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 d2866c2..26bd03e 100644
--- a/drivers/spi/spi_gpio.c
+++ b/drivers/spi/spi_gpio.c
@@ -178,8 +178,10 @@
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 @@
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 @@
{
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 0f01a0f..ca6782e 100644
--- 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
