spi_gpio driver
Generalize the old at91rm9200 "bootstrap" bitbanging SPI master driver as
"spi_gpio", so it works with arbitrary GPIOs and can be configured through
platform_data. Such SPI masters support:
- any number of bus instances (bus_num is the platform_device.id)
- any number of chipselects (one GPIO per spi_device)
- all four SPI_MODE values, and SPI_CS_HIGH
- i/o word sizes from 1 to 32 bits;
- devices configured as with any other spi_master controller
When configured using platform_data, this provides relatively low clock
rates. On platforms that support inlined GPIO calls, significantly
improved transfer speeds are also possible with a semi-custom driver.
(It's still painful when accessing flash memory, but less so.)
Sanity checked by using this version to replace both native controllers on
a board with six different SPI slaves, relying on three different
SPI_MODE_* values and both SPI_CS_HIGH settings for correct operation.
[akpm@linux-foundation.org: cleanups]
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Magnus Damm <damm@igel.co.jp>
Tested-by: Magnus Damm <damm@igel.co.jp>
Cc: Torgil Svensson <torgil.svensson@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/spi/spi_gpio.h b/include/linux/spi/spi_gpio.h
new file mode 100644
index 0000000..0f01a0f
--- /dev/null
+++ b/include/linux/spi/spi_gpio.h
@@ -0,0 +1,60 @@
+#ifndef __LINUX_SPI_GPIO_H
+#define __LINUX_SPI_GPIO_H
+
+/*
+ * For each bitbanged SPI bus, set up a platform_device node with:
+ * - name "spi_gpio"
+ * - id the same as the SPI bus number it implements
+ * - dev.platform data pointing to a struct spi_gpio_platform_data
+ *
+ * Or, see the driver code for information about speedups that are
+ * possible on platforms that support inlined access for GPIOs (no
+ * spi_gpio_platform_data is used).
+ *
+ * Use spi_board_info with these busses in the usual way, being sure
+ * that the controller_data being the GPIO used for each device's
+ * chipselect:
+ *
+ * static struct spi_board_info ... [] = {
+ * ...
+ * // this slave uses GPIO 42 for its chipselect
+ * .controller_data = (void *) 42,
+ * ...
+ * // this one uses GPIO 86 for its chipselect
+ * .controller_data = (void *) 86,
+ * ...
+ * };
+ *
+ * If the bitbanged bus is later switched to a "native" controller,
+ * that platform_device and controller_data should be removed.
+ */
+
+/**
+ * struct spi_gpio_platform_data - parameter for bitbanged SPI master
+ * @sck: number of the GPIO used for clock output
+ * @mosi: number of the GPIO used for Master Output, Slave In (MOSI) data
+ * @miso: number of the GPIO used for Master Input, Slave Output (MISO) data
+ * @num_chipselect: how many slaves to allow
+ *
+ * All GPIO signals used with the SPI bus managed through this driver
+ * (chipselects, MOSI, MISO, SCK) must be configured as GPIOs, instead
+ * of some alternate function.
+ *
+ * It can be convenient to use this driver with pins that have alternate
+ * functions associated with a "native" SPI controller if a driver for that
+ * controller is not available, or is missing important functionality.
+ *
+ * On platforms which can do so, configure MISO with a weak pullup unless
+ * there's an external pullup on that signal. That saves power by avoiding
+ * floating signals. (A weak pulldown would save power too, but many
+ * drivers expect to see all-ones data as the no slave "response".)
+ */
+struct spi_gpio_platform_data {
+ unsigned sck;
+ unsigned mosi;
+ unsigned miso;
+
+ u16 num_chipselect;
+};
+
+#endif /* __LINUX_SPI_GPIO_H */