drivers/spi/spi-bitbang.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * polling/bitbanging SPI master controller driver utilities
  */

 #include <linux/spinlock.h>
 #include <linux/workqueue.h>
 #include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/delay.h>
 #include <linux/errno.h>
 #include <linux/platform_device.h>
 #include <linux/slab.h>

 #include <linux/spi/spi.h>
 #include <linux/spi/spi_bitbang.h>

 #define SPI_BITBANG_CS_DELAY	100


 /*----------------------------------------------------------------------*/

 /*
  * FIRST PART (OPTIONAL):  word-at-a-time spi_transfer support.
  * Use this for GPIO or shift-register level hardware APIs.
  *
  * spi_bitbang_cs is in spi_device->controller_state, which is unavailable
  * to glue code.  These bitbang setup() and cleanup() routines are always
  * used, though maybe they're called from controller-aware code.
  *
  * chipselect() and friends may use spi_device->controller_data and
  * controller registers as appropriate.
  *
  *
  * NOTE:  SPI controller pins can often be used as GPIO pins instead,
  * which means you could use a bitbang driver either to get hardware
  * working quickly, or testing for differences that aren't speed related.
  */

 struct spi_bitbang_cs {
 	unsigned	nsecs;	/* (clock cycle time)/2 */
 	u32		(*txrx_word)(struct spi_device *spi, unsigned nsecs,
 					u32 word, u8 bits, unsigned flags);
 	unsigned	(*txrx_bufs)(struct spi_device *,
 					u32 (*txrx_word)(
 						struct spi_device *spi,
 						unsigned nsecs,
 						u32 word, u8 bits,
 						unsigned flags),
 					unsigned, struct spi_transfer *,
 					unsigned);
 };

 static unsigned bitbang_txrx_8(
 	struct spi_device	*spi,
 	u32			(*txrx_word)(struct spi_device *spi,
 					unsigned nsecs,
 					u32 word, u8 bits,
 					unsigned flags),
 	unsigned		ns,
 	struct spi_transfer	*t,
 	unsigned flags
 ) {
 	unsigned		bits = t->bits_per_word;
 	unsigned		count = t->len;
 	const u8		*tx = t->tx_buf;
 	u8			*rx = t->rx_buf;

 	while (likely(count > 0)) {
 		u8		word = 0;

 		if (tx)
 			word = *tx++;
 		word = txrx_word(spi, ns, word, bits, flags);
 		if (rx)
 			*rx++ = word;
 		count -= 1;
 	}
 	return t->len - count;
 }

 static unsigned bitbang_txrx_16(
 	struct spi_device	*spi,
 	u32			(*txrx_word)(struct spi_device *spi,
 					unsigned nsecs,
 					u32 word, u8 bits,
 					unsigned flags),
 	unsigned		ns,
 	struct spi_transfer	*t,
 	unsigned flags
 ) {
 	unsigned		bits = t->bits_per_word;
 	unsigned		count = t->len;
 	const u16		*tx = t->tx_buf;
 	u16			*rx = t->rx_buf;

 	while (likely(count > 1)) {
 		u16		word = 0;

 		if (tx)
 			word = *tx++;
 		word = txrx_word(spi, ns, word, bits, flags);
 		if (rx)
 			*rx++ = word;
 		count -= 2;
 	}
 	return t->len - count;
 }

 static unsigned bitbang_txrx_32(
 	struct spi_device	*spi,
 	u32			(*txrx_word)(struct spi_device *spi,
 					unsigned nsecs,
 					u32 word, u8 bits,
 					unsigned flags),
 	unsigned		ns,
 	struct spi_transfer	*t,
 	unsigned flags
 ) {
 	unsigned		bits = t->bits_per_word;
 	unsigned		count = t->len;
 	const u32		*tx = t->tx_buf;
 	u32			*rx = t->rx_buf;

 	while (likely(count > 3)) {
 		u32		word = 0;

 		if (tx)
 			word = *tx++;
 		word = txrx_word(spi, ns, word, bits, flags);
 		if (rx)
 			*rx++ = word;
 		count -= 4;
 	}
 	return t->len - count;
 }

 int spi_bitbang_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
 {
 	struct spi_bitbang_cs	*cs = spi->controller_state;
 	u8			bits_per_word;
 	u32			hz;

 	if (t) {
 		bits_per_word = t->bits_per_word;
 		hz = t->speed_hz;
 	} else {
 		bits_per_word = 0;
 		hz = 0;
 	}

 	/* spi_transfer level calls that work per-word */
 	if (!bits_per_word)
 		bits_per_word = spi->bits_per_word;
 	if (bits_per_word <= 8)
 		cs->txrx_bufs = bitbang_txrx_8;
 	else if (bits_per_word <= 16)
 		cs->txrx_bufs = bitbang_txrx_16;
 	else if (bits_per_word <= 32)
 		cs->txrx_bufs = bitbang_txrx_32;
 	else
 		return -EINVAL;

 	/* nsecs = (clock period)/2 */
 	if (!hz)
 		hz = spi->max_speed_hz;
 	if (hz) {
 		cs->nsecs = (1000000000/2) / hz;
 		if (cs->nsecs > (MAX_UDELAY_MS * 1000 * 1000))
 			return -EINVAL;
 	}

 	return 0;
 }
 EXPORT_SYMBOL_GPL(spi_bitbang_setup_transfer);

 /**
  * spi_bitbang_setup - default setup for per-word I/O loops
  */
 int spi_bitbang_setup(struct spi_device *spi)
 {
 	struct spi_bitbang_cs	*cs = spi->controller_state;
 	struct spi_bitbang	*bitbang;

 	bitbang = spi_master_get_devdata(spi->master);

 	if (!cs) {
 		cs = kzalloc(sizeof(*cs), GFP_KERNEL);
 		if (!cs)
 			return -ENOMEM;
 		spi->controller_state = cs;
 	}

 	/* per-word shift register access, in hardware or bitbanging */
 	cs->txrx_word = bitbang->txrx_word[spi->mode & (SPI_CPOL|SPI_CPHA)];
 	if (!cs->txrx_word)
 		return -EINVAL;

 	if (bitbang->setup_transfer) {
 		int retval = bitbang->setup_transfer(spi, NULL);
 		if (retval < 0)
 			return retval;
 	}

 	dev_dbg(&spi->dev, "%s, %u nsec/bit\n", __func__, 2 * cs->nsecs);

 	return 0;
 }
 EXPORT_SYMBOL_GPL(spi_bitbang_setup);

 /**
  * spi_bitbang_cleanup - default cleanup for per-word I/O loops
  */
 void spi_bitbang_cleanup(struct spi_device *spi)
 {
 	kfree(spi->controller_state);
 }
 EXPORT_SYMBOL_GPL(spi_bitbang_cleanup);

 static int spi_bitbang_bufs(struct spi_device *spi, struct spi_transfer *t)
 {
 	struct spi_bitbang_cs	*cs = spi->controller_state;
 	unsigned		nsecs = cs->nsecs;
 	struct spi_bitbang	*bitbang;

 	bitbang = spi_master_get_devdata(spi->master);
 	if (bitbang->set_line_direction) {
 		int err;

 		err = bitbang->set_line_direction(spi, !!(t->tx_buf));
 		if (err < 0)
 			return err;
 	}

 	if (spi->mode & SPI_3WIRE) {
 		unsigned flags;

 		flags = t->tx_buf ? SPI_MASTER_NO_RX : SPI_MASTER_NO_TX;
 		return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t, flags);
 	}
 	return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t, 0);
 }

 /*----------------------------------------------------------------------*/

 /*
  * SECOND PART ... simple transfer queue runner.
  *
  * This costs a task context per controller, running the queue by
  * performing each transfer in sequence.  Smarter hardware can queue
  * several DMA transfers at once, and process several controller queues
  * in parallel; this driver doesn't match such hardware very well.
  *
  * Drivers can provide word-at-a-time i/o primitives, or provide
  * transfer-at-a-time ones to leverage dma or fifo hardware.
  */

 static int spi_bitbang_prepare_hardware(struct spi_master *spi)
 {
 	struct spi_bitbang	*bitbang;

 	bitbang = spi_master_get_devdata(spi);

 	mutex_lock(&bitbang->lock);
 	bitbang->busy = 1;
 	mutex_unlock(&bitbang->lock);

 	return 0;
 }

 static int spi_bitbang_transfer_one(struct spi_master *master,
 				    struct spi_device *spi,
 				    struct spi_transfer *transfer)
 {
 	struct spi_bitbang *bitbang = spi_master_get_devdata(master);
 	int status = 0;

 	if (bitbang->setup_transfer) {
 		status = bitbang->setup_transfer(spi, transfer);
 		if (status < 0)
 			goto out;
 	}

 	if (transfer->len)
 		status = bitbang->txrx_bufs(spi, transfer);

 	if (status == transfer->len)
 		status = 0;
 	else if (status >= 0)
 		status = -EREMOTEIO;

 out:
 	spi_finalize_current_transfer(master);

 	return status;
 }

 static int spi_bitbang_unprepare_hardware(struct spi_master *spi)
 {
 	struct spi_bitbang	*bitbang;

 	bitbang = spi_master_get_devdata(spi);

 	mutex_lock(&bitbang->lock);
 	bitbang->busy = 0;
 	mutex_unlock(&bitbang->lock);

 	return 0;
 }

 static void spi_bitbang_set_cs(struct spi_device *spi, bool enable)
 {
 	struct spi_bitbang *bitbang = spi_master_get_devdata(spi->master);

 	/* SPI core provides CS high / low, but bitbang driver
 	 * expects CS active
 	 * spi device driver takes care of handling SPI_CS_HIGH
 	 */
 	enable = (!!(spi->mode & SPI_CS_HIGH) == enable);

 	ndelay(SPI_BITBANG_CS_DELAY);
 	bitbang->chipselect(spi, enable ? BITBANG_CS_ACTIVE :
 			    BITBANG_CS_INACTIVE);
 	ndelay(SPI_BITBANG_CS_DELAY);
 }

 /*----------------------------------------------------------------------*/

 int spi_bitbang_init(struct spi_bitbang *bitbang)
 {
 	struct spi_master *master = bitbang->master;

 	if (!master || !bitbang->chipselect)
 		return -EINVAL;

 	mutex_init(&bitbang->lock);

 	if (!master->mode_bits)
 		master->mode_bits = SPI_CPOL | SPI_CPHA | bitbang->flags;

 	if (master->transfer || master->transfer_one_message)
 		return -EINVAL;

 	master->prepare_transfer_hardware = spi_bitbang_prepare_hardware;
 	master->unprepare_transfer_hardware = spi_bitbang_unprepare_hardware;
 	master->transfer_one = spi_bitbang_transfer_one;
 	master->set_cs = spi_bitbang_set_cs;

 	if (!bitbang->txrx_bufs) {
 		bitbang->use_dma = 0;
 		bitbang->txrx_bufs = spi_bitbang_bufs;
 		if (!master->setup) {
 			if (!bitbang->setup_transfer)
 				bitbang->setup_transfer =
 					 spi_bitbang_setup_transfer;
 			master->setup = spi_bitbang_setup;
 			master->cleanup = spi_bitbang_cleanup;
 		}
 	}

 	return 0;
 }
 EXPORT_SYMBOL_GPL(spi_bitbang_init);

 /**
  * spi_bitbang_start - start up a polled/bitbanging SPI master driver
  * @bitbang: driver handle
  *
  * Caller should have zero-initialized all parts of the structure, and then
  * provided callbacks for chip selection and I/O loops.  If the master has
  * a transfer method, its final step should call spi_bitbang_transfer; or,
  * that's the default if the transfer routine is not initialized.  It should
  * also set up the bus number and number of chipselects.
  *
  * For i/o loops, provide callbacks either per-word (for bitbanging, or for
  * hardware that basically exposes a shift register) or per-spi_transfer
  * (which takes better advantage of hardware like fifos or DMA engines).
  *
  * Drivers using per-word I/O loops should use (or call) spi_bitbang_setup,
  * spi_bitbang_cleanup and spi_bitbang_setup_transfer to handle those spi
  * master methods.  Those methods are the defaults if the bitbang->txrx_bufs
  * routine isn't initialized.
  *
  * This routine registers the spi_master, which will process requests in a
  * dedicated task, keeping IRQs unblocked most of the time.  To stop
  * processing those requests, call spi_bitbang_stop().
  *
  * On success, this routine will take a reference to master. The caller is
  * responsible for calling spi_bitbang_stop() to decrement the reference and
  * spi_master_put() as counterpart of spi_alloc_master() to prevent a memory
  * leak.
  */
 int spi_bitbang_start(struct spi_bitbang *bitbang)
 {
 	struct spi_master *master = bitbang->master;
 	int ret;

 	ret = spi_bitbang_init(bitbang);
 	if (ret)
 		return ret;

 	/* driver may get busy before register() returns, especially
 	 * if someone registered boardinfo for devices
 	 */
 	ret = spi_register_master(spi_master_get(master));
 	if (ret)
 		spi_master_put(master);

 	return ret;
 }
 EXPORT_SYMBOL_GPL(spi_bitbang_start);

 /**
  * spi_bitbang_stop - stops the task providing spi communication
  */
 void spi_bitbang_stop(struct spi_bitbang *bitbang)
 {
 	spi_unregister_master(bitbang->master);
 }
 EXPORT_SYMBOL_GPL(spi_bitbang_stop);

 MODULE_LICENSE("GPL");
	// SPDX-License-Identifier: GPL-2.0-or-later
	/*
	* polling/bitbanging SPI master controller driver utilities
	*/

	#include <linux/spinlock.h>
	#include <linux/workqueue.h>
	#include <linux/interrupt.h>
	#include <linux/module.h>
	#include <linux/delay.h>
	#include <linux/errno.h>
	#include <linux/platform_device.h>
	#include <linux/slab.h>

	#include <linux/spi/spi.h>
	#include <linux/spi/spi_bitbang.h>

	#define SPI_BITBANG_CS_DELAY 100


	/----------------------------------------------------------------------/

	/*
	* FIRST PART (OPTIONAL): word-at-a-time spi_transfer support.
	* Use this for GPIO or shift-register level hardware APIs.
	*
	* spi_bitbang_cs is in spi_device->controller_state, which is unavailable
	* to glue code. These bitbang setup() and cleanup() routines are always
	* used, though maybe they're called from controller-aware code.
	*
	* chipselect() and friends may use spi_device->controller_data and
	* controller registers as appropriate.
	*
	*
	* NOTE: SPI controller pins can often be used as GPIO pins instead,
	* which means you could use a bitbang driver either to get hardware
	* working quickly, or testing for differences that aren't speed related.
	*/

	struct spi_bitbang_cs {
	unsigned nsecs; /* (clock cycle time)/2 */
	u32 (txrx_word)(struct spi_device spi, unsigned nsecs,
	u32 word, u8 bits, unsigned flags);
	unsigned (txrx_bufs)(struct spi_device ,
	u32 (*txrx_word)(
	struct spi_device *spi,
	unsigned nsecs,
	u32 word, u8 bits,
	unsigned flags),
	unsigned, struct spi_transfer *,
	unsigned);
	};

	static unsigned bitbang_txrx_8(
	struct spi_device *spi,
	u32 (txrx_word)(struct spi_device spi,
	unsigned nsecs,
	u32 word, u8 bits,
	unsigned flags),
	unsigned ns,
	struct spi_transfer *t,
	unsigned flags
	) {
	unsigned bits = t->bits_per_word;
	unsigned count = t->len;
	const u8 *tx = t->tx_buf;
	u8 *rx = t->rx_buf;

	while (likely(count > 0)) {
	u8 word = 0;

	if (tx)
	word = *tx++;
	word = txrx_word(spi, ns, word, bits, flags);
	if (rx)
	*rx++ = word;
	count -= 1;
	}
	return t->len - count;
	}

	static unsigned bitbang_txrx_16(
	struct spi_device *spi,
	u32 (txrx_word)(struct spi_device spi,
	unsigned nsecs,
	u32 word, u8 bits,
	unsigned flags),
	unsigned ns,
	struct spi_transfer *t,
	unsigned flags
	) {
	unsigned bits = t->bits_per_word;
	unsigned count = t->len;
	const u16 *tx = t->tx_buf;
	u16 *rx = t->rx_buf;

	while (likely(count > 1)) {
	u16 word = 0;

	if (tx)
	word = *tx++;
	word = txrx_word(spi, ns, word, bits, flags);
	if (rx)
	*rx++ = word;
	count -= 2;
	}
	return t->len - count;
	}

	static unsigned bitbang_txrx_32(
	struct spi_device *spi,
	u32 (txrx_word)(struct spi_device spi,
	unsigned nsecs,
	u32 word, u8 bits,
	unsigned flags),
	unsigned ns,
	struct spi_transfer *t,
	unsigned flags
	) {
	unsigned bits = t->bits_per_word;
	unsigned count = t->len;
	const u32 *tx = t->tx_buf;
	u32 *rx = t->rx_buf;

	while (likely(count > 3)) {
	u32 word = 0;

	if (tx)
	word = *tx++;
	word = txrx_word(spi, ns, word, bits, flags);
	if (rx)
	*rx++ = word;
	count -= 4;
	}
	return t->len - count;
	}

	int spi_bitbang_setup_transfer(struct spi_device spi, struct spi_transfer t)
	{
	struct spi_bitbang_cs *cs = spi->controller_state;
	u8 bits_per_word;
	u32 hz;

	if (t) {
	bits_per_word = t->bits_per_word;
	hz = t->speed_hz;
	} else {
	bits_per_word = 0;
	hz = 0;
	}

	/* spi_transfer level calls that work per-word */
	if (!bits_per_word)
	bits_per_word = spi->bits_per_word;
	if (bits_per_word <= 8)
	cs->txrx_bufs = bitbang_txrx_8;
	else if (bits_per_word <= 16)
	cs->txrx_bufs = bitbang_txrx_16;
	else if (bits_per_word <= 32)
	cs->txrx_bufs = bitbang_txrx_32;
	else
	return -EINVAL;

	/* nsecs = (clock period)/2 */
	if (!hz)
	hz = spi->max_speed_hz;
	if (hz) {
	cs->nsecs = (1000000000/2) / hz;
	if (cs->nsecs > (MAX_UDELAY_MS * 1000 * 1000))
	return -EINVAL;
	}

	return 0;
	}
	EXPORT_SYMBOL_GPL(spi_bitbang_setup_transfer);

	/**
	* spi_bitbang_setup - default setup for per-word I/O loops
	*/
	int spi_bitbang_setup(struct spi_device *spi)
	{
	struct spi_bitbang_cs *cs = spi->controller_state;
	struct spi_bitbang *bitbang;

	bitbang = spi_master_get_devdata(spi->master);

	if (!cs) {
	cs = kzalloc(sizeof(*cs), GFP_KERNEL);
	if (!cs)
	return -ENOMEM;
	spi->controller_state = cs;
	}

	/* per-word shift register access, in hardware or bitbanging */
	cs->txrx_word = bitbang->txrx_word[spi->mode & (SPI_CPOL\|SPI_CPHA)];
	if (!cs->txrx_word)
	return -EINVAL;

	if (bitbang->setup_transfer) {
	int retval = bitbang->setup_transfer(spi, NULL);
	if (retval < 0)
	return retval;
	}

	dev_dbg(&spi->dev, "%s, %u nsec/bit\n", __func__, 2 * cs->nsecs);

	return 0;
	}
	EXPORT_SYMBOL_GPL(spi_bitbang_setup);

	/**
	* spi_bitbang_cleanup - default cleanup for per-word I/O loops
	*/
	void spi_bitbang_cleanup(struct spi_device *spi)
	{
	kfree(spi->controller_state);
	}
	EXPORT_SYMBOL_GPL(spi_bitbang_cleanup);

	static int spi_bitbang_bufs(struct spi_device spi, struct spi_transfer t)
	{
	struct spi_bitbang_cs *cs = spi->controller_state;
	unsigned nsecs = cs->nsecs;
	struct spi_bitbang *bitbang;

	bitbang = spi_master_get_devdata(spi->master);
	if (bitbang->set_line_direction) {
	int err;

	err = bitbang->set_line_direction(spi, !!(t->tx_buf));
	if (err < 0)
	return err;
	}

	if (spi->mode & SPI_3WIRE) {
	unsigned flags;

	flags = t->tx_buf ? SPI_MASTER_NO_RX : SPI_MASTER_NO_TX;
	return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t, flags);
	}
	return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t, 0);
	}

	/----------------------------------------------------------------------/

	/*
	* SECOND PART ... simple transfer queue runner.
	*
	* This costs a task context per controller, running the queue by
	* performing each transfer in sequence. Smarter hardware can queue
	* several DMA transfers at once, and process several controller queues
	* in parallel; this driver doesn't match such hardware very well.
	*
	* Drivers can provide word-at-a-time i/o primitives, or provide
	* transfer-at-a-time ones to leverage dma or fifo hardware.
	*/

	static int spi_bitbang_prepare_hardware(struct spi_master *spi)
	{
	struct spi_bitbang *bitbang;

	bitbang = spi_master_get_devdata(spi);

	mutex_lock(&bitbang->lock);
	bitbang->busy = 1;
	mutex_unlock(&bitbang->lock);

	return 0;
	}

	static int spi_bitbang_transfer_one(struct spi_master *master,
	struct spi_device *spi,
	struct spi_transfer *transfer)
	{
	struct spi_bitbang *bitbang = spi_master_get_devdata(master);
	int status = 0;

	if (bitbang->setup_transfer) {
	status = bitbang->setup_transfer(spi, transfer);
	if (status < 0)
	goto out;
	}

	if (transfer->len)
	status = bitbang->txrx_bufs(spi, transfer);

	if (status == transfer->len)
	status = 0;
	else if (status >= 0)
	status = -EREMOTEIO;

	out:
	spi_finalize_current_transfer(master);

	return status;
	}

	static int spi_bitbang_unprepare_hardware(struct spi_master *spi)
	{
	struct spi_bitbang *bitbang;

	bitbang = spi_master_get_devdata(spi);

	mutex_lock(&bitbang->lock);
	bitbang->busy = 0;
	mutex_unlock(&bitbang->lock);

	return 0;
	}

	static void spi_bitbang_set_cs(struct spi_device *spi, bool enable)
	{
	struct spi_bitbang *bitbang = spi_master_get_devdata(spi->master);

	/* SPI core provides CS high / low, but bitbang driver
	* expects CS active
	* spi device driver takes care of handling SPI_CS_HIGH
	*/
	enable = (!!(spi->mode & SPI_CS_HIGH) == enable);

	ndelay(SPI_BITBANG_CS_DELAY);
	bitbang->chipselect(spi, enable ? BITBANG_CS_ACTIVE :
	BITBANG_CS_INACTIVE);
	ndelay(SPI_BITBANG_CS_DELAY);
	}

	/----------------------------------------------------------------------/

	int spi_bitbang_init(struct spi_bitbang *bitbang)
	{
	struct spi_master *master = bitbang->master;

	if (!master \|\| !bitbang->chipselect)
	return -EINVAL;

	mutex_init(&bitbang->lock);

	if (!master->mode_bits)
	master->mode_bits = SPI_CPOL \| SPI_CPHA \| bitbang->flags;

	if (master->transfer \|\| master->transfer_one_message)
	return -EINVAL;

	master->prepare_transfer_hardware = spi_bitbang_prepare_hardware;
	master->unprepare_transfer_hardware = spi_bitbang_unprepare_hardware;
	master->transfer_one = spi_bitbang_transfer_one;
	master->set_cs = spi_bitbang_set_cs;

	if (!bitbang->txrx_bufs) {
	bitbang->use_dma = 0;
	bitbang->txrx_bufs = spi_bitbang_bufs;
	if (!master->setup) {
	if (!bitbang->setup_transfer)
	bitbang->setup_transfer =
	spi_bitbang_setup_transfer;
	master->setup = spi_bitbang_setup;
	master->cleanup = spi_bitbang_cleanup;
	}
	}

	return 0;
	}
	EXPORT_SYMBOL_GPL(spi_bitbang_init);

	/**
	* spi_bitbang_start - start up a polled/bitbanging SPI master driver
	* @bitbang: driver handle
	*
	* Caller should have zero-initialized all parts of the structure, and then
	* provided callbacks for chip selection and I/O loops. If the master has
	* a transfer method, its final step should call spi_bitbang_transfer; or,
	* that's the default if the transfer routine is not initialized. It should
	* also set up the bus number and number of chipselects.
	*
	* For i/o loops, provide callbacks either per-word (for bitbanging, or for
	* hardware that basically exposes a shift register) or per-spi_transfer
	* (which takes better advantage of hardware like fifos or DMA engines).
	*
	* Drivers using per-word I/O loops should use (or call) spi_bitbang_setup,
	* spi_bitbang_cleanup and spi_bitbang_setup_transfer to handle those spi
	* master methods. Those methods are the defaults if the bitbang->txrx_bufs
	* routine isn't initialized.
	*
	* This routine registers the spi_master, which will process requests in a
	* dedicated task, keeping IRQs unblocked most of the time. To stop
	* processing those requests, call spi_bitbang_stop().
	*
	* On success, this routine will take a reference to master. The caller is
	* responsible for calling spi_bitbang_stop() to decrement the reference and
	* spi_master_put() as counterpart of spi_alloc_master() to prevent a memory
	* leak.
	*/
	int spi_bitbang_start(struct spi_bitbang *bitbang)
	{
	struct spi_master *master = bitbang->master;
	int ret;

	ret = spi_bitbang_init(bitbang);
	if (ret)
	return ret;

	/* driver may get busy before register() returns, especially
	* if someone registered boardinfo for devices
	*/
	ret = spi_register_master(spi_master_get(master));
	if (ret)
	spi_master_put(master);

	return ret;
	}
	EXPORT_SYMBOL_GPL(spi_bitbang_start);

	/**
	* spi_bitbang_stop - stops the task providing spi communication
	*/
	void spi_bitbang_stop(struct spi_bitbang *bitbang)
	{
	spi_unregister_master(bitbang->master);
	}
	EXPORT_SYMBOL_GPL(spi_bitbang_stop);

	MODULE_LICENSE("GPL");