[PATCH 3.16 19/99] mmc: spi: Toggle SPI polarity, do not hardcode it

[Ben Hutchings]

3.16.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Walleij <linus.walleij@xxxxxxxxxx>

commit af3ed119329cf9690598c5a562d95dfd128e91d6 upstream.

The code in mmc_spi_initsequence() tries to send a burst with
high chipselect and for this reason hardcodes the device into
SPI_CS_HIGH.

This is not good because the SPI_CS_HIGH flag indicates
logical "asserted" CS not always the physical level. In
some cases the signal is inverted in the GPIO library and
in that case SPI_CS_HIGH is already set, and enforcing
SPI_CS_HIGH again will actually drive it low.

Instead of hard-coding this, toggle the polarity so if the
default is LOW it goes high to assert chipselect but if it
is already high then toggle it low instead.

Cc: Phil Elwell <phil@xxxxxxxxxxxxxxx>
Reported-by: Mark Brown <broonie@xxxxxxxxxx>
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20191204152749.12652-1-linus.walleij@xxxxxxxxxx
Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/mmc/host/mmc_spi.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/drivers/mmc/host/mmc_spi.c
+++ b/drivers/mmc/host/mmc_spi.c
@@ -1149,17 +1149,22 @@ static void mmc_spi_initsequence(struct
 	 * SPI protocol.  Another is that when chipselect is released while
 	 * the card returns BUSY status, the clock must issue several cycles
 	 * with chipselect high before the card will stop driving its output.
+	 *
+	 * SPI_CS_HIGH means "asserted" here. In some cases like when using
+	 * GPIOs for chip select, SPI_CS_HIGH is set but this will be logically
+	 * inverted by gpiolib, so if we want to ascertain to drive it high
+	 * we should toggle the default with an XOR as we do here.
 	 */
-	host->spi->mode |= SPI_CS_HIGH;
+	host->spi->mode ^= SPI_CS_HIGH;
 	if (spi_setup(host->spi) != 0) {
 		/* Just warn; most cards work without it. */
 		dev_warn(&host->spi->dev,
 				"can't change chip-select polarity\n");
-		host->spi->mode &= ~SPI_CS_HIGH;
+		host->spi->mode ^= SPI_CS_HIGH;
 	} else {
 		mmc_spi_readbytes(host, 18);
 
-		host->spi->mode &= ~SPI_CS_HIGH;
+		host->spi->mode ^= SPI_CS_HIGH;
 		if (spi_setup(host->spi) != 0) {
 			/* Wot, we can't get the same setup we had before? */
 			dev_err(&host->spi->dev,