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From: Linus Walleij <linus.walleij@linaro.org>
To: linux-mmc@vger.kernel.org, Ulf Hansson <ulf.hansson@linaro.org>
Cc: Linus Walleij <linus.walleij@linaro.org>,
	Phil Elwell <phil@raspberrypi.org>,
	Mark Brown <broonie@kernel.org>
Subject: [PATCH] mmc: spi: Toggle SPI polarity, do not hardcode it
Date: Wed,  4 Dec 2019 16:27:49 +0100
Message-ID: <20191204152749.12652-1-linus.walleij@linaro.org> (raw)

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@raspberrypi.org>
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/mmc/host/mmc_spi.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/mmc/host/mmc_spi.c b/drivers/mmc/host/mmc_spi.c
index 74c6cfbf9172..1f02f54f09c0 100644
--- a/drivers/mmc/host/mmc_spi.c
+++ b/drivers/mmc/host/mmc_spi.c
@@ -1134,17 +1134,22 @@ static void mmc_spi_initsequence(struct mmc_spi_host *host)
 	 * 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,
-- 
2.23.0


             reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-04 15:27 Linus Walleij [this message]
2019-12-04 17:20 ` Mark Brown
2019-12-10  9:51 ` Ulf Hansson
2019-12-10 23:10   ` Linus Walleij
2019-12-16 12:09     ` Ulf Hansson

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