[PATCH v2 1/1] spi: spi-sun6i: Fix chipselect/clock bug

[Ralf Schlatterbeck <rsc@xxxxxxxxxx>]

From: Mirko Vogt <mirko-dev|linux@xxxxxxx>

The current sun6i SPI implementation initializes the transfer too early,
resulting in SCK going high before the transfer. When using an additional
(gpio) chipselect with sun6i, the chipselect is asserted at a time when
clock is high, making the SPI transfer fail.

This is due to SUN6I_GBL_CTL_BUS_ENABLE being written into
SUN6I_GBL_CTL_REG at an early stage. Moving that to the transfer
function, hence, right before the transfer starts, mitigates that
problem.

Fixes: 3558fe900e8af (spi: sunxi: Add Allwinner A31 SPI controller driver)
Signed-off-by: Mirko Vogt <mirko-dev|linux@xxxxxxx>
Signed-off-by: Ralf Schlatterbeck <rsc@xxxxxxxxxx>
---
For oscilloscope screenshots with/without the patch, see my blog post
https://blog.runtux.com/posts/2019/04/18/
or the discussion in the armbian forum at
https://forum.armbian.com/topic/4330-spi-gpio-chip-select-support/
(my logo there is a penguin).
As far as we can tell the driver never worked with gpio chipselects.
History:
Updated patch with suggested readability-improvements by Andre Przywara

 drivers/spi/spi-sun6i.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi-sun6i.c b/drivers/spi/spi-sun6i.c
index cc8401980125..23ad052528db 100644
--- a/drivers/spi/spi-sun6i.c
+++ b/drivers/spi/spi-sun6i.c
@@ -379,6 +379,10 @@ static int sun6i_spi_transfer_one(struct spi_master *master,
 	}
 
 	sun6i_spi_write(sspi, SUN6I_CLK_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)
@@ -504,7 +508,7 @@ static int sun6i_spi_runtime_resume(struct device *dev)
 	}
 
 	sun6i_spi_write(sspi, SUN6I_GBL_CTL_REG,
-			SUN6I_GBL_CTL_BUS_ENABLE | SUN6I_GBL_CTL_MASTER | SUN6I_GBL_CTL_TP);
+			SUN6I_GBL_CTL_MASTER | SUN6I_GBL_CTL_TP);
 
 	return 0;
 
-- 
2.20.1