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[1/4] spi: a3700: Clear DATA_OUT when performing a read

Message ID 1516205728-3256-2-git-send-email-maxime.chevallier@smile.fr (mailing list archive)
State Accepted
Commit 44a5f423e70374e5b42cecd85e78f2d79334e0f2
Headers show

Commit Message

Maxime Chevallier Jan. 17, 2018, 4:15 p.m. UTC 
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>
---
 drivers/spi/spi-armada-3700.c | 5 +++++
 1 file changed, 5 insertions(+)
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Patch

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);