emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
internal buffer, and this callback is useful only for such a purpose.
+The callback function may return 0 or a negative error. When the
+return value is ``-EPIPE``, PCM core treats as a buffer XRUN happens,
+and changes the state to ``SNDRV_PCM_STATE_XRUN`` automatically.
+
This callback is atomic as default.
page callback