GPIOMMC - Driver for an MMC/SD card on a bitbanging GPIO SPI bus
================================================================

The gpiommc module hooks up the mmc_spi and spi_gpio modules for running an
MMC or SD card on GPIO pins.

Two interfaces for registering a new MMC/SD card device are provided:
A static platform-device based mechanism and a dynamic configfs based interface.


Registering devices via platform-device
=======================================

The platform-device interface is used for registering MMC/SD devices that are
part of the hardware platform. This is most useful only for embedded machines
with MMC/SD devices statically connected to the platform GPIO bus.

The data structures are declared in <linux/mmc/gpiommc.h>.

To register a new device, define an instance of struct gpiommc_platform_data.
This structure holds any information about how the device is hooked up to the
GPIO pins and what hardware modes the device supports. See the docbook-style
documentation in the header file for more information on the struct fields.

Then allocate a new instance of a platform device by doing:

	pdev = platform_device_alloc(GPIOMMC_PLATDEV_NAME, gpiommc_next_id());

This will allocate the platform device data structures and hook it up to the
gpiommc driver.
Then add the gpiommc_platform_data to the platform device.

	err = platform_device_add_data(pdev, pdata, sizeof(struct gpiommc_platform_data));

You may free the local instance of struct gpiommc_platform_data now. (So the
struct may be allocated on the stack, too).
Now simply register the platform device.

	err = platform_device_add(pdev);

Done. The gpiommc probe routine will be invoked now and you should see a kernel
log message for the added device.


Registering devices via configfs
================================

MMC/SD cards connected via GPIO often are a pretty dynamic thing, as for example
selfmade hacks for soldering an MMC/SD card to standard GPIO pins on embedded
hardware are a common situation.
So we provide a dynamic interface to conveniently handle adding and removing
devices from userspace, without the need to recompile the kernel.

The "gpiommc" subdirectory at the configfs mountpoint is used for handling
the dynamic configuration.

To create a new device, it must first be allocated with mkdir.
The following command will allocate a device named "my_mmc":
	mkdir /config/gpiommc/my_mmc

There are several configuration files available in the new
/config/gpiommc/my_mmc/ directory:

gpio_data_in			= The SPI data-IN GPIO pin number.
gpio_data_out			= The SPI data-OUT GPIO pin number.
gpio_clock			= The SPI Clock GPIO pin number.
gpio_chipselect			= The SPI Chipselect GPIO pin number.
gpio_chipselect_activelow	= Boolean. If 0, Chipselect is active-HIGH.
				  If 1, Chipselect is active-LOW.
spi_mode			= The SPI data mode. Can be 0-3.
spi_delay			= Enable all delays in the lowlevel bitbanging.
max_bus_speed			= The maximum SPI bus speed. In Hertz.

register			= Not a configuration parameter.
				  Used to register the configured card
				  with the kernel.

The device must first get configured and then registered by writing "1" to
the "register" file.
The configuration parameters "gpio_data_in", "gpio_data_out", "gpio_clock"
and "gpio_chipselect" are essential and _must_ be configured before writing
"1" to the "register" file. The registration will fail, otherwise.

The default values for the other parameters are:
gpio_chipselect_activelow	= 1		(CS active-LOW)
spi_mode			= 0		(SPI_MODE_0)
spi_delay			= 1		(enabled)
max_bus_speed			= 5000000	(5 Mhz)

Configuration values can not be changed after registration. To unregister
the device, write a "0" to the "register" file. The configuration can be
changed again after unregistering.

To completely remove the device, simply rmdir the directory
(/config/gpiommc/my_mmc in this example).
There's no need to first unregister the device before removing it. That will
be done automatically.