ret;
+
+	props.type = BACKLIGHT_RAW;
+	props.brightness = current_level;
+	props.max_brightness = bl->info.max;
+
+	bl->base = devm_backlight_device_register(panel->dev, "dp_aux_backlight",
+						  panel->dev, bl,
+						  &dp_aux_bl_ops, &props);
+	if (IS_ERR(bl->base))
+		return PTR_ERR(bl->base);
+
+	panel->backlight = bl->base;
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_panel_dp_aux_backlight);
+
+#endif

drivers/gpu/drm/drm_panel.c

···
 #include <linux/module.h>
 
 #include <drm/drm_crtc.h>
-#include <drm/drm_dp_helper.h>
 #include <drm/drm_panel.h>
 #include <drm/drm_print.h>
 
 static DEFINE_MUTEX(panel_lock);
 static LIST_HEAD(panel_list);
-
-struct dp_aux_backlight {
-	struct backlight_device *base;
-	struct drm_dp_aux *aux;
-	struct drm_edp_backlight_info info;
-	bool enabled;
-};
 
 /**
  * DOC: drm panel
···
 	return 0;
 }
 EXPORT_SYMBOL(drm_panel_of_backlight);
-
-static int dp_aux_backlight_update_status(struct backlight_device *bd)
-{
-	struct dp_aux_backlight *bl = bl_get_data(bd);
-	u16 brightness = backlight_get_brightness(bd);
-	int ret = 0;
-
-	if (!backlight_is_blank(bd)) {
-		if (!bl->enabled) {
-			drm_edp_backlight_enable(bl->aux, &bl->info, brightness);
-			bl->enabled = true;
-			return 0;
-		}
-		ret = drm_edp_backlight_set_level(bl->aux, &bl->info, brightness);
-	} else {
-		if (bl->enabled) {
-			drm_edp_backlight_disable(bl->aux, &bl->info);
-			bl->enabled = false;
-		}
-	}
-
-	return ret;
-}
-
-static const struct backlight_ops dp_aux_bl_ops = {
-	.update_status = dp_aux_backlight_update_status,
-};
-
-/**
- * drm_panel_dp_aux_backlight - create and use DP AUX backlight
- * @panel: DRM panel
- * @aux: The DP AUX channel to use
- *
- * Use this function to create and handle backlight if your panel
- * supports backlight control over DP AUX channel using DPCD
- * registers as per VESA's standard backlight control interface.
- *
- * When the panel is enabled backlight will be enabled after a
- * successful call to &drm_panel_funcs.enable()
- *
- * When the panel is disabled backlight will be disabled before the
- * call to &drm_panel_funcs.disable().
- *
- * A typical implementation for a panel driver supporting backlight
- * control over DP AUX will call this function at probe time.
- * Backlight will then be handled transparently without requiring
- * any intervention from the driver.
- *
- * drm_panel_dp_aux_backlight() must be called after the call to drm_panel_init().
- *
- * Return: 0 on success or a negative error code on failure.
- */
-int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux)
-{
-	struct dp_aux_backlight *bl;
-	struct backlight_properties props = { 0 };
-	u16 current_level;
-	u8 current_mode;
-	u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE];
-	int ret;
-
-	if (!panel || !panel->dev || !aux)
-		return -EINVAL;
-
-	ret = drm_dp_dpcd_read(aux, DP_EDP_DPCD_REV, edp_dpcd,
-			       EDP_DISPLAY_CTL_CAP_SIZE);
-	if (ret < 0)
-		return ret;
-
-	if (!drm_edp_backlight_supported(edp_dpcd)) {
-		DRM_DEV_INFO(panel->dev, "DP AUX backlight is not supported\n");
-		return 0;
-	}
-
-	bl = devm_kzalloc(panel->dev, sizeof(*bl), GFP_KERNEL);
-	if (!bl)
-		return -ENOMEM;
-
-	bl->aux = aux;
-
-	ret = drm_edp_backlight_init(aux, &bl->info, 0, edp_dpcd,
-				     &current_level, &current_mode);
-	if (ret < 0)
-		return ret;
-
-	props.type = BACKLIGHT_RAW;
-	props.brightness = current_level;
-	props.max_brightness = bl->info.max;
-
-	bl->base = devm_backlight_device_register(panel->dev, "dp_aux_backlight",
-						  panel->dev, bl,
-						  &dp_aux_bl_ops, &props);
-	if (IS_ERR(bl->base))
-		return PTR_ERR(bl->base);
-
-	panel->backlight = bl->base;
-
-	return 0;
-}
-EXPORT_SYMBOL(drm_panel_dp_aux_backlight);
 #endif
 
 MODULE_AUTHOR("Thierry Reding <[e-mail]>");

include/drm/drm_dp_helper.h

···
 
 struct drm_device;
 struct drm_dp_aux;
+struct drm_panel;
 
 /*
  * Unless otherwise noted, all values are from the DP 1.1a spec.  Note that
···
 int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backlight_info *bl,
 			     u16 level);
 int drm_edp_backlight_disable(struct drm_dp_aux *aux, const struct drm_edp_backlight_info *bl);
+
+#if IS_ENABLED(CONFIG_DRM_KMS_HELPER) && (IS_BUILTIN(CONFIG_BACKLIGHT_CLASS_DEVICE) || \
+	(IS_MODULE(CONFIG_DRM_KMS_HELPER) && IS_MODULE(CONFIG_BACKLIGHT_CLASS_DEVICE)))
+
+int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux);
+
+#else
+
+static inline int drm_panel_dp_aux_backlight(struct drm_panel *panel,
+					     struct drm_dp_aux *aux)
+{
+	return 0;
+}
+
+#endif
 
 #ifdef CONFIG_DRM_DP_CEC
 void drm_dp_cec_irq(struct drm_dp_aux *aux);

include/drm/drm_panel.h

···
 struct device_node;
 struct drm_connector;
 struct drm_device;
-struct drm_dp_aux;
 struct drm_panel;
 struct display_timing;
 
···
 #if IS_ENABLED(CONFIG_DRM_PANEL) && (IS_BUILTIN(CONFIG_BACKLIGHT_CLASS_DEVICE) || \
 	(IS_MODULE(CONFIG_DRM) && IS_MODULE(CONFIG_BACKLIGHT_CLASS_DEVICE)))
 int drm_panel_of_backlight(struct drm_panel *panel);
-int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux);
 #else
 static inline int drm_panel_of_backlight(struct drm_panel *panel)
-{
-	return 0;
-}
-
-static inline int drm_panel_dp_aux_backlight(struct drm_panel *panel,
-					     struct drm_dp_aux *aux)
 {
 	return 0;
 }