Merge remote-tracking branch 'spi/for-5.8' into spi-linus

Author: broonie

drivers/spi/spi-dw-dma.c

@@ -372,8 +372,20 @@ static int dw_spi_dma_setup(struct dw_spi *dws, struct spi_transfer *xfer)
 {
 	u16 imr = 0, dma_ctrl = 0;
 
+	/*
+	 * Having a Rx DMA channel serviced with higher priority than a Tx DMA
+	 * channel might not be enough to provide a well balanced DMA-based
+	 * SPI transfer interface. There might still be moments when the Tx DMA
+	 * channel is occasionally handled faster than the Rx DMA channel.
+	 * That in its turn will eventually cause the SPI Rx FIFO overflow if
+	 * SPI bus speed is high enough to fill the SPI Rx FIFO in before it's
+	 * cleared by the Rx DMA channel. In order to fix the problem the Tx
+	 * DMA activity is intentionally slowed down by limiting the SPI Tx
+	 * FIFO depth with a value twice bigger than the Tx burst length
+	 * calculated earlier by the dw_spi_dma_maxburst_init() method.
+	 */
 	dw_writel(dws, DW_SPI_DMARDLR, dws->rxburst - 1);
-	dw_writel(dws, DW_SPI_DMATDLR, dws->fifo_len - dws->txburst);
+	dw_writel(dws, DW_SPI_DMATDLR, dws->txburst);
 
 	if (xfer->tx_buf)
 		dma_ctrl |= SPI_DMA_TDMAE;