[PATCH] rtc: ds1343: Do not hardcode SPI mode flags

[Linus Walleij <linus.walleij@linaro.org>]

The current use of mode flags to us SPI_MODE_3 and
SPI_CS_HIGH is fragile: it overwrites anything already
assigned by the SPI core. Change it thusly:

- Just |= the SPI_MODE_3 so we keep other flags
- Assign ^= SPI_CS_HIGH since we might be active high
  already, and that is usually the case with GPIOs used
  for chip select, even if they are in practice active low.

Add a comment clarifying why ^= SPI_CS_HIGH is the right
choice here.

Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/rtc/rtc-ds1343.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/rtc/rtc-ds1343.c b/drivers/rtc/rtc-ds1343.c
index fa6de31d5793..105a2fd93583 100644
--- a/drivers/rtc/rtc-ds1343.c
+++ b/drivers/rtc/rtc-ds1343.c
@@ -484,9 +484,12 @@ static int ds1343_probe(struct spi_device *spi)
 	mutex_init(&priv->mutex);
 
 	/* RTC DS1347 works in spi mode 3 and
-	 * its chip select is active high
+	 * its chip select is active high. Active high should be defined as
+	 * "inverse polarity" as GPIO-based chip selects can be logically
+	 * active high but inverted by the GPIO library.
 	 */
-	spi->mode = SPI_MODE_3 | SPI_CS_HIGH;
+	spi->mode |= SPI_MODE_3;
+	spi->mode ^= SPI_CS_HIGH;
 	spi->bits_per_word = 8;
 	res = spi_setup(spi);
 	if (res)
-- 
2.23.0