spi: dw: Clear DMAC register when done or stopped

If DMAC register is left uncleared any further DMAless transfers
may cause the DMAC hardware handshaking interface getting activated.
So the next DMA-based Rx/Tx transaction will be started right
after the dma_async_issue_pending() method is invoked even if no
DMATDLR/DMARDLR conditions are met. This at the same time may cause
the Tx/Rx FIFO buffers underrun/overrun. In order to fix this we
must clear DMAC register after a current DMA-based transaction is
finished.

Co-developed-by: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Signed-off-by: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200515104758.6934-4-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw-mid.c

@@ -106,6 +106,8 @@ static void mid_spi_dma_exit(struct dw_spi *dws)
 		dmaengine_terminate_sync(dws->rxchan);
 		dma_release_channel(dws->rxchan);
 	}
+
+	dw_writel(dws, DW_SPI_DMACR, 0);
 }
 
 static irqreturn_t dma_transfer(struct dw_spi *dws)
@@ -152,6 +154,8 @@ static void dw_spi_dma_tx_done(void *arg)
 	clear_bit(TX_BUSY, &dws->dma_chan_busy);
 	if (test_bit(RX_BUSY, &dws->dma_chan_busy))
 		return;
+
+	dw_writel(dws, DW_SPI_DMACR, 0);
 	spi_finalize_current_transfer(dws->master);
 }
 
@@ -199,6 +203,8 @@ static void dw_spi_dma_rx_done(void *arg)
 	clear_bit(RX_BUSY, &dws->dma_chan_busy);
 	if (test_bit(TX_BUSY, &dws->dma_chan_busy))
 		return;
+
+	dw_writel(dws, DW_SPI_DMACR, 0);
 	spi_finalize_current_transfer(dws->master);
 }
 
@@ -292,6 +298,8 @@ static void mid_spi_dma_stop(struct dw_spi *dws)
 		dmaengine_terminate_sync(dws->rxchan);
 		clear_bit(RX_BUSY, &dws->dma_chan_busy);
 	}
+
+	dw_writel(dws, DW_SPI_DMACR, 0);
 }
 
 static const struct dw_spi_dma_ops mfld_dma_ops = {