spi: spi-cadence: Interleave write of TX and read of RX FIFO

When working in slave mode it seems the timing is exceedingly tight.
The TX FIFO can never empty, because the master is driving the clock so
zeros would be sent for those bytes where the FIFO is empty.

Return to interleaving the writing of the TX FIFO and the reading
of the RX FIFO to try to ensure the data is available when required.

Fixes: a84c11e ("spi: spi-cadence: Avoid read of RX FIFO before its ready")
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230518093927.711358-1-ckeepax@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: charleskeepax

drivers/spi/spi-cadence.c

@@ -12,6 +12,7 @@
 #include <linux/gpio/consumer.h>
 #include <linux/interrupt.h>
 #include <linux/io.h>
+#include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/of_irq.h>
 #include <linux/of_address.h>
@@ -301,47 +302,43 @@ static int cdns_spi_setup_transfer(struct spi_device *spi,
 }
 
 /**
- * cdns_spi_fill_tx_fifo - Fills the TX FIFO with as many bytes as possible
+ * cdns_spi_process_fifo - Fills the TX FIFO, and drain the RX FIFO
  * @xspi:	Pointer to the cdns_spi structure
+ * @ntx:	Number of bytes to pack into the TX FIFO
+ * @nrx:	Number of bytes to drain from the RX FIFO
  */
-static void cdns_spi_fill_tx_fifo(struct cdns_spi *xspi, unsigned int avail)
+static void cdns_spi_process_fifo(struct cdns_spi *xspi, int ntx, int nrx)
 {
-	unsigned long trans_cnt = 0;
+	ntx = clamp(ntx, 0, xspi->tx_bytes);
+	nrx = clamp(nrx, 0, xspi->rx_bytes);
 
-	while ((trans_cnt < avail) && (xspi->tx_bytes > 0)) {
+	xspi->tx_bytes -= ntx;
+	xspi->rx_bytes -= nrx;
+
+	while (ntx || nrx) {
 		/* When xspi in busy condition, bytes may send failed,
 		 * then spi control did't work thoroughly, add one byte delay
 		 */
-		if (cdns_spi_read(xspi, CDNS_SPI_ISR) &
-		    CDNS_SPI_IXR_TXFULL)
+		if (cdns_spi_read(xspi, CDNS_SPI_ISR) & CDNS_SPI_IXR_TXFULL)
 			udelay(10);
 
-		if (xspi->txbuf)
-			cdns_spi_write(xspi, CDNS_SPI_TXD, *xspi->txbuf++);
-		else
-			cdns_spi_write(xspi, CDNS_SPI_TXD, 0);
+		if (ntx) {
+			if (xspi->txbuf)
+				cdns_spi_write(xspi, CDNS_SPI_TXD, *xspi->txbuf++);
+			else
+				cdns_spi_write(xspi, CDNS_SPI_TXD, 0);
 
-		xspi->tx_bytes--;
-		trans_cnt++;
-	}
-}
+			ntx--;
+		}
 
-/**
- * cdns_spi_read_rx_fifo - Reads the RX FIFO with as many bytes as possible
- * @xspi:       Pointer to the cdns_spi structure
- * @count:	Read byte count
- */
-static void cdns_spi_read_rx_fifo(struct cdns_spi *xspi, unsigned long count)
-{
-	u8 data;
-
-	/* Read out the data from the RX FIFO */
-	while (count > 0) {
-		data = cdns_spi_read(xspi, CDNS_SPI_RXD);
-		if (xspi->rxbuf)
-			*xspi->rxbuf++ = data;
-		xspi->rx_bytes--;
-		count--;
+		if (nrx) {
+			u8 data = cdns_spi_read(xspi, CDNS_SPI_RXD);
+
+			if (xspi->rxbuf)
+				*xspi->rxbuf++ = data;
+
+			nrx--;
+		}
 	}
 }
 
@@ -391,11 +388,10 @@ static irqreturn_t cdns_spi_irq(int irq, void *dev_id)
 		if (xspi->tx_bytes < xspi->tx_fifo_depth >> 1)
 			cdns_spi_write(xspi, CDNS_SPI_THLD, 1);
 
-		cdns_spi_read_rx_fifo(xspi, trans_cnt);
-
 		if (xspi->tx_bytes) {
-			cdns_spi_fill_tx_fifo(xspi, trans_cnt);
+			cdns_spi_process_fifo(xspi, trans_cnt, trans_cnt);
 		} else {
+			cdns_spi_process_fifo(xspi, 0, trans_cnt);
 			cdns_spi_write(xspi, CDNS_SPI_IDR,
 				       CDNS_SPI_IXR_DEFAULT);
 			spi_finalize_current_transfer(ctlr);
@@ -448,7 +444,7 @@ static int cdns_transfer_one(struct spi_controller *ctlr,
 			cdns_spi_write(xspi, CDNS_SPI_THLD, xspi->tx_fifo_depth >> 1);
 	}
 
-	cdns_spi_fill_tx_fifo(xspi, xspi->tx_fifo_depth);
+	cdns_spi_process_fifo(xspi, xspi->tx_fifo_depth, 0);
 	spi_transfer_delay_exec(transfer);
 
 	cdns_spi_write(xspi, CDNS_SPI_IER, CDNS_SPI_IXR_DEFAULT);