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>

drivers/spi/Kconfig

@@ -78,7 +78,7 @@ config SPI_AU1550
 	  will be called au1550_spi.
 
 config SPI_BITBANG
-	tristate "Bitbanging SPI master"
+	tristate "Utilities for Bitbanging SPI masters"
 	help
 	  With a few GPIO pins, your system can bitbang the SPI protocol.
 	  Select this to get SPI support through I/O pins (GPIO, parallel
@@ -100,6 +100,22 @@ config SPI_BUTTERFLY
 	  inexpensive battery powered microcontroller evaluation board.
 	  This same cable can be used to flash new firmware.
 
+config SPI_GPIO
+	tristate "GPIO-based bitbanging SPI Master"
+	depends on GENERIC_GPIO
+	select SPI_BITBANG
+	help
+	  This simple GPIO bitbanging SPI master uses the arch-neutral GPIO
+	  interface to manage MOSI, MISO, SCK, and chipselect signals.  SPI
+	  slaves connected to a bus using this driver are configured as usual,
+	  except that the spi_board_info.controller_data holds the GPIO number
+	  for the chipselect used by this controller driver.
+
+	  Note that this driver often won't achieve even 1 Mbit/sec speeds,
+	  making it unusually slow for SPI.  If your platform can inline
+	  GPIO operations, you should be able to leverage that for better
+	  speed with a custom version of this driver; see the source code.
+
 config SPI_IMX
 	tristate "Freescale iMX SPI controller"
 	depends on ARCH_IMX && EXPERIMENTAL

drivers/spi/Makefile

@@ -16,6 +16,7 @@ obj-$(CONFIG_SPI_BFIN)			+= spi_bfin5xx.o
 obj-$(CONFIG_SPI_BITBANG)		+= spi_bitbang.o
 obj-$(CONFIG_SPI_AU1550)		+= au1550_spi.o
 obj-$(CONFIG_SPI_BUTTERFLY)		+= spi_butterfly.o
+obj-$(CONFIG_SPI_GPIO)			+= spi_gpio.o
 obj-$(CONFIG_SPI_IMX)			+= spi_imx.o
 obj-$(CONFIG_SPI_LM70_LLP)		+= spi_lm70llp.o
 obj-$(CONFIG_SPI_PXA2XX)		+= pxa2xx_spi.o

include/linux/spi/spi_gpio.h

+#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 */