spi: mediatek: Fix fifo transfer

commit 0d5c395 upstream.

Commit 3a70dd2 ("spi: mediatek: fix fifo rx mode") claims that
fifo RX mode was never handled, and adds the presumably missing code
to the FIFO transfer function. However, the claim that receive data
was not handled is incorrect. It was handled as part of interrupt
handling after the transfer was complete. The code added with the above
mentioned commit reads data from the receive FIFO before the transfer
is started, which is wrong. This results in an actual transfer error
on a Hayato Chromebook.

Remove the code trying to handle receive data before the transfer is
started to fix the problem.

Fixes: 3a70dd2 ("spi: mediatek: fix fifo rx mode")
Cc: Peter Hess <peter.hess@ph-home.de>
Cc: Frank Wunderlich <frank-w@public-files.de>
Cc: Tzung-Bi Shih <tzungbi@google.com>
Cc: Hsin-Yi Wang <hsinyi@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Hsin-Yi Wang <hsinyi@google.com>
Tested-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20210802030023.1748777-1-linux@roeck-us.net
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/spi/spi-mt65xx.c

@@ -433,24 +433,15 @@ static int mtk_spi_fifo_transfer(struct spi_master *master,
 	mtk_spi_prepare_transfer(master, xfer);
 	mtk_spi_setup_packet(master);
 
-	cnt = xfer->len / 4;
-	if (xfer->tx_buf)
+	if (xfer->tx_buf) {
+		cnt = xfer->len / 4;
 		iowrite32_rep(mdata->base + SPI_TX_DATA_REG, xfer->tx_buf, cnt);
-
-	if (xfer->rx_buf)
-		ioread32_rep(mdata->base + SPI_RX_DATA_REG, xfer->rx_buf, cnt);
-
-	remainder = xfer->len % 4;
-	if (remainder > 0) {
-		reg_val = 0;
-		if (xfer->tx_buf) {
+		remainder = xfer->len % 4;
+		if (remainder > 0) {
+			reg_val = 0;
 			memcpy(&reg_val, xfer->tx_buf + (cnt * 4), remainder);
 			writel(reg_val, mdata->base + SPI_TX_DATA_REG);
 		}
-		if (xfer->rx_buf) {
-			reg_val = readl(mdata->base + SPI_RX_DATA_REG);
-			memcpy(xfer->rx_buf + (cnt * 4), &reg_val, remainder);
-		}
 	}
 
 	mtk_spi_enable_transfer(master);