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

drivers/spi/spi_gpio.c

+/*
+ * spi_gpio.c - SPI master driver using generic bitbanged GPIO
+ *
+ * Copyright (C) 2006,2008 David Brownell
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/platform_device.h>
+#include <linux/gpio.h>
+
+#include <linux/spi/spi.h>
+#include <linux/spi/spi_bitbang.h>
+#include <linux/spi/spi_gpio.h>
+
+
+/*
+ * This bitbanging SPI master driver should help make systems usable
+ * when a native hardware SPI engine is not available, perhaps because
+ * its driver isn't yet working or because the I/O pins it requires
+ * are used for other purposes.
+ *
+ * platform_device->driver_data ... points to spi_gpio
+ *
+ * spi->controller_state ... reserved for bitbang framework code
+ * spi->controller_data ... holds chipselect GPIO
+ *
+ * spi->master->dev.driver_data ... points to spi_gpio->bitbang
+ */
+
+struct spi_gpio {
+	struct spi_bitbang		bitbang;
+	struct spi_gpio_platform_data	pdata;
+	struct platform_device		*pdev;
+};
+
+/*----------------------------------------------------------------------*/
+
+/*
+ * Because the overhead of going through four GPIO procedure calls
+ * per transferred bit can make performance a problem, this code
+ * is set up so that you can use it in either of two ways:
+ *
+ *   - The slow generic way:  set up platform_data to hold the GPIO
+ *     numbers used for MISO/MOSI/SCK, and issue procedure calls for
+ *     each of them.  This driver can handle several such busses.
+ *
+ *   - The quicker inlined way:  only helps with platform GPIO code
+ *     that inlines operations for constant GPIOs.  This can give
+ *     you tight (fast!) inner loops, but each such bus needs a
+ *     new driver.  You'll define a new C file, with Makefile and
+ *     Kconfig support; the C code can be a total of six lines:
+ *
+ *		#define DRIVER_NAME	"myboard_spi2"
+ *		#define	SPI_MISO_GPIO	119
+ *		#define	SPI_MOSI_GPIO	120
+ *		#define	SPI_SCK_GPIO	121
+ *		#define	SPI_N_CHIPSEL	4
+ *		#include "spi_gpio.c"
+ */
+
+#ifndef DRIVER_NAME
+#define DRIVER_NAME	"spi_gpio"
+
+#define GENERIC_BITBANG	/* vs tight inlines */
+
+/* all functions referencing these symbols must define pdata */
+#define SPI_MISO_GPIO	((pdata)->miso)
+#define SPI_MOSI_GPIO	((pdata)->mosi)
+#define SPI_SCK_GPIO	((pdata)->sck)
+
+#define SPI_N_CHIPSEL	((pdata)->num_chipselect)
+
+#endif
+
+/*----------------------------------------------------------------------*/
+
+static inline const struct spi_gpio_platform_data * __pure
+spi_to_pdata(const struct spi_device *spi)
+{
+	const struct spi_bitbang	*bang;
+	const struct spi_gpio		*spi_gpio;
+
+	bang = spi_master_get_devdata(spi->master);
+	spi_gpio = container_of(bang, struct spi_gpio, bitbang);
+	return &spi_gpio->pdata;
+}
+
+/* this is #defined to avoid unused-variable warnings when inlining */
+#define pdata		spi_to_pdata(spi)
+
+static inline void setsck(const struct spi_device *spi, int is_on)
+{
+	gpio_set_value(SPI_SCK_GPIO, is_on);
+}
+
+static inline void setmosi(const struct spi_device *spi, int is_on)
+{
+	gpio_set_value(SPI_MOSI_GPIO, is_on);
+}
+
+static inline int getmiso(const struct spi_device *spi)
+{
+	return gpio_get_value(SPI_MISO_GPIO);
+}
+
+#undef pdata
+
+/*
+ * NOTE:  this clocks "as fast as we can".  It "should" be a function of the
+ * requested device clock.  Software overhead means we usually have trouble
+ * reaching even one Mbit/sec (except when we can inline bitops), so for now
+ * we'll just assume we never need additional per-bit slowdowns.
+ */
+#define spidelay(nsecs)	do {} while (0)
+
+#define	EXPAND_BITBANG_TXRX
+#include <linux/spi/spi_bitbang.h>
+
+/*
+ * These functions can leverage inline expansion of GPIO calls to shrink
+ * costs for a txrx bit, often by factors of around ten (by instruction
+ * count).  That is particularly visible for larger word sizes, but helps
+ * even with default 8-bit words.
+ *
+ * REVISIT overheads calling these functions for each word also have
+ * significant performance costs.  Having txrx_bufs() calls that inline
+ * the txrx_word() logic would help performance, e.g. on larger blocks
+ * used with flash storage or MMC/SD.  There should also be ways to make
+ * GCC be less stupid about reloading registers inside the I/O loops,
+ * even without inlined GPIO calls; __attribute__((hot)) on GCC 4.3?
+ */
+
+static u32 spi_gpio_txrx_word_mode0(struct spi_device *spi,
+		unsigned nsecs, u32 word, u8 bits)
+{
+	return bitbang_txrx_be_cpha0(spi, nsecs, 0, word, bits);
+}
+
+static u32 spi_gpio_txrx_word_mode1(struct spi_device *spi,
+		unsigned nsecs, u32 word, u8 bits)
+{
+	return bitbang_txrx_be_cpha1(spi, nsecs, 0, word, bits);
+}
+
+static u32 spi_gpio_txrx_word_mode2(struct spi_device *spi,
+		unsigned nsecs, u32 word, u8 bits)
+{
+	return bitbang_txrx_be_cpha0(spi, nsecs, 1, word, bits);
+}
+
+static u32 spi_gpio_txrx_word_mode3(struct spi_device *spi,
+		unsigned nsecs, u32 word, u8 bits)
+{
+	return bitbang_txrx_be_cpha1(spi, nsecs, 1, word, bits);
+}
+
+/*----------------------------------------------------------------------*/
+
+static void spi_gpio_chipselect(struct spi_device *spi, int is_active)
+{
+	unsigned long cs = (unsigned long) spi->controller_data;
+
+	/* set initial clock polarity */
+	if (is_active)
+		setsck(spi, spi->mode & SPI_CPOL);
+
+	/* SPI is normally active-low */
+	gpio_set_value(cs, (spi->mode & SPI_CS_HIGH) ? is_active : !is_active);
+}
+
+static int spi_gpio_setup(struct spi_device *spi)
+{
+	unsigned long	cs = (unsigned long) spi->controller_data;
+	int		status = 0;
+
+	if (spi->bits_per_word > 32)
+		return -EINVAL;
+
+	if (!spi->controller_state) {
+		status = gpio_request(cs, spi->dev.bus_id);
+		if (status)
+			return status;
+		status = gpio_direction_output(cs, spi->mode & SPI_CS_HIGH);
+	}
+	if (!status)
+		status = spi_bitbang_setup(spi);
+	if (status) {
+		if (!spi->controller_state)
+			gpio_free(cs);
+	}
+	return status;
+}
+
+static void spi_gpio_cleanup(struct spi_device *spi)
+{
+	unsigned long	cs = (unsigned long) spi->controller_data;
+
+	gpio_free(cs);
+	spi_bitbang_cleanup(spi);
+}
+
+static int __init spi_gpio_alloc(unsigned pin, const char *label, bool is_in)
+{
+	int value;
+
+	value = gpio_request(pin, label);
+	if (value == 0) {
+		if (is_in)
+			value = gpio_direction_input(pin);
+		else
+			value = gpio_direction_output(pin, 0);
+	}
+	return value;
+}
+
+static int __init
+spi_gpio_request(struct spi_gpio_platform_data *pdata, const char *label)
+{
+	int value;
+
+	/* NOTE:  SPI_*_GPIO symbols may reference "pdata" */
+
+	value = spi_gpio_alloc(SPI_MOSI_GPIO, label, false);
+	if (value)
+		goto done;
+
+	value = spi_gpio_alloc(SPI_MISO_GPIO, label, true);
+	if (value)
+		goto free_mosi;
+
+	value = spi_gpio_alloc(SPI_SCK_GPIO, label, false);
+	if (value)
+		goto free_miso;
+
+	goto done;
+
+free_miso:
+	gpio_free(SPI_MISO_GPIO);
+free_mosi:
+	gpio_free(SPI_MOSI_GPIO);
+done:
+	return value;
+}
+
+static int __init spi_gpio_probe(struct platform_device *pdev)
+{
+	int				status;
+	struct spi_master		*master;
+	struct spi_gpio			*spi_gpio;
+	struct spi_gpio_platform_data	*pdata;
+
+	pdata = pdev->dev.platform_data;
+#ifdef GENERIC_BITBANG
+	if (!pdata || !pdata->num_chipselect)
+		return -ENODEV;
+#endif
+
+	status = spi_gpio_request(pdata, dev_name(&pdev->dev));
+	if (status < 0)
+		return status;
+
+	master = spi_alloc_master(&pdev->dev, sizeof *spi_gpio);
+	if (!master) {
+		status = -ENOMEM;
+		goto gpio_free;
+	}
+	spi_gpio = spi_master_get_devdata(master);
+	platform_set_drvdata(pdev, spi_gpio);
+
+	spi_gpio->pdev = pdev;
+	if (pdata)
+		spi_gpio->pdata = *pdata;
+
+	master->bus_num = pdev->id;
+	master->num_chipselect = SPI_N_CHIPSEL;
+	master->setup = spi_gpio_setup;
+	master->cleanup = spi_gpio_cleanup;
+
+	spi_gpio->bitbang.master = spi_master_get(master);
+	spi_gpio->bitbang.chipselect = spi_gpio_chipselect;
+	spi_gpio->bitbang.txrx_word[SPI_MODE_0] = spi_gpio_txrx_word_mode0;
+	spi_gpio->bitbang.txrx_word[SPI_MODE_1] = spi_gpio_txrx_word_mode1;
+	spi_gpio->bitbang.txrx_word[SPI_MODE_2] = spi_gpio_txrx_word_mode2;
+	spi_gpio->bitbang.txrx_word[SPI_MODE_3] = spi_gpio_txrx_word_mode3;
+	spi_gpio->bitbang.setup_transfer = spi_bitbang_setup_transfer;
+	spi_gpio->bitbang.flags = SPI_CS_HIGH;
+
+	status = spi_bitbang_start(&spi_gpio->bitbang);
+	if (status < 0) {
+		spi_master_put(spi_gpio->bitbang.master);
+gpio_free:
+		gpio_free(SPI_MISO_GPIO);
+		gpio_free(SPI_MOSI_GPIO);
+		gpio_free(SPI_SCK_GPIO);
+		spi_master_put(master);
+	}
+
+	return status;
+}
+
+static int __exit spi_gpio_remove(struct platform_device *pdev)
+{
+	struct spi_gpio			*spi_gpio;
+	struct spi_gpio_platform_data	*pdata;
+	int				status;
+
+	spi_gpio = platform_get_drvdata(pdev);
+	pdata = pdev->dev.platform_data;
+
+	/* stop() unregisters child devices too */
+	status = spi_bitbang_stop(&spi_gpio->bitbang);
+	spi_master_put(spi_gpio->bitbang.master);
+
+	platform_set_drvdata(pdev, NULL);
+
+	gpio_free(SPI_MISO_GPIO);
+	gpio_free(SPI_MOSI_GPIO);
+	gpio_free(SPI_SCK_GPIO);
+
+	return status;
+}
+
+MODULE_ALIAS("platform:" DRIVER_NAME);
+
+static struct platform_driver spi_gpio_driver = {
+	.driver.name	= DRIVER_NAME,
+	.driver.owner	= THIS_MODULE,
+	.remove		= __exit_p(spi_gpio_remove),
+};
+
+static int __init spi_gpio_init(void)
+{
+	return platform_driver_probe(&spi_gpio_driver, spi_gpio_probe);
+}
+module_init(spi_gpio_init);
+
+static void __exit spi_gpio_exit(void)
+{
+	platform_driver_unregister(&spi_gpio_driver);
+}
+module_exit(spi_gpio_exit);
+
+
+MODULE_DESCRIPTION("SPI master driver using generic bitbanged GPIO ");
+MODULE_AUTHOR("David Brownell");
+MODULE_LICENSE("GPL");

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 */