staging: fbtft: Do not hardcode SPI CS polarity inversion

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@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191204233230.22309-1-linus.walleij@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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;
 	}
 

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 */