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Subject[PATCH 5.2 56/56] spi: bcm2835: Fix 3-wire mode if DMA is enabled
From: Lukas Wunner <>

commit 8d8bef50365847134b51c1ec46786bc2873e4e47 upstream.

Commit 6935224da248 ("spi: bcm2835: enable support of 3-wire mode")
added 3-wire support to the BCM2835 SPI driver by setting the REN bit
(Read Enable) in the CS register when receiving data. The REN bit puts
the transmitter in high-impedance state. The driver recognizes that
data is to be received by checking whether the rx_buf of a transfer is

Commit 3ecd37edaa2a ("spi: bcm2835: enable dma modes for transfers
meeting certain conditions") subsequently broke 3-wire support because
it set the SPI_MASTER_MUST_RX flag which causes spi_map_msg() to replace
rx_buf with a dummy buffer if it is NULL. As a result, rx_buf is
*always* non-NULL if DMA is enabled.

Reinstate 3-wire support by not only checking whether rx_buf is non-NULL,
but also checking that it is not the dummy buffer.

Fixes: 3ecd37edaa2a ("spi: bcm2835: enable dma modes for transfers meeting certain conditions")
Reported-by: Nuno Sá <>
Signed-off-by: Lukas Wunner <>
Cc: # v4.2+
Cc: Martin Sperl <>
Acked-by: Stefan Wahren <>
Signed-off-by: Mark Brown <>
Signed-off-by: Greg Kroah-Hartman <>

drivers/spi/spi-bcm2835.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -764,7 +764,8 @@ static int bcm2835_spi_transfer_one(stru
bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);

/* handle all the 3-wire mode */
- if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
+ if (spi->mode & SPI_3WIRE && tfr->rx_buf &&
+ tfr->rx_buf != master->dummy_rx)
cs |= BCM2835_SPI_CS_REN;
cs &= ~BCM2835_SPI_CS_REN;

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