text: /*
   * spi.c - SPI init/core code
   *
   * Copyright (C) 2005 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/device.h>
  #include <linux/init.h>
  #include <linux/cache.h>
  #include <linux/mutex.h>
  #include <linux/mod_devicetable.h>
  #include <linux/spi/spi.h>
  
  
  /* SPI bustype and spi_master class are registered after board init code
   * provides the SPI device tables, ensuring that both are present by the
   * time controller driver registration causes spi_devices to "enumerate".
   */
  static void spidev_release(struct device *dev)
  {
  	struct spi_device	*spi = to_spi_device(dev);
  
  	/* spi masters may cleanup for released devices */
  	if (spi->master->cleanup)
  		spi->master->cleanup(spi);
  
  	spi_master_put(spi->master);
  	kfree(dev);
  }
  
  static ssize_t
  modalias_show(struct device *dev, struct device_attribute *a, char *buf)
  {
  	const struct spi_device	*spi = to_spi_device(dev);
  
  	return sprintf(buf, "%s\n", spi->modalias);
  }
  
  static struct device_attribute spi_dev_attrs[] = {
  	__ATTR_RO(modalias),
  	__ATTR_NULL,
  };
  
  /* modalias support makes "modprobe $MODALIAS" new-style hotplug work,
   * and the sysfs version makes coldplug work too.
   */
  
  static const struct spi_device_id *spi_match_id(const struct spi_device_id *id,
  						const struct spi_device *sdev)
  {
  	while (id->name[0]) {
  		if (!strcmp(sdev->modalias, id->name))
  			return id;
  		id++;
  	}
  	return NULL;
  }
  
  const struct spi_device_id *spi_get_device_id(const struct spi_device *sdev)
  {
  	const struct spi_driver *sdrv = to_spi_driver(sdev->dev.driver);
  
  	return spi_match_id(sdrv->id_table, sdev);
  }
  EXPORT_SYMBOL_GPL(spi_get_device_id);
  
  static int spi_match_device(struct device *dev, struct device_driver *drv)
  {
  	const struct spi_device	*spi = to_spi_device(dev);
  	const struct spi_driver	*sdrv = to_spi_driver(drv);
  
  	if (sdrv->id_table)
  		return !!spi_match_id(sdrv->id_table, spi);
  
  	return strcmp(spi->modalias, drv->name) == 0;
  }
  
  static int spi_uevent(struct device *dev, struct kobj_uevent_env *env)
  {
  	const struct spi_device		*spi = to_spi_device(dev);
  
  	add_uevent_var(env, "MODALIAS=%s%s", SPI_MODULE_PREFIX, spi->modalias);
  	return 0;
  }
  
  #ifdef	CONFIG_PM
  
  static int spi_suspend(struct device *dev, pm_message_t message)
  {
  	int			value = 0;
  	struct spi_driver	*drv = to_spi_driver(dev->driver);
  
  	/* suspend will stop irqs and dma; no more i/o */
  	if (drv) {
  		if (drv->suspend)
  			value = drv->suspend(to_spi_device(dev), message);
  		else
  			dev_dbg(dev, "... can't suspend\n");
  	}
  	return value;
  }
  
  static int spi_resume(struct device *dev)
  {
  	int			value = 0;
  	struct spi_driver	*drv = to_spi_driver(dev->driver);
  
  	/* resume may restart the i/o queue */
  	if (drv) {
  		if (drv->resume)
  			value = drv->resume(to_spi_device(dev));
  		else
  			dev_dbg(dev, "... can't resume\n");
  	}
  	return value;
  }
  
  #else
  #define spi_suspend	NULL
  #define spi_resume	NULL
  #endif
  
  struct bus_type spi_bus_type = {
  	.name		= "spi",
  	.dev_attrs	= spi_dev_attrs,
  	.match		= spi_match_device,
  	.uevent		= spi_uevent,
  	.suspend	= spi_suspend,
  	.resume		= spi_resume,
  };
  EXPORT_SYMBOL_GPL(spi_bus_type);
  
  
  static int spi_drv_probe(struct device *dev)
  {
  	const struct spi_driver		*sdrv = to_spi_driver(dev->driver);
  
  	return sdrv->probe(to_spi_device(dev));
  }
  
  static int spi_drv_remove(struct device *dev)
  {
  	const struct spi_driver		*sdrv = to_spi_driver(dev->driver);
  
  	return sdrv->remove(to_spi_device(dev));
  }
  
  static void spi_drv_shutdown(struct device *dev)
  {
  	const struct spi_driver		*sdrv = to_spi_driver(dev->driver);
  
  	sdrv->shutdown(to_spi_device(dev));
  }
  
  /**
   * spi_register_driver - register a SPI driver
   * @sdrv: the driver to register
   * Context: can sleep
   */
  int spi_register_driver(struct spi_driver *sdrv)
  {
  	sdrv->driver.bus = &spi_bus_type;
  	if (sdrv->probe)
  		sdrv->driver.probe = spi_drv_probe;
  	if (sdrv->remove)
  		sdrv->driver.remove = spi_drv_remove;
  	if (sdrv->shutdown)
  		sdrv->driver.shutdown = spi_drv_shutdown;
  	return driver_register(&sdrv->driver);
  }
  EXPORT_SYMBOL_GPL(spi_register_driver);
  
  /*-------------------------------------------------------------------------*/
  
  /* SPI devices should normally not be created by SPI device drivers; that
   * would make them board-specific.  Similarly with SPI master drivers.
   * Device registration normally goes into like arch/.../mach.../board-YYY.c
   * with other readonly (flashable) information about mainboard devices.
   */
  
  struct boardinfo {
  	struct list_head	list;
  	unsigned		n_board_info;
  	struct spi_board_info	board_info[0];
  };
  
  static LIST_HEAD(board_list);
  static DEFINE_MUTEX(board_lock);
  
  /**
   * spi_alloc_device - Allocate a new SPI device
   * @master: Controller to which device is connected
   * Context: can sleep
   *
   * Allows a driver to allocate and initialize a spi_device without
   * registering it immediately.  This allows a driver to directly
   * fill the spi_device with device parameters before calling
   * spi_add_device() on it.
   *
   * Caller is responsible to call spi_add_device() on the returned
   * spi_device structure to add it to the SPI master.  If the caller
   * needs to discard the spi_device without adding it, then it should
   * call spi_dev_put() on it.
   *
   * Returns a pointer to the new device, or NULL.
   */
  struct spi_device *spi_alloc_device(struct spi_master *master)
  {
  	struct spi_device	*spi;
  	struct device		*dev = master->dev.parent;
  
  	if (!spi_master_get(master))
  		return NULL;
  
  	spi = kzalloc(sizeof *spi, GFP_KERNEL);
  	if (!spi) {
  		dev_err(dev, "cannot alloc spi_device\n");
  		spi_master_put(master);
  		return NULL;
  	}
  
  	spi->master = master;
  	spi->dev.parent = dev;
  	spi->dev.bus = &spi_bus_type;
  	spi->dev.release = spidev_release;
  	device_initialize(&spi->dev);
  	return spi;
  }
  EXPORT_SYMBOL_GPL(spi_alloc_device);
  
  /**
   * spi_add_device - Add spi_device allocated with spi_alloc_device
   * @spi: spi_device to register
   *
   * Companion function to spi_alloc_device.  Devices allocated with
   * spi_alloc_device can be added onto the spi bus with this function.
   *
   * Returns 0 on success; negative errno on failure
   */
  int spi_add_device(struct spi_device *spi)
  {
  	static DEFINE_MUTEX(spi_add_lock);
  	struct device *dev = spi->master->dev.parent;
  	int status;
  
  	/* Chipselects are numbered 0..max; validate. */
  	if (spi->chip_select >= spi->master->num_chipselect) {
  		dev_err(dev, "cs%d >= max %d\n",
  			spi->chip_select,
  			spi->master->num_chipselect);
  		return -EINVAL;
  	}
  
  	/* Set the bus ID string */
  	dev_set_name(&spi->dev, "%s.%u", dev_name(&spi->master->dev),
  			spi->chip_select);
  
  
  	/* We need to make sure there's no other device with this
  	 * chipselect **BEFORE** we call setup(), else we'll trash
  	 * its configuration.  Lock against concurrent add() calls.
  	 */
  	mutex_lock(&spi_add_lock);
  
  	if (bus_find_device_by_name(&spi_bus_type, NULL, dev_name(&spi->dev))
  			!= NULL) {
  		dev_err(dev, "chipselect %d already in use\n",
  				spi->chip_select);
  		status = -EBUSY;
  		goto done;
  	}
  
  	/* Drivers may modify this initial i/o setup, but will
  	 * normally rely on the device being setup.  Devices
  	 * using SPI_CS_HIGH can't coexist well otherwise...
  	 */
  	status = spi_setup(spi);
  	if (status < 0) {
  		dev_err(dev, "can't %s %s, status %d\n",
  				"setup", dev_name(&spi->dev), status);
  		goto done;
  	}
  
  	/* Device may be bound to an active driver when this returns */
  	status = device_add(&spi->dev);
  	if (status < 0)
  		dev_err(dev, "can't %s %s, status %d\n",
  				"add", dev_name(&spi->dev), status);
  	else
  		dev_dbg(dev, "registered child %s\n", dev_name(&spi->dev));
  
  done:
  	mutex_unlock(&spi_add_lock);
  	return status;
  }
  EXPORT_SYMBOL_GPL(spi_add_device);
  
  /**
   * spi_new_device - instantiate one new SPI device
   * @master: Controller to which device is connected
   * @chip: Describes the SPI device
   * Context: can sleep
   *
   * On typical mainboards, this is purely internal; and it's not needed
   * after board init creates the hard-wired devices.  Some development
   * platforms may not be able to use spi_register_board_info though, and
   * this is exported so that for example a USB or parport based adapter
   * driver could add devices (which it would learn about out-of-band).
   *
   * Returns the new device, or NULL.
   */
  struct spi_device *spi_new_device(struct spi_master *master,
  				  struct spi_board_info *chip)
  {
  	struct spi_device	*proxy;
  	int			status;
  
  	/* NOTE:  caller did any chip->bus_num checks necessary.
  	 *
  	 * Also, unless we change the return value convention to use
  	 * error-or-pointer (not NULL-or-pointer), troubleshootability
  	 * suggests syslogged diagnostics are best here (ugh).
  	 */
  
  	proxy = spi_alloc_device(master);
  	if (!proxy)
  		return NULL;
  
  	WARN_ON(strlen(chip->modalias) >= sizeof(proxy->modalias));
  
  	proxy->chip_select = chip->chip_select;
  	proxy->max_speed_hz = chip->max_speed_hz;
  	proxy->mode = chip->mode;
  	proxy->irq = chip->irq;
  	strlcpy(proxy->modalias, chip->modalias, sizeof(proxy->modalias));
  	proxy->dev.platform_data = (void *) chip->platform_data;
  	proxy->controller_data = chip->controller_data;
  	proxy->controller_state = NULL;
  
  	status = spi_add_device(proxy);
  	if (status < 0) {
  		spi_dev_put(proxy);
  		return NULL;
  	}
  
  	return proxy;
  }
  EXPORT_SYMBOL_GPL(spi_new_device);
  
  /**
   * spi_register_board_info - register SPI devices for a given board
   * @info: array of chip descriptors
   * @n: how many descriptors are provided
   * Context: can sleep
   *
   * Board-specific early init code calls this (probably during arch_initcall)
   * with segments of the SPI device table.  Any device nodes are created later,
   * after the relevant parent SPI controller (bus_num) is defined.  We keep
   * this table of devices forever, so that reloading a controller driver will
   * not make Linux forget about these hard-wired devices.
   *
   * Other code can also call this, e.g. a particular add-on board might provide
   * SPI devices through its expansion connector, so code initializing that board
   * would naturally declare its SPI devices.
   *
   * The board info passed can safely be __initdata ... but be careful of
   * any embedded pointers (platform_data, etc), they're copied as-is.
   */
  int __init
  spi_register_board_info(struct spi_board_info const *info, unsigned n)
  {
  	struct boardinfo	*bi;
  
  	bi = kmalloc(sizeof(*bi) + n * sizeof *info, GFP_KERNEL);
  	if (!bi)
  		return -ENOMEM;
  	bi->n_board_info = n;
  	memcpy(bi->board_info, info, n * sizeof *info);
  
  	mutex_lock(&board_lock);
  	list_add_tail(&bi->list, &board_list);
  	mutex_unlock(&board_lock);
  	return 0;
  }
  
  /* FIXME someone should add support for a __setup("spi", ...) that
   * creates board info from kernel command lines
   */
  
  static void scan_boardinfo(struct spi_master *master)
  {
  	struct boardinfo	*bi;
  
  	mutex_lock(&board_lock);
  	list_for_each_entry(bi, &board_list, list) {
  		struct spi_board_info	*chip = bi->board_info;
  		unsigned		n;
  
  		for (n = bi->n_board_info; n > 0; n--, chip++) {
  			if (chip->bus_num != master->bus_num)
  				continue;
  			/* NOTE: this relies on spi_new_device to
  			 * issue diagnostics when given bogus inputs
  			 */
  			(void) spi_new_device(master, chip);
  		}
  	}
  	mutex_unlock(&board_lock);
  }
  
  /*-------------------------------------------------------------------------*/
  
  static void spi_master_release(struct device *dev)
  {
  	struct spi_master *master;
  
  	master = container_of(dev, struct spi_master, dev);
  	kfree(master);
  }
  
  static struct class spi_master_class = {
  	.name		= "spi_master",
  	.owner		= THIS_MODULE,
  	.dev_release	= spi_master_release,
  };
  
  
  /**
   * spi_alloc_master - allocate SPI master controller
   * @dev: the controller, possibly using the platform_bus
   * @size: how much zeroed driver-private data to allocate; the pointer to this
   *	memory is in the driver_data field of the returned device,
   *	accessible with spi_master_get_devdata().
   * Context: can sleep
   *
   * This call is used only by SPI master controller drivers, which are the
   * only ones directly touching chip registers.  It's how they allocate
   * an spi_master structure, prior to calling spi_register_master().
   *
   * This must be called from context that can sleep.  It returns the SPI
   * master structure on success, else NULL.
   *
   * The caller is responsible for assigning the bus number and initializing
   * the master's methods before calling spi_register_master(); and (after errors
   * adding the device) calling spi_master_put() to prevent a memory leak.
   */
  struct spi_master *spi_alloc_master(struct device *dev, unsigned size)
  {
  	struct spi_master	*master;
  
  	if (!dev)
  		return NULL;
  
  	master = kzalloc(size + sizeof *master, GFP_KERNEL);
  	if (!master)
  		return NULL;
  
  	device_initialize(&master->dev);
  	master->dev.class = &spi_master_class;
  	master->dev.parent = get_device(dev);
  	spi_master_set_devdata(master, &master[1]);
  
  	return master;
  }
  EXPORT_SYMBOL_GPL(spi_alloc_master);
  
  /**
   * spi_register_master - register SPI master controller
   * @master: initialized master, originally from spi_alloc_master()
   * Context: can sleep
   *
   * SPI master controllers connect to their drivers using some non-SPI bus,
   * such as the platform bus.  The final stage of probe() in that code
   * includes calling spi_register_master() to hook up to this SPI bus glue.
   *
   * SPI controllers use board specific (often SOC specific) bus numbers,
   * and board-specific addressing for SPI devices combines those numbers
   * with chip select numbers.  Since SPI does not directly support dynamic
   * device identification, boards need configuration tables telling which
   * chip is at which address.
   *
   * This must be called from context that can sleep.  It returns zero on
   * success, else a negative error code (dropping the master's refcount).
   * After a successful return, the caller is responsible for calling
   * spi_unregister_master().
   */
  int spi_register_master(struct spi_master *master)
  {
  	static atomic_t		dyn_bus_id = ATOMIC_INIT((1<<15) - 1);
  	struct device		*dev = master->dev.parent;
  	int			status = -ENODEV;
  	int			dynamic = 0;
  
  	if (!dev)
  		return -ENODEV;
  
  	/* even if it's just one always-selected device, there must
  	 * be at least one chipselect
  	 */
  	if (master->num_chipselect == 0)
  		return -EINVAL;
  
  	/* convention:  dynamically assigned bus IDs count down from the max */
  	if (master->bus_num < 0) {
  		/* FIXME switch to an IDR based scheme, something like
  		 * I2C now uses, so we can't run out of "dynamic" IDs
  		 */
  		master->bus_num = atomic_dec_return(&dyn_bus_id);
  		dynamic = 1;
  	}
  
  	/* register the device, then userspace will see it.
  	 * registration fails if the bus ID is in use.
  	 */
  	dev_set_name(&master->dev, "spi%u", master->bus_num);
  	status = device_add(&master->dev);
  	if (status < 0)
  		goto done;
  	dev_dbg(dev, "registered master %s%s\n", dev_name(&master->dev),
  			dynamic ? " (dynamic)" : "");
  
  	/* populate children from any spi device tables */
  	scan_boardinfo(master);
  	status = 0;
  done:
  	return status;
  }
  EXPORT_SYMBOL_GPL(spi_register_master);
  
  
  static int __unregister(struct device *dev, void *master_dev)
  {
  	/* note: before about 2.6.14-rc1 this would corrupt memory: */
  	if (dev != master_dev)
  		spi_unregister_device(to_spi_device(dev));
  	return 0;
  }
  
  /**
   * spi_unregister_master - unregister SPI master controller
   * @master: the master being unregistered
   * Context: can sleep
   *
   * This call is used only by SPI master controller drivers, which are the
   * only ones directly touching chip registers.
   *
   * This must be called from context that can sleep.
   */
  void spi_unregister_master(struct spi_master *master)
  {
  	int dummy;
  
  	dummy = device_for_each_child(master->dev.parent, &master->dev,
  					__unregister);
  	device_unregister(&master->dev);
  }
  EXPORT_SYMBOL_GPL(spi_unregister_master);
  
  static int __spi_master_match(struct device *dev, void *data)
  {
  	struct spi_master *m;
  	u16 *bus_num = data;
  
  	m = container_of(dev, struct spi_master, dev);
  	return m->bus_num == *bus_num;
  }
  
  /**
   * spi_busnum_to_master - look up master associated with bus_num
   * @bus_num: the master's bus number
   * Context: can sleep
   *
   * This call may be used with devices that are registered after
   * arch init time.  It returns a refcounted pointer to the relevant
   * spi_master (which the caller must release), or NULL if there is
   * no such master registered.
   */
  struct spi_master *spi_busnum_to_master(u16 bus_num)
  {
  	struct device		*dev;
  	struct spi_master	*master = NULL;
  
  	dev = class_find_device(&spi_master_class, NULL, &bus_num,
  				__spi_master_match);
  	if (dev)
  		master = container_of(dev, struct spi_master, dev);
  	/* reference got in class_find_device */
  	return master;
  }
  EXPORT_SYMBOL_GPL(spi_busnum_to_master);
  
  
  /*-------------------------------------------------------------------------*/
  
  /* Core methods for SPI master protocol drivers.  Some of the
   * other core methods are currently defined as inline functions.
   */
  
  /**
   * spi_setup - setup SPI mode and clock rate
   * @spi: the device whose settings are being modified
   * Context: can sleep, and no requests are queued to the device
   *
   * SPI protocol drivers may need to update the transfer mode if the
   * device doesn't work with its default.  They may likewise need
   * to update clock rates or word sizes from initial values.  This function
   * changes those settings, and must be called from a context that can sleep.
   * Except for SPI_CS_HIGH, which takes effect immediately, the changes take
   * effect the next time the device is selected and data is transferred to
   * or from it.  When this function returns, the spi device is deselected.
   *
   * Note that this call will fail if the protocol driver specifies an option
   * that the underlying controller or its driver does not support.  For
   * example, not all hardware supports wire transfers using nine bit words,
   * LSB-first wire encoding, or active-high chipselects.
   */
  int spi_setup(struct spi_device *spi)
  {
  	unsigned	bad_bits;
  	int		status;
  
  	/* help drivers fail *cleanly* when they need options
  	 * that aren't supported with their current master
  	 */
  	bad_bits = spi->mode & ~spi->master->mode_bits;
  	if (bad_bits) {
  		dev_dbg(&spi->dev, "setup: unsupported mode bits %x\n",
  			bad_bits);
  		return -EINVAL;
  	}
  
  	if (!spi->bits_per_word)
  		spi->bits_per_word = 8;
  
  	status = spi->master->setup(spi);
  
  	dev_dbg(&spi->dev, "setup mode %d, %s%s%s%s"
  				"%u bits/w, %u Hz max --> %d\n",
  			(int) (spi->mode & (SPI_CPOL | SPI_CPHA)),
  			(spi->mode & SPI_CS_HIGH) ? "cs_high, " : "",
  			(spi->mode & SPI_LSB_FIRST) ? "lsb, " : "",
  			(spi->mode & SPI_3WIRE) ? "3wire, " : "",
  			(spi->mode & SPI_LOOP) ? "loopback, " : "",
  			spi->bits_per_word, spi->max_speed_hz,
  			status);
  
  	return status;
  }
  EXPORT_SYMBOL_GPL(spi_setup);
  
  /**
   * spi_async - asynchronous SPI transfer
   * @spi: device with which data will be exchanged
   * @message: describes the data transfers, including completion callback
   * Context: any (irqs may be blocked, etc)
   *
   * This call may be used in_irq and other contexts which can't sleep,
   * as well as from task contexts which can sleep.
   *
   * The completion callback is invoked in a context which can't sleep.
   * Before that invocation, the value of message->status is undefined.
   * When the callback is issued, message->status holds either zero (to
   * indicate complete success) or a negative error code.  After that
   * callback returns, the driver which issued the transfer request may
   * deallocate the associated memory; it's no longer in use by any SPI
   * core or controller driver code.
   *
   * Note that although all messages to a spi_device are handled in
   * FIFO order, messages may go to different devices in other orders.
   * Some device might be higher priority, or have various "hard" access
   * time requirements, for example.
   *
   * On detection of any fault during the transfer, processing of
   * the entire message is aborted, and the device is deselected.
   * Until returning from the associated message completion callback,
   * no other spi_message queued to that device will be processed.
   * (This rule applies equally to all the synchronous transfer calls,
   * which are wrappers around this core asynchronous primitive.)
   */
  int spi_async(struct spi_device *spi, struct spi_message *message)
  {
  	struct spi_master *master = spi->master;
  
  	/* Half-duplex links include original MicroWire, and ones with
  	 * only one data pin like SPI_3WIRE (switches direction) or where
  	 * either MOSI or MISO is missing.  They can also be caused by
  	 * software limitations.
  	 */
  	if ((master->flags & SPI_MASTER_HALF_DUPLEX)
  			|| (spi->mode & SPI_3WIRE)) {
  		struct spi_transfer *xfer;
  		unsigned flags = master->flags;
  
  		list_for_each_entry(xfer, &message->transfers, transfer_list) {
  			if (xfer->rx_buf && xfer->tx_buf)
  				return -EINVAL;
  			if ((flags & SPI_MASTER_NO_TX) && xfer->tx_buf)
  				return -EINVAL;
  			if ((flags & SPI_MASTER_NO_RX) && xfer->rx_buf)
  				return -EINVAL;
  		}
  	}
  
  	message->spi = spi;
  	message->status = -EINPROGRESS;
  	return master->transfer(spi, message);
  }
  EXPORT_SYMBOL_GPL(spi_async);
  
  
  /*-------------------------------------------------------------------------*/
  
  /* Utility methods for SPI master protocol drivers, layered on
   * top of the core.  Some other utility methods are defined as
   * inline functions.
   */
  
  static void spi_complete(void *arg)
  {
  	complete(arg);
  }
  
  /**
   * spi_sync - blocking/synchronous SPI data transfers
   * @spi: device with which data will be exchanged
   * @message: describes the data transfers
   * Context: can sleep
   *
   * This call may only be used from a context that may sleep.  The sleep
   * is non-interruptible, and has no timeout.  Low-overhead controller
   * drivers may DMA directly into and out of the message buffers.
   *
   * Note that the SPI device's chip select is active during the message,
   * and then is normally disabled between messages.  Drivers for some
   * frequently-used devices may want to minimize costs of selecting a chip,
   * by leaving it selected in anticipation that the next message will go
   * to the same chip.  (That may increase power usage.)
   *
   * Also, the caller is guaranteeing that the memory associated with the
   * message will not be freed before this call returns.
   *
   * It returns zero on success, else a negative error code.
   */
  int spi_sync(struct spi_device *spi, struct spi_message *message)
  {
  	DECLARE_COMPLETION_ONSTACK(done);
  	int status;
  
  	message->complete = spi_complete;
  	message->context = &done;
  	status = spi_async(spi, message);
  	if (status == 0) {
  		wait_for_completion(&done);
  		status = message->status;
  	}
  	message->context = NULL;
  	return status;
  }
  EXPORT_SYMBOL_GPL(spi_sync);
  
  /* portable code must never pass more than 32 bytes */
  #define	SPI_BUFSIZ	max(32,SMP_CACHE_BYTES)
  
  static u8	*buf;
  
  /**
   * spi_write_then_read - SPI synchronous write followed by read
   * @spi: device with which data will be exchanged
   * @txbuf: data to be written (need not be dma-safe)
   * @n_tx: size of txbuf, in bytes
   * @rxbuf: buffer into which data will be read (need not be dma-safe)
   * @n_rx: size of rxbuf, in bytes
   * Context: can sleep
   *
   * This performs a half duplex MicroWire style transaction with the
   * device, sending txbuf and then reading rxbuf.  The return value
   * is zero for success, else a negative errno status code.
   * This call may only be used from a context that may sleep.
   *
   * Parameters to this routine are always copied using a small buffer;
   * portable code should never use this for more than 32 bytes.
   * Performance-sensitive or bulk transfer code should instead use
   * spi_{async,sync}() calls with dma-safe buffers.
   */
  int spi_write_then_read(struct spi_device *spi,
  		const u8 *txbuf, unsigned n_tx,
  		u8 *rxbuf, unsigned n_rx)
  {
  	static DEFINE_MUTEX(lock);
  
  	int			status;
  	struct spi_message	message;
  	struct spi_transfer	x[2];
  	u8			*local_buf;
  
  	/* Use preallocated DMA-safe buffer.  We can't avoid copying here,
  	 * (as a pure convenience thing), but we can keep heap costs
  	 * out of the hot path ...
  	 */
  	if ((n_tx + n_rx) > SPI_BUFSIZ)
  		return -EINVAL;
  
  	spi_message_init(&message);
  	memset(x, 0, sizeof x);
  	if (n_tx) {
  		x[0].len = n_tx;
  		spi_message_add_tail(&x[0], &message);
  	}
  	if (n_rx) {
  		x[1].len = n_rx;
  		spi_message_add_tail(&x[1], &message);
  	}
  
  	/* ... unless someone else is using the pre-allocated buffer */
  	if (!mutex_trylock(&lock)) {
  		local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
  		if (!local_buf)
  			return -ENOMEM;
  	} else
  		local_buf = buf;
  
  	memcpy(local_buf, txbuf, n_tx);
  	x[0].tx_buf = local_buf;
  	x[1].rx_buf = local_buf + n_tx;
  
  	/* do the i/o */
  	status = spi_sync(spi, &message);
  	if (status == 0)
  		memcpy(rxbuf, x[1].rx_buf, n_rx);
  
  	if (x[0].tx_buf == buf)
  		mutex_unlock(&lock);
  	else
  		kfree(local_buf);
  
  	return status;
  }
  EXPORT_SYMBOL_GPL(spi_write_then_read);
  
  /*-------------------------------------------------------------------------*/
  
  static int __init spi_init(void)
  {
  	int	status;
  
  	buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
  	if (!buf) {
  		status = -ENOMEM;
  		goto err0;
  	}
  
  	status = bus_register(&spi_bus_type);
  	if (status < 0)
  		goto err1;
  
  	status = class_register(&spi_master_class);
  	if (status < 0)
  		goto err2;
  	return 0;
  
  err2:
  	bus_unregister(&spi_bus_type);
  err1:
  	kfree(buf);
  	buf = NULL;
  err0:
  	return status;
  }
  
  /* board_info is normally registered in arch_initcall(),
   * but even essential drivers wait till later
   *
   * REVISIT only boardinfo really needs static linking. the rest (device and
   * driver registration) _could_ be dynamically linked (modular) ... costs
   * include needing to have boardinfo data structures be much more public.
   */
  postcore_initcall(spi_init);
  
  