[PATCH v2 2/2] spi: sun6i: Set SPI mode in prepare_message

[Kevin Mehall]

With a GPIO chip select, CS is asserted before entering transfer_one.
The spi-sun6i driver previously configured the SPI mode (including clock
polarity) and enabled the bus in transfer_one, which can cause an
extraneous SCK transition with CS asserted, corrupting the transferred
data.

This patch moves the SPI mode configuration and bus enable to the
spi_prepare_message callback, ensuring that SCK is driven to the correct
level prior to asserting CS.

A previous fix for a related issue (0d7993b234c9f) was incomplete in
that it only delayed enabling the SCK output drive to prevent it from
being driven at the wrong level when resuming from autosuspend, but
didn't help if switching CPOL modes between chip selects while active,
or if SCK floats to the opposite level when suspended.

Fixes: 0d7993b234c9 ("spi: spi-sun6i: Fix chipselect/clock bug")
Signed-off-by: Kevin Mehall <km at kevinmehall.net>
---
 drivers/spi/spi-sun6i.c | 67 +++++++++++++++++++++++++----------------
 1 file changed, 41 insertions(+), 26 deletions(-)

diff --git a/drivers/spi/spi-sun6i.c b/drivers/spi/spi-sun6i.c
index fc228574ed38..983e791e3396 100644
--- a/drivers/spi/spi-sun6i.c
+++ b/drivers/spi/spi-sun6i.c
@@ -205,6 +205,44 @@ static size_t sun6i_spi_max_transfer_size(struct spi_device *spi)
 	return SUN6I_MAX_XFER_SIZE - 1;
 }
 
+static int sun6i_spi_prepare_message(struct spi_controller *ctlr,
+				     struct spi_message *msg)
+{
+	struct sun6i_spi *sspi = spi_controller_get_devdata(ctlr);
+	struct spi_device *spi = msg->spi;
+	u32 reg;
+
+	/* Set the mode bits in the transfer control register */
+	reg = sun6i_spi_read(sspi, SUN6I_TFR_CTL_REG);
+
+	if (spi->mode & SPI_CPOL)
+		reg |= SUN6I_TFR_CTL_CPOL;
+	else
+		reg &= ~SUN6I_TFR_CTL_CPOL;
+
+	if (spi->mode & SPI_CPHA)
+		reg |= SUN6I_TFR_CTL_CPHA;
+	else
+		reg &= ~SUN6I_TFR_CTL_CPHA;
+
+	if (spi->mode & SPI_LSB_FIRST)
+		reg |= SUN6I_TFR_CTL_FBS;
+	else
+		reg &= ~SUN6I_TFR_CTL_FBS;
+
+	sun6i_spi_write(sspi, SUN6I_TFR_CTL_REG, reg);
+
+	/*
+	 * Now that the clock polarity is configured, enable the bus if the
+	 * controller was previously suspended.
+	 */
+	reg = sun6i_spi_read(sspi, SUN6I_GBL_CTL_REG);
+	reg |= SUN6I_GBL_CTL_BUS_ENABLE;
+	sun6i_spi_write(sspi, SUN6I_GBL_CTL_REG, reg);
+
+	return 0;
+}
+
 static void sun6i_spi_dma_rx_cb(void *param)
 {
 	struct sun6i_spi *sspi = param;
@@ -336,31 +374,12 @@ static int sun6i_spi_transfer_one(struct spi_controller *host,
 
 	sun6i_spi_write(sspi, SUN6I_FIFO_CTL_REG, reg);
 
-	/*
-	 * Setup the transfer control register: Chip Select,
-	 * polarities, etc.
-	 */
-	reg = sun6i_spi_read(sspi, SUN6I_TFR_CTL_REG);
-
-	if (spi->mode & SPI_CPOL)
-		reg |= SUN6I_TFR_CTL_CPOL;
-	else
-		reg &= ~SUN6I_TFR_CTL_CPOL;
-
-	if (spi->mode & SPI_CPHA)
-		reg |= SUN6I_TFR_CTL_CPHA;
-	else
-		reg &= ~SUN6I_TFR_CTL_CPHA;
-
-	if (spi->mode & SPI_LSB_FIRST)
-		reg |= SUN6I_TFR_CTL_FBS;
-	else
-		reg &= ~SUN6I_TFR_CTL_FBS;
-
 	/*
 	 * If it's a TX only transfer, we don't want to fill the RX
 	 * FIFO with bogus data
 	 */
+	reg = sun6i_spi_read(sspi, SUN6I_TFR_CTL_REG);
+
 	if (sspi->rx_buf) {
 		reg &= ~SUN6I_TFR_CTL_DHB;
 		rx_len = tfr->len;
@@ -429,11 +448,6 @@ static int sun6i_spi_transfer_one(struct spi_controller *host,
 		sun6i_spi_write(sspi, SUN6I_TFR_CTL_REG, reg);
 	}
 
-	/* Finally enable the bus - doing so before might raise SCK to HIGH */
-	reg = sun6i_spi_read(sspi, SUN6I_GBL_CTL_REG);
-	reg |= SUN6I_GBL_CTL_BUS_ENABLE;
-	sun6i_spi_write(sspi, SUN6I_GBL_CTL_REG, reg);
-
 	/* Setup the transfer now... */
 	if (sspi->tx_buf) {
 		tx_len = tfr->len;
@@ -668,6 +682,7 @@ static int sun6i_spi_probe(struct platform_device *pdev)
 	host->max_speed_hz = 100 * 1000 * 1000;
 	host->min_speed_hz = 3 * 1000;
 	host->use_gpio_descriptors = true;
+	host->prepare_message = sun6i_spi_prepare_message;
 	host->set_cs = sun6i_spi_set_cs;
 	host->transfer_one = sun6i_spi_transfer_one;
 	host->num_chipselect = 4;
-- 
2.53.0