[PATCH 18/30] spi: dw: Refactor IRQ-based SPI transfer procedure

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

Current IRQ-based SPI transfer execution procedure doesn't work well at
the final stage of the execution. If all the Tx data is sent out (written
to the Tx FIFO) but there is some data left to receive, the Tx FIFO Empty
IRQ will constantly happen until all of the requested inbound data is
received. Though for a short period of time, but it will make the system
less responsive. In order to fix that let's refactor the SPI transfer
execution procedure by taking the Rx FIFO Full IRQ into account. We'll read
and write SPI transfer data each time the IRQ happens as before. If all
the outbound data is sent out, we'll disable the Tx FIFO Empty IRQ. If
there is still some data to receive, we'll adjust the Rx FIFO Threshold
level, so the next IRQ would be raised at the moment of all incoming data
being available in the Rx FIFO.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
---
 drivers/spi/spi-dw-core.c | 33 ++++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/drivers/spi/spi-dw-core.c b/drivers/spi/spi-dw-core.c
index 84c1fdfd6e52..682463b2f68b 100644
--- a/drivers/spi/spi-dw-core.c
+++ b/drivers/spi/spi-dw-core.c
@@ -189,17 +189,30 @@ static irqreturn_t interrupt_transfer(struct dw_spi *dws)
 		return IRQ_HANDLED;
 	}
 
+	/*
+	 * Read data from the Rx FIFO every time we've got a chance executing
+	 * this method. If there is nothing left to receive, terminate the
+	 * procedure. Otherwise adjust the Rx FIFO Threshold level if it's a
+	 * final stage of the transfer. By doing so we'll get the next IRQ
+	 * right when the leftover incoming data is received.
+	 */
 	dw_reader(dws);
 	if (!dws->rx_len) {
 		spi_mask_intr(dws, 0xff);
 		spi_finalize_current_transfer(dws->master);
-		return IRQ_HANDLED;
+	} else if (dws->rx_len <= dw_readl(dws, DW_SPI_RXFTLR)) {
+		dw_writel(dws, DW_SPI_RXFTLR, dws->rx_len - 1);
 	}
+
+	/*
+	 * Send data out if Tx FIFO Empty IRQ is received. The IRQ will be
+	 * disabled after the data transmission is finished so not to
+	 * have the TXE IRQ flood at the final stage of the transfer.
+	 */
 	if (irq_status & SPI_INT_TXEI) {
-		spi_mask_intr(dws, SPI_INT_TXEI);
 		dw_writer(dws);
-		/* Enable TX irq always, it will be disabled when RX finished */
-		spi_umask_intr(dws, SPI_INT_TXEI);
+		if (!dws->tx_len)
+			spi_mask_intr(dws, SPI_INT_TXEI);
 	}
 
 	return IRQ_HANDLED;
@@ -317,10 +330,6 @@ static int dw_spi_transfer_one(struct spi_controller *master,
 	/* For poll mode just disable all interrupts */
 	spi_mask_intr(dws, 0xff);
 
-	/*
-	 * Interrupt mode
-	 * we only need set the TXEI IRQ, as TX/RX always happen syncronizely
-	 */
 	if (dws->dma_mapped) {
 		ret = dws->dma_ops->dma_setup(dws, transfer);
 		if (ret < 0) {
@@ -328,12 +337,18 @@ static int dw_spi_transfer_one(struct spi_controller *master,
 			return ret;
 		}
 	} else {
+		/*
+		 * Originally Tx and Rx data lengths match. Rx FIFO Threshold level
+		 * will be adjusted at the final stage of the IRQ-based SPI transfer
+		 * execution so not to lose the leftover of the incoming data.
+		 */
 		txlevel = min_t(u16, dws->fifo_len / 2, dws->tx_len);
 		dw_writel(dws, DW_SPI_TXFTLR, txlevel);
+		dw_writel(dws, DW_SPI_RXFTLR, txlevel - 1);
 
 		/* Set the interrupt mask */
 		imask |= SPI_INT_TXEI | SPI_INT_TXOI |
-			 SPI_INT_RXUI | SPI_INT_RXOI;
+			 SPI_INT_RXUI | SPI_INT_RXOI | SPI_INT_RXFI;
 		spi_umask_intr(dws, imask);
 
 		dws->transfer_handler = interrupt_transfer;
-- 
2.27.0