drm/ast: Fix modeset failed on DisplayPort

If we switch display and update cursor together, it could lead to
modeset failed because of concurrent access to IO registers.

Add lock protection in DP's edid access to avoid this problem.
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230601004847.1115-1-jammy_huang@aspeedtech.com

drivers/gpu/drm/ast/ast_mode.c

@@ -1647,6 +1647,8 @@ static int ast_dp501_output_init(struct ast_device *ast)
 static int ast_astdp_connector_helper_get_modes(struct drm_connector *connector)
 {
 	void *edid;
+	struct drm_device *dev = connector->dev;
+	struct ast_device *ast = to_ast_device(dev);
 
 	int succ;
 	int count;
@@ -1655,9 +1657,17 @@ static int ast_astdp_connector_helper_get_modes(struct drm_connector *connector)
 	if (!edid)
 		goto err_drm_connector_update_edid_property;
 
+	/*
+	 * Protect access to I/O registers from concurrent modesetting
+	 * by acquiring the I/O-register lock.
+	 */
+	mutex_lock(&ast->ioregs_lock);
+
 	succ = ast_astdp_read_edid(connector->dev, edid);
 	if (succ < 0)
-		goto err_kfree;
+		goto err_mutex_unlock;
+
+	mutex_unlock(&ast->ioregs_lock);
 
 	drm_connector_update_edid_property(connector, edid);
 	count = drm_add_edid_modes(connector, edid);
@@ -1665,7 +1675,8 @@ static int ast_astdp_connector_helper_get_modes(struct drm_connector *connector)
 
 	return count;
 
-err_kfree:
+err_mutex_unlock:
+	mutex_unlock(&ast->ioregs_lock);
 	kfree(edid);
 err_drm_connector_update_edid_property:
 	drm_connector_update_edid_property(connector, NULL);