spi: spi-cadence: Reverse the order of interleaved write and read operations

In the existing implementation, when executing interleaved write and read
operations in the ISR for a transfer length greater than the FIFO size,
the TXFIFO write precedes the RXFIFO read. Consequently, the initially
received data in the RXFIFO is pushed out and lost, leading to a failure
in data integrity. To address this issue, reverse the order of interleaved
operations and conduct the RXFIFO read followed by the TXFIFO write.

Fixes: 6afe2ae ("spi: spi-cadence: Interleave write of TX and read of RX FIFO")
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@amd.com>
Link: https://msgid.link/r/20231218090652.18403-1-amit.kumar-mahapatra@amd.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Amit Kumar Mahapatra

drivers/spi/spi-cadence.c

@@ -317,6 +317,15 @@ static void cdns_spi_process_fifo(struct cdns_spi *xspi, int ntx, int nrx)
 	xspi->rx_bytes -= nrx;
 
 	while (ntx || nrx) {
+		if (nrx) {
+			u8 data = cdns_spi_read(xspi, CDNS_SPI_RXD);
+
+			if (xspi->rxbuf)
+				*xspi->rxbuf++ = data;
+
+			nrx--;
+		}
+
 		if (ntx) {
 			if (xspi->txbuf)
 				cdns_spi_write(xspi, CDNS_SPI_TXD, *xspi->txbuf++);
@@ -326,14 +335,6 @@ static void cdns_spi_process_fifo(struct cdns_spi *xspi, int ntx, int nrx)
 			ntx--;
 		}
 
-		if (nrx) {
-			u8 data = cdns_spi_read(xspi, CDNS_SPI_RXD);
-
-			if (xspi->rxbuf)
-				*xspi->rxbuf++ = data;
-
-			nrx--;
-		}
 	}
 }