[PATCH] spi: spi-bcm2835.c: Fix 3-wire mode

["Nuno Sá" <nuno.sa@analog.com>]

As stated in
https://www.raspberrypi.org/documentation/hardware/raspberrypi/spi/README.md,
one of rx or tx buffer's must be null. However, if DMA is enabled, the
driver sets the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX on the
controller flags. Hence, the spi core will provide dummy buffers even if
one of the buffers was set to null by the device driver. Thus, the
communication with the 3-wire device fails.

This patch uses the prepare_message callback to look for the device mode
and sets/clears the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX on a
per spi message basis. It also assumes that DMA is not supported on
half-duplex devices.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
---
 drivers/spi/spi-bcm2835.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 6f243a90c844..8993a15a4684 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -674,6 +674,10 @@ static bool bcm2835_spi_can_dma(struct spi_controller *ctlr,
 	if (tfr->len < BCM2835_SPI_DMA_MIN_LENGTH)
 		return false;
 
+	/* Do not DMA on half-duplex devices */
+	if (spi->mode & SPI_3WIRE)
+		return false;
+
 	/* return OK */
 	return true;
 }
@@ -902,6 +906,15 @@ static int bcm2835_spi_prepare_message(struct spi_controller *ctlr,
 		cs |= BCM2835_SPI_CS_CPOL;
 	if (spi->mode & SPI_CPHA)
 		cs |= BCM2835_SPI_CS_CPHA;
+	/*
+	 * For half-duplex devices, one of tx or rx buffers must be null
+	 * for one spi transfer. Hence, we need to clear the spi controller
+	 * flags so that we don't get dummy buffers...
+	 */
+	if (spi->mode & SPI_3WIRE)
+		master->flags &= ~(SPI_MASTER_MUST_RX | SPI_MASTER_MUST_TX);
+	else if (master->can_dma)
+		master->flags |= (SPI_MASTER_MUST_RX | SPI_MASTER_MUST_TX);
 
 	bcm2835_wr(bs, BCM2835_SPI_CS, cs);
 
-- 
2.22.0