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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Maxime Chevallier <maxime.chevallier@smile.fr>,
        Mark Brown <broonie@kernel.org>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.14 052/183] spi: a3700: Clear DATA_OUT when performing a read
Date:   Wed, 25 Apr 2018 12:34:32 +0200
Message-Id: <20180425103244.645304344@linuxfoundation.org>
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4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Maxime Chevallier <maxime.chevallier@smile.fr>


[ Upstream commit 44a5f423e70374e5b42cecd85e78f2d79334e0f2 ]

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@smile.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/spi/spi-armada-3700.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/drivers/spi/spi-armada-3700.c
+++ b/drivers/spi/spi-armada-3700.c
@@ -624,6 +624,11 @@ static int a3700_spi_transfer_one(struct
 	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);