drm/ast: Disable planes while switching display modes

The ast HW cursor requires the primary plane and CRTC to display at
a valid mode and format. This is not the case while switching
display modes, which can lead to the screen turing permanently dark.

As a workaround, the ast driver now disables active planes while the
mode or format switch takes place. It also synchronizes with the vertical
refresh to give CRTC and planes some time to catch up on each other.
The active planes planes (primary or cursor) will be re-enabled by
each plane's atomic_update() function.

v3:
	* move the logic into the CRTC's atomic_disable function
v2:
	* move the logic into the commit-tail function
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200914072236.19398-3-tzimmermann@suse.de

drivers/gpu/drm/ast/ast_drv.h

@@ -177,6 +177,8 @@ struct ast_private *ast_device_create(struct drm_driver *drv,
 
 #define AST_IO_MM_OFFSET		(0x380)
 
+#define AST_IO_VGAIR1_VREFRESH		BIT(3)
+
 #define __ast_read(x) \
 static inline u##x ast_read##x(struct ast_private *ast, u32 reg) { \
 u##x val = 0;\

drivers/gpu/drm/ast/ast_mode.c

@@ -514,6 +514,16 @@ static void ast_set_start_address_crt1(struct ast_private *ast,
 
 }
 
+static void ast_wait_for_vretrace(struct ast_private *ast)
+{
+	unsigned long timeout = jiffies + HZ;
+	u8 vgair1;
+
+	do {
+		vgair1 = ast_io_read8(ast, AST_IO_INPUT_STATUS1_READ);
+	} while (!(vgair1 & AST_IO_VGAIR1_VREFRESH) && time_before(jiffies, timeout));
+}
+
 /*
  * Primary plane
  */
@@ -809,7 +819,28 @@ static void
 ast_crtc_helper_atomic_disable(struct drm_crtc *crtc,
 			       struct drm_crtc_state *old_crtc_state)
 {
+	struct drm_device *dev = crtc->dev;
+	struct ast_private *ast = to_ast_private(dev);
+
 	ast_crtc_dpms(crtc, DRM_MODE_DPMS_OFF);
+
+	/*
+	 * HW cursors require the underlying primary plane and CRTC to
+	 * display a valid mode and image. This is not the case during
+	 * full modeset operations. So we temporarily disable any active
+	 * plane, including the HW cursor. Each plane's atomic_update()
+	 * helper will re-enable it if necessary.
+	 *
+	 * We only do this during *full* modesets. It does not affect
+	 * simple pageflips on the planes.
+	 */
+	drm_atomic_helper_disable_planes_on_crtc(old_crtc_state, false);
+
+	/*
+	 * Ensure that no scanout takes place before reprogramming mode
+	 * and format registers.
+	 */
+	ast_wait_for_vretrace(ast);
 }
 
 static const struct drm_crtc_helper_funcs ast_crtc_helper_funcs = {