spi-gpio: allow operation without CS signal

Change spi-gpio so that it is possible to drive SPI communications over
GPIO without the need for a chipselect signal.

This is useful in very small setups where there's only one slave device
on the bus.

This patch does not affect existing setups.

I use this for a tiny communication channel between an embedded device and
a microcontroller. There are not enough GPIOs available for chipselect
and it's not needed anyway in this case.

Signed-off-by: Michael Buesch <[e-mail]>
Cc: David Brownell <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/spi/spi_gpio.c

···
 	if (is_active)
 		setsck(spi, spi->mode & SPI_CPOL);
 
-	/* SPI is normally active-low */
-	gpio_set_value(cs, (spi->mode & SPI_CS_HIGH) ? is_active : !is_active);
+	if (cs != SPI_GPIO_NO_CHIPSELECT) {
+		/* SPI is normally active-low */
+		gpio_set_value(cs, (spi->mode & SPI_CS_HIGH) ? is_active : !is_active);
+	}
 }
 
 static int spi_gpio_setup(struct spi_device *spi)
···
 		return -EINVAL;
 
 	if (!spi->controller_state) {
-		status = gpio_request(cs, dev_name(&spi->dev));
-		if (status)
-			return status;
-		status = gpio_direction_output(cs, spi->mode & SPI_CS_HIGH);
+		if (cs != SPI_GPIO_NO_CHIPSELECT) {
+			status = gpio_request(cs, dev_name(&spi->dev));
+			if (status)
+				return status;
+			status = gpio_direction_output(cs, spi->mode & SPI_CS_HIGH);
+		}
 	}
 	if (!status)
 		status = spi_bitbang_setup(spi);
 	if (status) {
-		if (!spi->controller_state)
+		if (!spi->controller_state && cs != SPI_GPIO_NO_CHIPSELECT)
 			gpio_free(cs);
 	}
 	return status;
···
 {
 	unsigned long	cs = (unsigned long) spi->controller_data;
 
-	gpio_free(cs);
+	if (cs != SPI_GPIO_NO_CHIPSELECT)
+		gpio_free(cs);
 	spi_bitbang_cleanup(spi);
 }
 

include/linux/spi/spi_gpio.h

···
  *	...
  *	};
  *
+ * If chipselect is not used (there's only one device on the bus), assign
+ * SPI_GPIO_NO_CHIPSELECT to the controller_data:
+ *		.controller_data = (void *) SPI_GPIO_NO_CHIPSELECT;
+ *
  * If the bitbanged bus is later switched to a "native" controller,
  * that platform_device and controller_data should be removed.
  */
+
+#define SPI_GPIO_NO_CHIPSELECT		((unsigned long)-1l)
 
 /**
  * struct spi_gpio_platform_data - parameter for bitbanged SPI master