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

[Ralf Schlatterbeck]

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.

Signed-off-by: Ralf Schlatterbeck <rsc at runtux.com>
Signed-off-by: Mirko Vogt <mirko-dev|linux at nanl.de>
---
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).

 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..2db075c87f51 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 */
+	sun6i_spi_write(sspi, SUN6I_GBL_CTL_REG,
+			sun6i_spi_read(sspi, SUN6I_GBL_CTL_REG)
+			| SUN6I_GBL_CTL_BUS_ENABLE);
 
 	/* 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