[PATCH AUTOSEL for 4.14 038/161] spi: a3700: Clear DATA_OUT when performing a read

Sasha Levin
2018-04-09T00:22:57Z

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>
---
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 fe3fa1e8517a..4903f15177cf 100644
--- 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 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);
--
2.15.1