+ /*
+ * When processing the residue, we need to read two registers but we
+ * can't do it in an atomic way. AT_XDMAC_CNDA is used to find where
+ * we stand in the descriptor list and AT_XDMAC_CUBC is used
+ * to know how many data are remaining for the current descriptor.
+ * Since the dma channel is not paused to not loose data, between the
+ * AT_XDMAC_CNDA and AT_XDMAC_CUBC read, we may have change of
+ * descriptor.
+ * For that reason, after reading AT_XDMAC_CUBC, we check if we are
+ * still using the same descriptor by reading a second time
+ * AT_XDMAC_CNDA. If AT_XDMAC_CNDA has changed, it means we have to
+ * read again AT_XDMAC_CUBC.
+ * Memory barriers are used to ensure the read order of the registers.
+ * A max number of retries is set because unlikely it can never ends if
+ * we are transferring a lot of data with small buffers.
+ */