spi: lantiq: fix: Rx overflow error in full duplex mode

In full duplex mode, rx overflow error is observed. To overcome the error,
wait until the complete data got received and proceed further.

Fixes: 17f84b793c ("spi: lantiq-ssc: add support for Lantiq SSC SPI controller")
Signed-off-by: Dilip Kota <eswara.kota@linux.intel.com>
Link: https://lore.kernel.org/r/efb650b0faa49a00788c4e0ca8ef7196bdba851d.1594957019.git.eswara.kota@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-lantiq-ssc.c

@@ -183,6 +183,7 @@ struct lantiq_ssc_spi {
 	unsigned int			tx_fifo_size;
 	unsigned int			rx_fifo_size;
 	unsigned int			base_cs;
+	unsigned int			fdx_tx_level;
 };
 
 static u32 lantiq_ssc_readl(const struct lantiq_ssc_spi *spi, u32 reg)
@@ -480,6 +481,7 @@ static void tx_fifo_write(struct lantiq_ssc_spi *spi)
 	u32 data;
 	unsigned int tx_free = tx_fifo_free(spi);
 
+	spi->fdx_tx_level = 0;
 	while (spi->tx_todo && tx_free) {
 		switch (spi->bits_per_word) {
 		case 2 ... 8:
@@ -508,6 +510,7 @@ static void tx_fifo_write(struct lantiq_ssc_spi *spi)
 
 		lantiq_ssc_writel(spi, data, LTQ_SPI_TB);
 		tx_free--;
+		spi->fdx_tx_level++;
 	}
 }
 
@@ -519,6 +522,13 @@ static void rx_fifo_read_full_duplex(struct lantiq_ssc_spi *spi)
 	u32 data;
 	unsigned int rx_fill = rx_fifo_level(spi);
 
+	/*
+	 * Wait until all expected data to be shifted in.
+	 * Otherwise, rx overrun may occur.
+	 */
+	while (rx_fill != spi->fdx_tx_level)
+		rx_fill = rx_fifo_level(spi);
+
 	while (rx_fill) {
 		data = lantiq_ssc_readl(spi, LTQ_SPI_RB);