diff mbox series

[v2] spi: gpio: No MISO does not imply no RX

Message ID 20180909222805.20805-1-linus.walleij@linaro.org
State Accepted
Commit abf5feef3ff0cefade0c76be53b59e55fdd46093
Headers show
Series [v2] spi: gpio: No MISO does not imply no RX | expand

Commit Message

Linus Walleij Sept. 9, 2018, 10:28 p.m. UTC 
There is a logical problem in spi-gpio with host just
assigning a MOSI line and no MISO: this is interpreted
as the host cannot do RX and the host is flagged with
SPI_MASTER_NO_RX.

This is wrong: since GPIO lines can switch direction,
in 3WIRE operation the host will simply reverse the
direction of the GPIO line and start reading from it,
there is even code for doing this in the driver, but
it went unnoticed because it was tested by using a
master with 4 wires but a device using just 3 wires.

Remove the offending flag.

Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

---
ChangeLog v1->v2:
- Totally changed approach on how to handle this
  3wire proble, one simple patch: instead of trying
  to interpret 3wire as "can't do TX or RX for
  certain transfers" we interpret 3wire setup with
  just MOSI as both TX and RX capable.
---
 drivers/spi/spi-gpio.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

-- 
2.17.1
diff mbox series

Patch

diff --git a/drivers/spi/spi-gpio.c b/drivers/spi/spi-gpio.c
index 421bfc7dda67..088772ebef9b 100644
--- a/drivers/spi/spi-gpio.c
+++ b/drivers/spi/spi-gpio.c
@@ -295,9 +295,11 @@  static int spi_gpio_request(struct device *dev,
 	spi_gpio->miso = devm_gpiod_get_optional(dev, "miso", GPIOD_IN);
 	if (IS_ERR(spi_gpio->miso))
 		return PTR_ERR(spi_gpio->miso);
-	if (!spi_gpio->miso)
-		/* HW configuration without MISO pin */
-		*mflags |= SPI_MASTER_NO_RX;
+	/*
+	 * No setting SPI_MASTER_NO_RX here - if there is only a MOSI
+	 * pin connected the host can still do RX by changing the
+	 * direction of the line.
+	 */
 
 	spi_gpio->sck = devm_gpiod_get(dev, "sck", GPIOD_OUT_LOW);
 	if (IS_ERR(spi_gpio->sck))
@@ -423,7 +425,7 @@  static int spi_gpio_probe(struct platform_device *pdev)
 	spi_gpio->bitbang.chipselect = spi_gpio_chipselect;
 	spi_gpio->bitbang.set_line_direction = spi_gpio_set_direction;
 
-	if ((master_flags & (SPI_MASTER_NO_TX | SPI_MASTER_NO_RX)) == 0) {
+	if ((master_flags & SPI_MASTER_NO_TX) == 0) {
 		spi_gpio->bitbang.txrx_word[SPI_MODE_0] = spi_gpio_txrx_word_mode0;
 		spi_gpio->bitbang.txrx_word[SPI_MODE_1] = spi_gpio_txrx_word_mode1;
 		spi_gpio->bitbang.txrx_word[SPI_MODE_2] = spi_gpio_txrx_word_mode2;