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 <mb@bu3sch.de>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi_gpio.c

@@ -178,8 +178,10 @@ static void spi_gpio_chipselect(struct spi_device *spi, int is_active)
 	if (is_active)
 		setsck(spi, spi->mode & SPI_CPOL);
 
+	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)
@@ -191,15 +193,17 @@ static int spi_gpio_setup(struct spi_device *spi)
 		return -EINVAL;
 
 	if (!spi->controller_state) {
+		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;
@@ -209,6 +213,7 @@ static void spi_gpio_cleanup(struct spi_device *spi)
 {
 	unsigned long	cs = (unsigned long) spi->controller_data;
 
+	if (cs != SPI_GPIO_NO_CHIPSELECT)
 		gpio_free(cs);
 	spi_bitbang_cleanup(spi);
 }

include/linux/spi/spi_gpio.h

@@ -25,10 +25,16 @@
  *	...
  *	};
  *
+ * 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
  * @sck: number of the GPIO used for clock output