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