According to description about "Transmit Data Level",

This bit field controls the level at which a DMA request
is made by the transmit logic.

It is equal to the watermark level.

That is, the dma_tx_req signal is generated when the number
of valid data entries in the TXFIFO
(TXFIFO0 if CSR=00
 TXFIFO1 if CSR=01
 TXFIFO2 if CSR=10
 TXFIFO3 if CSR=11)
is equal to or below this field value.

Different to receive data level, transmit data level does not need
to "-1".
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>