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>

1 files changed, 13 insertions, 8 deletions

diff --git a/drivers/spi/spi_gpio.c b/drivers/spi/spi_gpio.c
index d2866c293dee..26bd03e61855 100644
--- 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);
 }