linux-kernel - [PATCH 4.9 27/32] spi: bcm2835: Fix 3-wire mode if DMA is enabled

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Date:   Fri,  9 Aug 2019 15:45:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Nuno Sá <nuno.sa@...log.com>,
        Lukas Wunner <lukas@...ner.de>,
        Martin Sperl <kernel@...tin.sperl.org>,
        Stefan Wahren <wahrenst@....net>,
        Mark Brown <broonie@...nel.org>
Subject: [PATCH 4.9 27/32] spi: bcm2835: Fix 3-wire mode if DMA is enabled

From: Lukas Wunner <lukas@...ner.de>

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
non-NULL.

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 <nuno.sa@...log.com>
Signed-off-by: Lukas Wunner <lukas@...ner.de>
Cc: stable@...r.kernel.org # v4.2+
Cc: Martin Sperl <kernel@...tin.sperl.org>
Acked-by: Stefan Wahren <wahrenst@....net>
Link: https://lore.kernel.org/r/328318841455e505370ef8ecad97b646c033dc8a.1562148527.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 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
@@ -554,7 +554,8 @@ 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;
 	else
 		cs &= ~BCM2835_SPI_CS_REN;