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>

diff --git a/drivers/spi/spi_gpio.c b/drivers/spi/spi_gpio.c
index d2866c293dee0a5f160e5342755ffc8be525f351..26bd03e61855fba412726f23b638066a90abcaa9 100644 (file)
--- a/drivers/spi/spi_gpio.c
+++ b/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);
 
-       /* 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)
@@ -191,15 +193,17 @@ 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;
@@ -209,7 +213,8 @@ static void spi_gpio_cleanup(struct spi_device *spi)
 {
        unsigned long   cs = (unsigned long) spi->controller_data;
 
-       gpio_free(cs);
+       if (cs != SPI_GPIO_NO_CHIPSELECT)
+               gpio_free(cs);
        spi_bitbang_cleanup(spi);
 }
 

diff --git a/include/linux/spi/spi_gpio.h b/include/linux/spi/spi_gpio.h
index 0f01a0f1f40c9718a67dfc08f2561a0c13521fc7..ca6782ee4b9fce4f9208020e552129db6f0fbd5a 100644 (file)
--- a/include/linux/spi/spi_gpio.h
+++ b/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