[PATCH] staging: fbtft: Do not hardcode SPI CS polarity inversion

Linus Walleij linus.walleij at linaro.org
Wed Dec 4 23:32:30 UTC 2019


The current use of the mode flag SPI_CS_HIGH is fragile: it
overwrites anything already assigned by the SPI core.

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 at kernel.org>
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
---
 drivers/staging/fbtft/fb_uc1611.c    | 12 +++++++++---
 drivers/staging/fbtft/fb_watterott.c | 13 ++++++++++---
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/staging/fbtft/fb_uc1611.c b/drivers/staging/fbtft/fb_uc1611.c
index e763205e9e4f..f61e373c75e9 100644
--- a/drivers/staging/fbtft/fb_uc1611.c
+++ b/drivers/staging/fbtft/fb_uc1611.c
@@ -63,11 +63,17 @@ static int init_display(struct fbtft_par *par)
 {
 	int ret;
 
-	/* Set CS active high */
-	par->spi->mode |= SPI_CS_HIGH;
+	/*
+	 * Set CS active inverse polarity: just setting SPI_CS_HIGH does not
+	 * work with GPIO based chip selects that are logically active high
+	 * but inverted inside the GPIO library, so enforce inverted
+	 * semantics.
+	 */
+	par->spi->mode ^= SPI_CS_HIGH;
 	ret = spi_setup(par->spi);
 	if (ret) {
-		dev_err(par->info->device, "Could not set SPI_CS_HIGH\n");
+		dev_err(par->info->device,
+			"Could not set inverse CS polarity\n");
 		return ret;
 	}
 
diff --git a/drivers/staging/fbtft/fb_watterott.c b/drivers/staging/fbtft/fb_watterott.c
index 27cc8eabcbe9..76b25df376b8 100644
--- a/drivers/staging/fbtft/fb_watterott.c
+++ b/drivers/staging/fbtft/fb_watterott.c
@@ -150,10 +150,17 @@ static int init_display(struct fbtft_par *par)
 
 	/* enable SPI interface by having CS and MOSI low during reset */
 	save_mode = par->spi->mode;
-	par->spi->mode |= SPI_CS_HIGH;
-	ret = spi_setup(par->spi); /* set CS inactive low */
+	/*
+	 * Set CS active inverse polarity: just setting SPI_CS_HIGH does not
+	 * work with GPIO based chip selects that are logically active high
+	 * but inverted inside the GPIO library, so enforce inverted
+	 * semantics.
+	 */
+	par->spi->mode ^= SPI_CS_HIGH;
+	ret = spi_setup(par->spi);
 	if (ret) {
-		dev_err(par->info->device, "Could not set SPI_CS_HIGH\n");
+		dev_err(par->info->device,
+			"Could not set inverse CS polarity\n");
 		return ret;
 	}
 	write_reg(par, 0x00); /* make sure mode is set */
-- 
2.23.0



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