spi: dw-dma: Fail DMA-based transfer if no Tx-buffer specified

Since commit 46164fde6b ("spi: dw: Fix Rx-only DMA transfers") if DMA
interface is enabled, then Tx-buffer must be available in each SPI
transfer. It's required since in order to activate the incoming data
reception either DMA or CPU must be pushing data out to the SPI bus.
But the DW APB SSI DMA driver code is still left in state as if Tx-buffer
might be optional, which is no longer true. Let's fix it so an error would
be returned if no Tx-buffer detected and DMA Tx would be always
enabled.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920112322.24585-3-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw-dma.c

@@ -262,9 +262,6 @@ dw_spi_dma_prepare_tx(struct dw_spi *dws, struct spi_transfer *xfer)
 	struct dma_slave_config txconf;
 	struct dma_async_tx_descriptor *txdesc;
 
-	if (!xfer->tx_buf)
-		return NULL;
-
 	memset(&txconf, 0, sizeof(txconf));
 	txconf.direction = DMA_MEM_TO_DEV;
 	txconf.dst_addr = dws->dma_addr;
@@ -383,17 +380,19 @@ static struct dma_async_tx_descriptor *dw_spi_dma_prepare_rx(struct dw_spi *dws,
 
 static int dw_spi_dma_setup(struct dw_spi *dws, struct spi_transfer *xfer)
 {
-	u16 imr = 0, dma_ctrl = 0;
+	u16 imr, dma_ctrl;
 
-	if (xfer->tx_buf)
-		dma_ctrl |= SPI_DMA_TDMAE;
+	if (!xfer->tx_buf)
+		return -EINVAL;
+
+	/* Set the DMA handshaking interface */
+	dma_ctrl = SPI_DMA_TDMAE;
 	if (xfer->rx_buf)
 		dma_ctrl |= SPI_DMA_RDMAE;
 	dw_writel(dws, DW_SPI_DMACR, dma_ctrl);
 
 	/* Set the interrupt mask */
-	if (xfer->tx_buf)
-		imr |= SPI_INT_TXOI;
+	imr = SPI_INT_TXOI;
 	if (xfer->rx_buf)
 		imr |= SPI_INT_RXUI | SPI_INT_RXOI;
 	spi_umask_intr(dws, imr);
@@ -412,6 +411,8 @@ static int dw_spi_dma_transfer(struct dw_spi *dws, struct spi_transfer *xfer)
 
 	/* Prepare the TX dma transfer */
 	txdesc = dw_spi_dma_prepare_tx(dws, xfer);
+	if (!txdesc)
+		return -EINVAL;
 
 	/* Prepare the RX dma transfer */
 	rxdesc = dw_spi_dma_prepare_rx(dws, xfer);
@@ -423,17 +424,15 @@ static int dw_spi_dma_transfer(struct dw_spi *dws, struct spi_transfer *xfer)
 		dma_async_issue_pending(dws->rxchan);
 	}
 
-	if (txdesc) {
 	set_bit(TX_BUSY, &dws->dma_chan_busy);
 	dmaengine_submit(txdesc);
 	dma_async_issue_pending(dws->txchan);
-	}
 
 	ret = dw_spi_dma_wait(dws, xfer);
 	if (ret)
 		return ret;
 
-	if (txdesc && dws->master->cur_msg->status == -EINPROGRESS) {
+	if (dws->master->cur_msg->status == -EINPROGRESS) {
 		ret = dw_spi_dma_wait_tx_done(dws, xfer);
 		if (ret)
 			return ret;