Committer: Michael Beasley <mike@snafu.setup>

[mikesnafu-overlay.git] / include / linux / spi / spi.h

/*
 * 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.
 */

#ifndef __LINUX_SPI_H
#define __LINUX_SPI_H

/*
 * INTERFACES between SPI master-side drivers and SPI infrastructure.
 * (There's no SPI slave support for Linux yet...)
 */
extern struct bus_type spi_bus_type;

/**
 * struct spi_device - Master side proxy for an SPI slave device
 * @dev: Driver model representation of the device.
 * @master: SPI controller used with the device.
 * @max_speed_hz: Maximum clock rate to be used with this chip
 *      (on this board); may be changed by the device's driver.
 *      The spi_transfer.speed_hz can override this for each transfer.
 * @chip_select: Chipselect, distinguishing chips handled by @master.
 * @mode: The spi mode defines how data is clocked out and in.
 *      This may be changed by the device's driver.
 *      The "active low" default for chipselect mode can be overridden
 *      (by specifying SPI_CS_HIGH) as can the "MSB first" default for
 *      each word in a transfer (by specifying SPI_LSB_FIRST).
 * @bits_per_word: Data transfers involve one or more words; word sizes
 *      like eight or 12 bits are common.  In-memory wordsizes are
 *      powers of two bytes (e.g. 20 bit samples use 32 bits).
 *      This may be changed by the device's driver, or left at the
 *      default (0) indicating protocol words are eight bit bytes.
 *      The spi_transfer.bits_per_word can override this for each transfer.
 * @irq: Negative, or the number passed to request_irq() to receive
 *      interrupts from this device.
 * @controller_state: Controller's runtime state
 * @controller_data: Board-specific definitions for controller, such as
 *      FIFO initialization parameters; from board_info.controller_data
 * @modalias: Name of the driver to use with this device, or an alias
 *      for that name.  This appears in the sysfs "modalias" attribute
 *      for driver coldplugging, and in uevents used for hotplugging
 *
 * A @spi_device is used to interchange data between an SPI slave
 * (usually a discrete chip) and CPU memory.
 *
 * In @dev, the platform_data is used to hold information about this
 * device that's meaningful to the device's protocol driver, but not
 * to its controller.  One example might be an identifier for a chip
 * variant with slightly different functionality; another might be
 * information about how this particular board wires the chip's pins.
 */
struct spi_device {
        struct device           dev;
        struct spi_master       *master;
        u32                     max_speed_hz;
        u8                      chip_select;
        u8                      mode;
#define SPI_CPHA        0x01                    /* clock phase */
#define SPI_CPOL        0x02                    /* clock polarity */
#define SPI_MODE_0      (0|0)                   /* (original MicroWire) */
#define SPI_MODE_1      (0|SPI_CPHA)
#define SPI_MODE_2      (SPI_CPOL|0)
#define SPI_MODE_3      (SPI_CPOL|SPI_CPHA)
#define SPI_CS_HIGH     0x04                    /* chipselect active high? */
#define SPI_LSB_FIRST   0x08                    /* per-word bits-on-wire */
#define SPI_3WIRE       0x10                    /* SI/SO signals shared */
#define SPI_LOOP        0x20                    /* loopback mode */
        u8                      bits_per_word;
        int                     irq;
        void                    *controller_state;
        void                    *controller_data;
        const char              *modalias;

        /*
         * likely need more hooks for more protocol options affecting how
         * the controller talks to each chip, like:
         *  - memory packing (12 bit samples into low bits, others zeroed)
         *  - priority
         *  - drop chipselect after each word
         *  - chipselect delays
         *  - ...
         */
};

static inline struct spi_device *to_spi_device(struct device *dev)
{
        return dev ? container_of(dev, struct spi_device, dev) : NULL;
}

/* most drivers won't need to care about device refcounting */
static inline struct spi_device *spi_dev_get(struct spi_device *spi)
{
        return (spi && get_device(&spi->dev)) ? spi : NULL;
}

static inline void spi_dev_put(struct spi_device *spi)
{
        if (spi)
                put_device(&spi->dev);
}

/* ctldata is for the bus_master driver's runtime state */
static inline void *spi_get_ctldata(struct spi_device *spi)
{
        return spi->controller_state;
}

static inline void spi_set_ctldata(struct spi_device *spi, void *state)
{
        spi->controller_state = state;
}

/* device driver data */

static inline void spi_set_drvdata(struct spi_device *spi, void *data)
{
        dev_set_drvdata(&spi->dev, data);
}

static inline void *spi_get_drvdata(struct spi_device *spi)
{
        return dev_get_drvdata(&spi->dev);
}

struct spi_message;



/**
 * struct spi_driver - Host side "protocol" driver
 * @probe: Binds this driver to the spi device.  Drivers can verify
 *      that the device is actually present, and may need to configure
 *      characteristics (such as bits_per_word) which weren't needed for
 *      the initial configuration done during system setup.
 * @remove: Unbinds this driver from the spi device
 * @shutdown: Standard shutdown callback used during system state
 *      transitions such as powerdown/halt and kexec
 * @suspend: Standard suspend callback used during system state transitions
 * @resume: Standard resume callback used during system state transitions
 * @driver: SPI device drivers should initialize the name and owner
 *      field of this structure.
 *
 * This represents the kind of device driver that uses SPI messages to
 * interact with the hardware at the other end of a SPI link.  It's called
 * a "protocol" driver because it works through messages rather than talking
 * directly to SPI hardware (which is what the underlying SPI controller
 * driver does to pass those messages).  These protocols are defined in the
 * specification for the device(s) supported by the driver.
 *
 * As a rule, those device protocols represent the lowest level interface
 * supported by a driver, and it will support upper level interfaces too.
 * Examples of such upper levels include frameworks like MTD, networking,
 * MMC, RTC, filesystem character device nodes, and hardware monitoring.
 */
struct spi_driver {
        int                     (*probe)(struct spi_device *spi);
        int                     (*remove)(struct spi_device *spi);
        void                    (*shutdown)(struct spi_device *spi);
        int                     (*suspend)(struct spi_device *spi, pm_message_t mesg);
        int                     (*resume)(struct spi_device *spi);
        struct device_driver    driver;
};

static inline struct spi_driver *to_spi_driver(struct device_driver *drv)
{
        return drv ? container_of(drv, struct spi_driver, driver) : NULL;
}

extern int spi_register_driver(struct spi_driver *sdrv);

/**
 * spi_unregister_driver - reverse effect of spi_register_driver
 * @sdrv: the driver to unregister
 * Context: can sleep
 */
static inline void spi_unregister_driver(struct spi_driver *sdrv)
{
        if (sdrv)
                driver_unregister(&sdrv->driver);
}


/**
 * struct spi_master - interface to SPI master controller
 * @dev: device interface to this driver
 * @bus_num: board-specific (and often SOC-specific) identifier for a
 *      given SPI controller.
 * @num_chipselect: chipselects are used to distinguish individual
 *      SPI slaves, and are numbered from zero to num_chipselects.
 *      each slave has a chipselect signal, but it's common that not
 *      every chipselect is connected to a slave.
 * @setup: updates the device mode and clocking records used by a
 *      device's SPI controller; protocol code may call this.  This
 *      must fail if an unrecognized or unsupported mode is requested.
 *      It's always safe to call this unless transfers are pending on
 *      the device whose settings are being modified.
 * @transfer: adds a message to the controller's transfer queue.
 * @cleanup: frees controller-specific state
 *
 * Each SPI master controller can communicate with one or more @spi_device
 * children.  These make a small bus, sharing MOSI, MISO and SCK signals
 * but not chip select signals.  Each device may be configured to use a
 * different clock rate, since those shared signals are ignored unless
 * the chip is selected.
 *
 * The driver for an SPI controller manages access to those devices through
 * a queue of spi_message transactions, copying data between CPU memory and
 * an SPI slave device.  For each such message it queues, it calls the
 * message's completion function when the transaction completes.
 */
struct spi_master {
        struct device   dev;

        /* other than negative (== assign one dynamically), bus_num is fully
         * board-specific.  usually that simplifies to being SOC-specific.
         * example:  one SOC has three SPI controllers, numbered 0..2,
         * and one board's schematics might show it using SPI-2.  software
         * would normally use bus_num=2 for that controller.
         */
        s16                     bus_num;

        /* chipselects will be integral to many controllers; some others
         * might use board-specific GPIOs.
         */
        u16                     num_chipselect;

        /* setup mode and clock, etc (spi driver may call many times) */
        int                     (*setup)(struct spi_device *spi);

        /* bidirectional bulk transfers
         *
         * + The transfer() method may not sleep; its main role is
         *   just to add the message to the queue.
         * + For now there's no remove-from-queue operation, or
         *   any other request management
         * + To a given spi_device, message queueing is pure fifo
         *
         * + The master's main job is to process its message queue,
         *   selecting a chip then transferring data
         * + If there are multiple spi_device children, the i/o queue
         *   arbitration algorithm is unspecified (round robin, fifo,
         *   priority, reservations, preemption, etc)
         *
         * + Chipselect stays active during the entire message
         *   (unless modified by spi_transfer.cs_change != 0).
         * + The message transfers use clock and SPI mode parameters
         *   previously established by setup() for this device
         */
        int                     (*transfer)(struct spi_device *spi,
                                                struct spi_message *mesg);

        /* called on release() to free memory provided by spi_master */
        void                    (*cleanup)(struct spi_device *spi);
};

static inline void *spi_master_get_devdata(struct spi_master *master)
{
        return dev_get_drvdata(&master->dev);
}

static inline void spi_master_set_devdata(struct spi_master *master, void *data)
{
        dev_set_drvdata(&master->dev, data);
}

static inline struct spi_master *spi_master_get(struct spi_master *master)
{
        if (!master || !get_device(&master->dev))
                return NULL;
        return master;
}

static inline void spi_master_put(struct spi_master *master)
{
        if (master)
                put_device(&master->dev);
}


/* the spi driver core manages memory for the spi_master classdev */
extern struct spi_master *
spi_alloc_master(struct device *host, unsigned size);

extern int spi_register_master(struct spi_master *master);
extern void spi_unregister_master(struct spi_master *master);

extern struct spi_master *spi_busnum_to_master(u16 busnum);

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

/*
 * I/O INTERFACE between SPI controller and protocol drivers
 *
 * Protocol drivers use a queue of spi_messages, each transferring data
 * between the controller and memory buffers.
 *
 * The spi_messages themselves consist of a series of read+write transfer
 * segments.  Those segments always read the same number of bits as they
 * write; but one or the other is easily ignored by passing a null buffer
 * pointer.  (This is unlike most types of I/O API, because SPI hardware
 * is full duplex.)
 *
 * NOTE:  Allocation of spi_transfer and spi_message memory is entirely
 * up to the protocol driver, which guarantees the integrity of both (as
 * well as the data buffers) for as long as the message is queued.
 */

/**
 * struct spi_transfer - a read/write buffer pair
 * @tx_buf: data to be written (dma-safe memory), or NULL
 * @rx_buf: data to be read (dma-safe memory), or NULL
 * @tx_dma: DMA address of tx_buf, if @spi_message.is_dma_mapped
 * @rx_dma: DMA address of rx_buf, if @spi_message.is_dma_mapped
 * @len: size of rx and tx buffers (in bytes)
 * @speed_hz: Select a speed other then the device default for this
 *      transfer. If 0 the default (from @spi_device) is used.
 * @bits_per_word: select a bits_per_word other then the device default
 *      for this transfer. If 0 the default (from @spi_device) is used.
 * @cs_change: affects chipselect after this transfer completes
 * @delay_usecs: microseconds to delay after this transfer before
 *      (optionally) changing the chipselect status, then starting
 *      the next transfer or completing this @spi_message.
 * @transfer_list: transfers are sequenced through @spi_message.transfers
 *
 * SPI transfers always write the same number of bytes as they read.
 * Protocol drivers should always provide @rx_buf and/or @tx_buf.
 * In some cases, they may also want to provide DMA addresses for
 * the data being transferred; that may reduce overhead, when the
 * underlying driver uses dma.
 *
 * If the transmit buffer is null, zeroes will be shifted out
 * while filling @rx_buf.  If the receive buffer is null, the data
 * shifted in will be discarded.  Only "len" bytes shift out (or in).
 * It's an error to try to shift out a partial word.  (For example, by
 * shifting out three bytes with word size of sixteen or twenty bits;
 * the former uses two bytes per word, the latter uses four bytes.)
 *
 * In-memory data values are always in native CPU byte order, translated
 * from the wire byte order (big-endian except with SPI_LSB_FIRST).  So
 * for example when bits_per_word is sixteen, buffers are 2N bytes long
 * (@len = 2N) and hold N sixteen bit words in CPU byte order.
 *
 * When the word size of the SPI transfer is not a power-of-two multiple
 * of eight bits, those in-memory words include extra bits.  In-memory
 * words are always seen by protocol drivers as right-justified, so the
 * undefined (rx) or unused (tx) bits are always the most significant bits.
 *
 * All SPI transfers start with the relevant chipselect active.  Normally
 * it stays selected until after the last transfer in a message.  Drivers
 * can affect the chipselect signal using cs_change.
 *
 * (i) If the transfer isn't the last one in the message, this flag is
 * used to make the chipselect briefly go inactive in the middle of the
 * message.  Toggling chipselect in this way may be needed to terminate
 * a chip command, letting a single spi_message perform all of group of
 * chip transactions together.
 *
 * (ii) When the transfer is the last one in the message, the chip may
 * stay selected until the next transfer.  On multi-device SPI busses
 * with nothing blocking messages going to other devices, this is just
 * a performance hint; starting a message to another device deselects
 * this one.  But in other cases, this can be used to ensure correctness.
 * Some devices need protocol transactions to be built from a series of
 * spi_message submissions, where the content of one message is determined
 * by the results of previous messages and where the whole transaction
 * ends when the chipselect goes intactive.
 *
 * The code that submits an spi_message (and its spi_transfers)
 * to the lower layers is responsible for managing its memory.
 * Zero-initialize every field you don't set up explicitly, to
 * insulate against future API updates.  After you submit a message
 * and its transfers, ignore them until its completion callback.
 */
struct spi_transfer {
        /* it's ok if tx_buf == rx_buf (right?)
         * for MicroWire, one buffer must be null
         * buffers must work with dma_*map_single() calls, unless
         *   spi_message.is_dma_mapped reports a pre-existing mapping
         */
        const void      *tx_buf;
        void            *rx_buf;
        unsigned        len;

        dma_addr_t      tx_dma;
        dma_addr_t      rx_dma;

        unsigned        cs_change:1;
        u8              bits_per_word;
        u16             delay_usecs;
        u32             speed_hz;

        struct list_head transfer_list;
};

/**
 * struct spi_message - one multi-segment SPI transaction
 * @transfers: list of transfer segments in this transaction
 * @spi: SPI device to which the transaction is queued
 * @is_dma_mapped: if true, the caller provided both dma and cpu virtual
 *      addresses for each transfer buffer
 * @complete: called to report transaction completions
 * @context: the argument to complete() when it's called
 * @actual_length: the total number of bytes that were transferred in all
 *      successful segments
 * @status: zero for success, else negative errno
 * @queue: for use by whichever driver currently owns the message
 * @state: for use by whichever driver currently owns the message
 *
 * A @spi_message is used to execute an atomic sequence of data transfers,
 * each represented by a struct spi_transfer.  The sequence is "atomic"
 * in the sense that no other spi_message may use that SPI bus until that
 * sequence completes.  On some systems, many such sequences can execute as
 * as single programmed DMA transfer.  On all systems, these messages are
 * queued, and might complete after transactions to other devices.  Messages
 * sent to a given spi_device are alway executed in FIFO order.
 *
 * The code that submits an spi_message (and its spi_transfers)
 * to the lower layers is responsible for managing its memory.
 * Zero-initialize every field you don't set up explicitly, to
 * insulate against future API updates.  After you submit a message
 * and its transfers, ignore them until its completion callback.
 */
struct spi_message {
        struct list_head        transfers;

        struct spi_device       *spi;

        unsigned                is_dma_mapped:1;

        /* REVISIT:  we might want a flag affecting the behavior of the
         * last transfer ... allowing things like "read 16 bit length L"
         * immediately followed by "read L bytes".  Basically imposing
         * a specific message scheduling algorithm.
         *
         * Some controller drivers (message-at-a-time queue processing)
         * could provide that as their default scheduling algorithm.  But
         * others (with multi-message pipelines) could need a flag to
         * tell them about such special cases.
         */

        /* completion is reported through a callback */
        void                    (*complete)(void *context);
        void                    *context;
        unsigned                actual_length;
        int                     status;

        /* for optional use by whatever driver currently owns the
         * spi_message ...  between calls to spi_async and then later
         * complete(), that's the spi_master controller driver.
         */
        struct list_head        queue;
        void                    *state;
};

static inline void spi_message_init(struct spi_message *m)
{
        memset(m, 0, sizeof *m);
        INIT_LIST_HEAD(&m->transfers);
}

static inline void
spi_message_add_tail(struct spi_transfer *t, struct spi_message *m)
{
        list_add_tail(&t->transfer_list, &m->transfers);
}

static inline void
spi_transfer_del(struct spi_transfer *t)
{
        list_del(&t->transfer_list);
}

/* It's fine to embed message and transaction structures in other data
 * structures so long as you don't free them while they're in use.
 */

static inline struct spi_message *spi_message_alloc(unsigned ntrans, gfp_t flags)
{
        struct spi_message *m;

        m = kzalloc(sizeof(struct spi_message)
                        + ntrans * sizeof(struct spi_transfer),
                        flags);
        if (m) {
                int i;
                struct spi_transfer *t = (struct spi_transfer *)(m + 1);

                INIT_LIST_HEAD(&m->transfers);
                for (i = 0; i < ntrans; i++, t++)
                        spi_message_add_tail(t, m);
        }
        return m;
}

static inline void spi_message_free(struct spi_message *m)
{
        kfree(m);
}

/**
 * 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.
 */
static inline int
spi_setup(struct spi_device *spi)
{
        return spi->master->setup(spi);
}


/**
 * 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.)
 */
static inline int
spi_async(struct spi_device *spi, struct spi_message *message)
{
        message->spi = spi;
        return spi->master->transfer(spi, message);
}

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

/* All these synchronous SPI transfer routines are utilities layered
 * over the core async transfer primitive.  Here, "synchronous" means
 * they will sleep uninterruptibly until the async transfer completes.
 */

extern int spi_sync(struct spi_device *spi, struct spi_message *message);

/**
 * spi_write - SPI synchronous write
 * @spi: device to which data will be written
 * @buf: data buffer
 * @len: data buffer size
 * Context: can sleep
 *
 * This writes the buffer and returns zero or a negative error code.
 * Callable only from contexts that can sleep.
 */
static inline int
spi_write(struct spi_device *spi, const u8 *buf, size_t len)
{
        struct spi_transfer     t = {
                        .tx_buf         = buf,
                        .len            = len,
                };
        struct spi_message      m;

        spi_message_init(&m);
        spi_message_add_tail(&t, &m);
        return spi_sync(spi, &m);
}

/**
 * spi_read - SPI synchronous read
 * @spi: device from which data will be read
 * @buf: data buffer
 * @len: data buffer size
 * Context: can sleep
 *
 * This reads the buffer and returns zero or a negative error code.
 * Callable only from contexts that can sleep.
 */
static inline int
spi_read(struct spi_device *spi, u8 *buf, size_t len)
{
        struct spi_transfer     t = {
                        .rx_buf         = buf,
                        .len            = len,
                };
        struct spi_message      m;

        spi_message_init(&m);
        spi_message_add_tail(&t, &m);
        return spi_sync(spi, &m);
}

/* this copies txbuf and rxbuf data; for small transfers only! */
extern int spi_write_then_read(struct spi_device *spi,
                const u8 *txbuf, unsigned n_tx,
                u8 *rxbuf, unsigned n_rx);

/**
 * spi_w8r8 - SPI synchronous 8 bit write followed by 8 bit read
 * @spi: device with which data will be exchanged
 * @cmd: command to be written before data is read back
 * Context: can sleep
 *
 * This returns the (unsigned) eight bit number returned by the
 * device, or else a negative error code.  Callable only from
 * contexts that can sleep.
 */
static inline ssize_t spi_w8r8(struct spi_device *spi, u8 cmd)
{
        ssize_t                 status;
        u8                      result;

        status = spi_write_then_read(spi, &cmd, 1, &result, 1);

        /* return negative errno or unsigned value */
        return (status < 0) ? status : result;
}

/**
 * spi_w8r16 - SPI synchronous 8 bit write followed by 16 bit read
 * @spi: device with which data will be exchanged
 * @cmd: command to be written before data is read back
 * Context: can sleep
 *
 * This returns the (unsigned) sixteen bit number returned by the
 * device, or else a negative error code.  Callable only from
 * contexts that can sleep.
 *
 * The number is returned in wire-order, which is at least sometimes
 * big-endian.
 */
static inline ssize_t spi_w8r16(struct spi_device *spi, u8 cmd)
{
        ssize_t                 status;
        u16                     result;

        status = spi_write_then_read(spi, &cmd, 1, (u8 *) &result, 2);

        /* return negative errno or unsigned value */
        return (status < 0) ? status : result;
}

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

/*
 * INTERFACE between board init code and SPI infrastructure.
 *
 * No SPI driver ever sees these SPI device table segments, but
 * it's how the SPI core (or adapters that get hotplugged) grows
 * the driver model tree.
 *
 * As a rule, SPI devices can't be probed.  Instead, board init code
 * provides a table listing the devices which are present, with enough
 * information to bind and set up the device's driver.  There's basic
 * support for nonstatic configurations too; enough to handle adding
 * parport adapters, or microcontrollers acting as USB-to-SPI bridges.
 */

/**
 * struct spi_board_info - board-specific template for a SPI device
 * @modalias: Initializes spi_device.modalias; identifies the driver.
 * @platform_data: Initializes spi_device.platform_data; the particular
 *      data stored there is driver-specific.
 * @controller_data: Initializes spi_device.controller_data; some
 *      controllers need hints about hardware setup, e.g. for DMA.
 * @irq: Initializes spi_device.irq; depends on how the board is wired.
 * @max_speed_hz: Initializes spi_device.max_speed_hz; based on limits
 *      from the chip datasheet and board-specific signal quality issues.
 * @bus_num: Identifies which spi_master parents the spi_device; unused
 *      by spi_new_device(), and otherwise depends on board wiring.
 * @chip_select: Initializes spi_device.chip_select; depends on how
 *      the board is wired.
 * @mode: Initializes spi_device.mode; based on the chip datasheet, board
 *      wiring (some devices support both 3WIRE and standard modes), and
 *      possibly presence of an inverter in the chipselect path.
 *
 * When adding new SPI devices to the device tree, these structures serve
 * as a partial device template.  They hold information which can't always
 * be determined by drivers.  Information that probe() can establish (such
 * as the default transfer wordsize) is not included here.
 *
 * These structures are used in two places.  Their primary role is to
 * be stored in tables of board-specific device descriptors, which are
 * declared early in board initialization and then used (much later) to
 * populate a controller's device tree after the that controller's driver
 * initializes.  A secondary (and atypical) role is as a parameter to
 * spi_new_device() call, which happens after those controller drivers
 * are active in some dynamic board configuration models.
 */
struct spi_board_info {
        /* the device name and module name are coupled, like platform_bus;
         * "modalias" is normally the driver name.
         *
         * platform_data goes to spi_device.dev.platform_data,
         * controller_data goes to spi_device.controller_data,
         * irq is copied too
         */
        char            modalias[KOBJ_NAME_LEN];
        const void      *platform_data;
        void            *controller_data;
        int             irq;

        /* slower signaling on noisy or low voltage boards */
        u32             max_speed_hz;


        /* bus_num is board specific and matches the bus_num of some
         * spi_master that will probably be registered later.
         *
         * chip_select reflects how this chip is wired to that master;
         * it's less than num_chipselect.
         */
        u16             bus_num;
        u16             chip_select;

        /* mode becomes spi_device.mode, and is essential for chips
         * where the default of SPI_CS_HIGH = 0 is wrong.
         */
        u8              mode;

        /* ... may need additional spi_device chip config data here.
         * avoid stuff protocol drivers can set; but include stuff
         * needed to behave without being bound to a driver:
         *  - quirks like clock rate mattering when not selected
         */
};

#ifdef  CONFIG_SPI
extern int
spi_register_board_info(struct spi_board_info const *info, unsigned n);
#else
/* board init code may ignore whether SPI is configured or not */
static inline int
spi_register_board_info(struct spi_board_info const *info, unsigned n)
        { return 0; }
#endif


/* If you're hotplugging an adapter with devices (parport, usb, etc)
 * use spi_new_device() to describe each device.  You can also call
 * spi_unregister_device() to start making that device vanish, but
 * normally that would be handled by spi_unregister_master().
 */
extern struct spi_device *
spi_new_device(struct spi_master *, struct spi_board_info *);

static inline void
spi_unregister_device(struct spi_device *spi)
{
        if (spi)
                device_unregister(&spi->dev);
}

#endif /* __LINUX_SPI_H */