I don't want ajax submit / callback validate form '#required' => TRUE,
'#limit_validation_errors' => array();

$form['actions']['previous'] = array(
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Previous'),
    '#limit_validation_errors' =&gt; array(
      // Validate $form_state-&gt;getValue('step1').
      array('step1'),
      // Validate $form_state-&gt;getValue(array('foo', 'bar')).
      array('foo', 'bar'),
    ),
    '#submit' =&gt; array('some_submit_function'), // #submit required.
  );