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spi_gpio driver

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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>
This commit is contained in:
David Brownell 2009-01-06 14:41:41 -08:00 • committed by Linus Torvalds
parent c2bacfc44f
commit d29389de0b
4 changed files with 438 additions and 1 deletions
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60
include/linux/spi/spi_gpio.h Normal file
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#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 */