linux-kernel - [PATCH 1/4] spi: a3700: Clear DATA

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Message-Id: <1516205728-3256-2-git-send-email-maxime.chevallier@smile.fr>
Date:   Wed, 17 Jan 2018 17:15:25 +0100
From:   Maxime Chevallier <maxime.chevallier@...le.fr>
To:     broonie@...nel.org
Cc:     linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        gregory.clement@...e-electrons.com,
        Maxime Chevallier <maxime.chevallier@...le.fr>
Subject: [PATCH 1/4] spi: a3700: Clear DATA_OUT when performing a read

When performing a read using FIFO mode, the spi controller shifts out
the last 2 bytes that were written in a previous transfer on MOSI.

This undocumented behaviour can cause devices to misinterpret the
transfer, so we explicitly clear the WFIFO before each read.

This behaviour was noticed on EspressoBin.

Signed-off-by: Maxime Chevallier <maxime.chevallier@...le.fr>
---
 drivers/spi/spi-armada-3700.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/spi/spi-armada-3700.c b/drivers/spi/spi-armada-3700.c
index d653453..c11ea6c 100644
--- a/drivers/spi/spi-armada-3700.c
+++ b/drivers/spi/spi-armada-3700.c
@@ -615,6 +615,11 @@ static int a3700_spi_transfer_one(struct spi_master *master,
 	a3700_spi_header_set(a3700_spi);
 
 	if (xfer->rx_buf) {
+		/* Clear WFIFO, since it's last 2 bytes are shifted out during
+		 * a read operation
+		 */
+		spireg_write(a3700_spi, A3700_SPI_DATA_OUT_REG, 0);
+
 		/* Set read data length */
 		spireg_write(a3700_spi, A3700_SPI_IF_DIN_CNT_REG,
 			     a3700_spi->buf_len);
-- 
2.1.4