drm: sti: clear all mixer control

Make sure that mixer control register is correctly reset
before use it.
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>

drivers/gpu/drm/sti/sti_drm_crtc.c

@@ -38,6 +38,8 @@ static void sti_drm_crtc_prepare(struct drm_crtc *crtc)
 		if (clk_prepare_enable(compo->clk_compo_aux))
 			DRM_INFO("Failed to prepare/enable compo_aux clk\n");
 	}
+
+	sti_mixer_clear_all_layers(mixer);
 }
 
 static void sti_drm_crtc_commit(struct drm_crtc *crtc)

drivers/gpu/drm/sti/sti_mixer.c

@@ -215,6 +215,15 @@ int sti_mixer_set_layer_status(struct sti_mixer *mixer,
 	return 0;
 }
 
+void sti_mixer_clear_all_layers(struct sti_mixer *mixer)
+{
+	u32 val;
+
+	DRM_DEBUG_DRIVER("%s clear all layer\n", sti_mixer_to_str(mixer));
+	val = sti_mixer_reg_read(mixer, GAM_MIXER_CTL) & 0xFFFF0000;
+	sti_mixer_reg_write(mixer, GAM_MIXER_CTL, val);
+}
+
 void sti_mixer_set_matrix(struct sti_mixer *mixer)
 {
 	unsigned int i;

drivers/gpu/drm/sti/sti_mixer.h

@@ -39,6 +39,7 @@ struct sti_mixer *sti_mixer_create(struct device *dev, int id,
 
 int sti_mixer_set_layer_status(struct sti_mixer *mixer,
 		struct sti_layer *layer, bool status);
+void sti_mixer_clear_all_layers(struct sti_mixer *mixer);
 int sti_mixer_set_layer_depth(struct sti_mixer *mixer, struct sti_layer *layer);
 int sti_mixer_active_video_area(struct sti_mixer *mixer,
 		struct drm_display_mode *mode);