linux spi驱动开发学习(一)-----spi子系统架构

linux spi驱动开发学习(一)-----spi子系统架构

一.spi设备(各定义在include/linux/spi.h)

struct spi_device {
    struct device   dev;    //设备文件
    struct spi_master   *master;    //spi主机
    u32 max_speed_hz;   //最大速率
    u8  chip_select;    //片选
    u8  mode;   //模式
    u8  bits_per_word;  //一个字有多少位
    int irq;    //中断号
    void    *controller_state;  //控制器状态
    void    *controller_data;   //控制器数据
    char    modalias[SPI_NAME_SIZE];//名字
};

2.spi传输模式:

#define SPI_CPHA        0x01            //时钟相位
#define SPI_CPOL        0x02            //时钟继续
#define SPI_MODE_0  (0|0)           //模式0
#define SPI_MODE_1  (0|SPI_CPHA)    //模式1
#define SPI_MODE_2  (SPI_CPOL|0)    //模式2
#define SPI_MODE_3  (SPI_CPOL|SPI_CPHA) //模式3
#define SPI_CS_HIGH 0x04            //片选高电平
#define SPI_LSB_FIRST   0x08            //LSB
#define SPI_3WIRE       0x10            //3线模式 SI和SO同一根线
#define SPI_LOOP        0x20            //回送模式
#define SPI_NO_CS       0x40            //单个设备占用一根SPI总线,所以没片选
#define SPI_READY       0x80            //从机拉低电平停止数据传输

3.spi设备的添加spi_new_device

struct spi_device *spi_new_device(struct spi_master *master,struct spi_board_info *chip)
{
    struct spi_device   *proxy;
    int status;

    proxy = spi_alloc_device(master);   //3.1 spi设备初始化
    if (!proxy)
        return NULL;
    WARN_ON(strlen(chip->modalias) >= sizeof(proxy->modalias));
    proxy->chip_select = chip->chip_select;   //片选
    proxy->max_speed_hz = chip->max_speed_hz; //最大速率
    proxy->mode = chip->mode; //模式
    proxy->irq = chip->irq;   //中断号
    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); //3.2 添加spi设备
    if (status < 0) {
        spi_dev_put(proxy); //增加spi设备引用计数
        return NULL;
    }
    return proxy;
}
EXPORT_SYMBOL_GPL(spi_new_device);

 

3.1.分配spi设备

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))    //判断spi主机是否存在
        return NULL;
    spi = kzalloc(sizeof *spi, GFP_KERNEL); //分配内存
    if (!spi) {
        dev_err(dev, "cannot alloc spi_device\n");
        spi_master_put(master); //增加主机引用计数
        return NULL;
    }
    spi->master = master;    //设置spi主机
    spi->dev.parent = dev;   //spi设备文件的父设备为spi主机设备文件的父设备
    spi->dev.bus = &spi_bus_type;    //总线类型
    spi->dev.release = spidev_release;   //释放方法
    device_initialize(&spi->dev);    //设备初始化
    return spi;
}
EXPORT_SYMBOL_GPL(spi_alloc_device);

3.2 添加spi设备

int spi_add_device(struct spi_device *spi)
{
    static DEFINE_MUTEX(spi_add_lock);
    struct device *dev = spi->master->dev.parent;
    struct device *d;
    int status;

    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;
    }
    dev_set_name(&spi->dev, "%s.%u", dev_name(&spi->master->dev),spi->chip_select);
    mutex_lock(&spi_add_lock);
    d = bus_find_device_by_name(&spi_bus_type, NULL, dev_name(&spi->dev));   //查找总线上的spi设备
    if (d != NULL) {    //判断是否已经在使用了
        dev_err(dev, "chipselect %d already in use\n",spi->chip_select);
        put_device(d);
        status = -EBUSY;
        goto done;
    }
    status = spi_setup(spi);    //调用spi主机 setup方法
    if (status < 0) {
        dev_err(dev, "can't setup %s, status %d\n",dev_name(&spi->dev), status);
        goto done;
    }
    status = device_add(&spi->dev);  //添加设备
    if (status < 0)
        dev_err(dev, "can't add %s, status %d\n",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);

3.2.1 spi setup方法

int spi_setup(struct spi_device *spi)
{
    unsigned    bad_bits;
    int status;
    bad_bits = spi->mode & ~spi->master->mode_bits;    //比较spi设备的模式和spi主机支持的模式
    if (bad_bits) { //存在不支持的模式
        dev_err(&spi->dev, "setup: unsupported mode bits %x\n",bad_bits);
        return -EINVAL;
    }
    if (!spi->bits_per_word) //若没设置设备的每个字含多少位
        spi->bits_per_word = 8;  //则默认设置为8

    status = spi->master->setup(spi); //调用spi主机的setup方法

    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);

二.spi板级设备

1.板级设备结构体

struct spi_board_info {
    char    modalias[SPI_NAME_SIZE];    //名字
    const void  *platform_data; //平台数据
    void    *controller_data;   //控制器数据
    int irq;            //中断号
    u32 max_speed_hz;   //最大速率
    u16 bus_num;        //spi总线编号
    u16 chip_select;    //片选
    u8  mode;           //模式
};

 

2.板级设备注册(静态注册,一般在板级初始化函数中调用)

int __init spi_register_board_info(struct spi_board_info const *info, unsigned n)
{
    struct boardinfo *bi;
    int i;

    bi = kzalloc(n * sizeof(*bi), GFP_KERNEL);  //分配内存
    if (!bi)
        return -ENOMEM;
    for (i = 0; i < n; i++, bi++, info++) {
        struct spi_master *master;
        memcpy(&bi->board_info, info, sizeof(*info));    //设置bi的板级信息
        mutex_lock(&board_lock);
        list_add_tail(&bi->list, &board_list);   //添加bi->list到全局board_list链表
        list_for_each_entry(master, &spi_master_list, list) //遍历spi主机链表
            spi_match_master_to_boardinfo(master, &bi->board_info);
        mutex_unlock(&board_lock);
    }
    return 0;
}

2.1.spi板级设备与spi主机匹配

static void spi_match_master_to_boardinfo(struct spi_master *master,struct spi_board_info *bi)
{
    struct spi_device *dev;
    if (master->bus_num != bi->bus_num)
        return;
    dev = spi_new_device(master, bi);
    if (!dev)
        dev_err(master->dev.parent, "can't create new device for %s\n",bi->modalias);
}

三.spi设备驱动

1.spi设备驱动结构体

struct spi_driver {
    const struct spi_device_id *id_table;   //spi设备id表
    int     (*probe)(struct spi_device *spi);   //probe方法(探测到设备)
    int     (*remove)(struct spi_device *spi);  //remove方法(设备移除)
    void    (*shutdown)(struct spi_device *spi);    //shutdown方法(关闭设备)
    int     (*suspend)(struct spi_device *spi, pm_message_t mesg);  //suspend方法(挂起设备)
    int     (*resume)(struct spi_device *spi);  //resume方法(唤醒设备)
    struct device_driver    driver; //设备驱动文件
};

2.spi设备驱动注册

int spi_register_driver(struct spi_driver *sdrv)
{
    sdrv->driver.bus = &spi_bus_type;    //总线类型
    if (sdrv->probe) //若存在probe方法
        sdrv->driver.probe = spi_drv_probe;  //设置其设备驱动文件的probe方法为spi_drv_probe
    if (sdrv->remove)    //若存在remove方法
        sdrv->driver.remove = spi_drv_remove;    //设置其设备驱动文件的remove方法为spi_drv_remove
    if (sdrv->shutdown)  ////若存在shutdown方法
        sdrv->driver.shutdown = spi_drv_shutdown;    //设置其设备驱动文件的shutdown方法为spi_drv_shutdown
    return driver_register(&sdrv->driver);   //注册设备驱动
}
EXPORT_SYMBOL_GPL(spi_register_driver);

这里的probe方法会在设备与驱动匹配的时候给调用
参看really_probe函数的部分代码

if (dev->bus->probe) {        //若总线有probe方法(spi子系统的没有)
    ret = dev->bus->probe(dev);   //则调用总线的probe方法
    if (ret)
        goto probe_failed;
}
else if (drv->probe) {   //若存在设备驱动的probe方法
    ret = drv->probe(dev);       //则调用设备驱动的probe方法
    if (ret)
        goto probe_failed;
}

 

2.1 spi_drv_probe

static int spi_drv_probe(struct device *dev)
{
    const struct spi_driver *sdrv = to_spi_driver(dev->driver);  //根据设备文件的设备驱动找到spi设备驱动
    return sdrv->probe(to_spi_device(dev));  //调用spi设备驱动的probe方法
}

3.spi设备驱动注销

static inline void spi_unregister_driver(struct spi_driver *sdrv)
{
    if (sdrv)
        driver_unregister(&sdrv->driver);    //注销设备驱动
}

四.spi主机

1.spi主机结构体

struct spi_master {
    struct device   dev;    //spi主机设备文件
    struct list_head list;
    s16 bus_num;    //spi总线号
    u16 num_chipselect; //片选号
    u16 dma_alignment;  //dma算法
    u16 mode_bits;  //模式位
    u16 flags;  //传输类型标志
    spinlock_t  bus_lock_spinlock;  //spi总线自旋锁
    struct mutex    bus_lock_mutex; //spi总线互斥锁
    bool    bus_lock_flag;  //上锁标志
    int (*setup)(struct spi_device *spi);   //setup方法
    int (*transfer)(struct spi_device *spi,struct spi_message *mesg);   //传输方法
    void    (*cleanup)(struct spi_device *spi); //cleanup方法
};

1.2.flags标志

#define SPI_MASTER_HALF_DUPLEX  BIT(0)  //半双工
#define SPI_MASTER_NO_RX    BIT(1)      //不读
#define SPI_MASTER_NO_TX    BIT(2)      //不写

2.spi主机初始化spi_alloc_master

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);    //分配内存
    if (!master)
        return NULL;
    device_initialize(&master->dev); //初始化主机设备文件
    master->dev.class = &spi_master_class;   //指定设备类spi_master_class
    master->dev.parent = get_device(dev);    //设置spi主机设备的父设备
    spi_master_set_devdata(master, &master[1]); //设置设备数据
    return master;
}
EXPORT_SYMBOL_GPL(spi_alloc_master);

3.注册spi主机

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;   //获得spi主机设备的父设备
    struct boardinfo    *bi;
    int status = -ENODEV;
    int dynamic = 0;

    if (!dev)
        return -ENODEV;
    if (master->num_chipselect == 0) //判断片选个数
        return -EINVAL;
    if (master->bus_num < 0) {    //验证spi总线编号
        master->bus_num = atomic_dec_return(&dyn_bus_id);
        dynamic = 1;
    }
    spin_lock_init(&master->bus_lock_spinlock);
    mutex_init(&master->bus_lock_mutex);

    master->bus_lock_flag = 0;
    dev_set_name(&master->dev, "spi%u", master->bus_num); //设置spi主机设备名
    status = device_add(&master->dev);   //添加spi主机设备
    if (status < 0)
        goto done;
    dev_dbg(dev, "registered master %s%s\n", dev_name(&master->dev),dynamic ? " (dynamic)" : "");
    mutex_lock(&board_lock);
    list_add_tail(&master->list, &spi_master_list);  //spi主机list链表添加进全局spi_master_list链表
    list_for_each_entry(bi, &board_list, list)      //遍历全局board_list查找bi结构体
        spi_match_master_to_boardinfo(master, &bi->board_info);  //找到匹配的板级spi设备

    mutex_unlock(&board_lock);
    status = 0;
    of_register_spi_devices(master);
done:
    return status;
}
EXPORT_SYMBOL_GPL(spi_register_master);

3.1 匹配spi主机和板级spi设备

static void spi_match_master_to_boardinfo(struct spi_master *master,struct spi_board_info *bi)
{
    struct spi_device *dev;
    if (master->bus_num != bi->bus_num)   //判断是否所属的spi总线
        return;
    dev = spi_new_device(master, bi);   //添加新的spi设备
    if (!dev)
        dev_err(master->dev.parent, "can't create new device for %s\n",bi->modalias);
}

在注册板级设备或主机设备的时候都会添加
spi板级设备添加进board_list链表,spi主机设备添加进spi_master_list链表
不管是先注册spi板级设备还是先注册spi主机设备
都会调用list_for_each_entry遍历对应的要匹配的设备的链表,查找是否有匹配的例子
若找到都会调用spi_match_master_to_boardinfo函数添加spi设备

4.注销spi主机

void spi_unregister_master(struct spi_master *master)
{
    int dummy;
    mutex_lock(&board_lock);
    list_del(&master->list); //删除链表
    mutex_unlock(&board_lock);
    dummy = device_for_each_child(&master->dev, NULL, __unregister); //调用__unregister函数注销子设备
    device_unregister(&master->dev); //注销设备
}
EXPORT_SYMBOL_GPL(spi_unregister_master);

4.1 注销挂载该总线上的spi子设备

static int __unregister(struct device *dev, void *null)
{
    spi_unregister_device(to_spi_device(dev));
    return 0;
}

5.spi主机设备类

static struct class spi_master_class = {
    .name   = "spi_master",
    .owner  = THIS_MODULE,
    .dev_release    = spi_master_release,
};

五.spi总线

1.spi总线结构体

struct bus_type spi_bus_type = {
    .name       = "spi",
    .dev_attrs  = spi_dev_attrs,
    .match      = spi_match_device, //匹配方法
    .uevent     = spi_uevent,
    .suspend    = spi_suspend,
    .resume     = spi_resume,
};
EXPORT_SYMBOL_GPL(spi_bus_type);

2.设备匹配方法spi_match_device

前面的匹配方法是spi板级设备与spi主机设备的匹配方法,匹配的结果是添加新spi设备spi_new_device
这里的匹配是spi设备和spi驱动的匹配,匹配的结果是会调用spi驱动的设备驱动文件probe方法,既spi_drv_probe

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 (of_driver_match_device(dev, drv))   //设备文件驱动表的匹配
        return 1;
    if (sdrv->id_table)  //spi设备驱动存在支持id表
        return !!spi_match_id(sdrv->id_table, spi);  //spi设备驱动表的匹配
    return strcmp(spi->modalias, drv->name) == 0; //比较spi设备的名字和spi设备驱动的名字
}

2.1 of_driver_match_device

static inline int of_driver_match_device(const struct device *dev,const struct device_driver *drv)
{
    return of_match_device(drv->of_match_table, dev) != NULL;    //调用of_match_device函数
}

2.1.1 of_match_device

const struct of_device_id *of_match_device(const struct of_device_id *matches,const struct device *dev)
{
    if ((!matches) || (!dev->of_node))   //id表和设备节点都不存在
        return NULL;    //则返回
    return of_match_node(matches, dev->of_node); //调用of_match_node函数
}
EXPORT_SYMBOL(of_match_device);

2.1.1.1 of_match_node //drv->of_match_table,dev->of_node

const struct of_device_id *of_match_node(const struct of_device_id *matches,const struct device_node *node)
{
    while (matches->name[0] || matches->type[0] || matches->compatible[0]) {   //名字,类型,兼容方法有一个存在
        int match = 1;
        if (matches->name[0])    //判断名字
            match &= node->name && !strcmp(matches->name, node->name);
        if (matches->type[0])    //判断类型
            match &= node->type && !strcmp(matches->type, node->type);
        if (matches->compatible[0])  //兼容方法
            match &= of_device_is_compatible(node,matches->compatible);
        if (match)  //匹配
            return matches; //返回匹配的id
        matches++;  //matches指针++,指向下一个id
    }
    return NULL;
}
EXPORT_SYMBOL(of_match_node);

2.2 spi_match_id

static const struct spi_device_id *spi_match_id(const struct spi_device_id *id,const struct spi_device *sdev)
{
    while (id->name[0]) {    //id表的成员的名字域不为空
        if (!strcmp(sdev->modalias, id->name))    //则判断其名字是否与spi设备的名字一样
            return id;  //一样则返回该id
        id++;   //id表指针++,指向下一个id
    }
    return NULL;
}

六 spi消息和spi传输

1.spi消息结构体

struct spi_message {
    struct list_head    transfers;  //spi传输事务链表头
    struct spi_device   *spi;   //所属spi设备
    unsigned    is_dma_mapped:1;
    void    (*complete)(void *context);
    void    *context;
    unsigned    actual_length;
    int     status; //传输状态
    struct list_head    queue;
    void    *state;
};

2.初始化spi消息

static inline void spi_message_init(struct spi_message *m)
{
    memset(m, 0, sizeof *m);
    INIT_LIST_HEAD(&m->transfers);   //初始化spi消息的事务链表头
}

3.添加传输事务到spi传输链表

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

4.spi传输结构体

struct spi_transfer {
    const void  *tx_buf;    //发送缓冲区指针
    void        *rx_buf;    //接收缓冲区指针
    unsigned    len;    //消息长度
    dma_addr_t  tx_dma; //DMA发送地址
    dma_addr_t  rx_dma; //DMA接收地址
    unsigned    cs_change:1;
    u8      bits_per_word;  //一个字多少位
    u16     delay_usecs;    //毫秒级延时
    u32     speed_hz;   //速率
    struct list_head transfer_list; //传输链表头
};

 

七.spi子系统的初始化spi_init

static int __init spi_init(void)
{
    int status;
    buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);  //分配数据收发缓冲区
    if (!buf) {
        status = -ENOMEM;
        goto err0;
    }
    status = bus_register(&spi_bus_type);   //注册spi总线
    if (status < 0)
        goto err1;
    status = class_register(&spi_master_class); //注册spi主机类 "/sys/class/spi_master"
    if (status < 0)
        goto err2;
    return 0;
err2:
    bus_unregister(&spi_bus_type);
err1:
    kfree(buf);
    buf = NULL;
err0:
    return status;
}
postcore_initcall(spi_init);    //入口声明 #define postcore_initcall(fn)    __define_initcall("2",fn,2)

八.spi子系统的API

1.spi读 spi_read

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消息初始化(初始化传输事务链表头)
    spi_message_add_tail(&t, &m);   //添加spi传输到spi消息传输链表
    return spi_sync(spi, &m);   //spi同步传输
}

 

2.spi写 spi_write

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消息初始化(初始化传输事务链表头)
    spi_message_add_tail(&t, &m);   //添加spi传输到spi消息传输链表
    return spi_sync(spi, &m);   //spi同步传输
}

spi的读写操作都是初始化一个spi_transfer传输结构体,并将其添加进spi消息传输事务链表中
然后通过spi_sync来同步读写操作,接着看下spi_sync的具体代码

2.1 spi_sync

int spi_sync(struct spi_device *spi, struct spi_message *message)
{
    return __spi_sync(spi, message, 0); //调用__spi_sync函数
}
EXPORT_SYMBOL_GPL(spi_sync);

2.1.1 __spi_sync函数

static int __spi_sync(struct spi_device *spi, struct spi_message *message,int bus_locked)
{
    DECLARE_COMPLETION_ONSTACK(done);   //声明一个工作队列
    int status;
    struct spi_master *master = spi->master; //获取spi主机

    message->complete = spi_complete;    //设置spi消息的complete方法为spi_complete
    message->context = &done;    //设置spi消息的context
    if (!bus_locked)
        mutex_lock(&master->bus_lock_mutex); //上锁
    status = spi_async_locked(spi, message);
    if (!bus_locked)
        mutex_unlock(&master->bus_lock_mutex);   //解锁
    if (status == 0) {
        wait_for_completion(&done); //等待完成
        status = message->status;    //设置spi消息传输状态
    }
    message->context = NULL;
    return status;
}

 

2.1.1.1 spi_async_locked 函数

int spi_async_locked(struct spi_device *spi, struct spi_message *message)
{
    struct spi_master *master = spi->master; //获取spi主机
    int ret;
    unsigned long flags;
    spin_lock_irqsave(&master->bus_lock_spinlock, flags);    //上自旋锁
    ret = __spi_async(spi, message);    //调用了spi异步同步方法
    spin_unlock_irqrestore(&master->bus_lock_spinlock, flags);   //解自旋锁
    return ret;
}
EXPORT_SYMBOL_GPL(spi_async_locked);

2.1.1.1.1 __spi_async函数(异步)

static int __spi_async(struct spi_device *spi, struct spi_message *message)
{
    struct spi_master *master = spi->master;
    //主机为半双工或spi设备为3线设备
    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) {  //遍历spi消息的传输事务链表
            if (xfer->rx_buf && xfer->tx_buf) //判断接收或发送缓冲区是否为空
                return -EINVAL;
            if ((flags & SPI_MASTER_NO_TX) && xfer->tx_buf)  //检验无spi数据发送的情况
                return -EINVAL;
            if ((flags & SPI_MASTER_NO_RX) && xfer->rx_buf)  //检验无spi数据接收的情况
                return -EINVAL;
        }
    }
    message->spi = spi;  //设置spi消息所属的spi设备
    message->status = -EINPROGRESS;  //设置spi消息的传输状态
    return master->transfer(spi, message);   //调用spi主机的transfer方法,收发spi信息给spi设备
}

3.spi先写后读 spi_write_then_read

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];   //声明两个spi传输结构体
    u8  *local_buf;

    if ((n_tx + n_rx) > SPI_BUFSIZ)  //验证发送和接收的数据总和是否溢出
        return -EINVAL;
    spi_message_init(&message);     //spi消息初始化(初始化传输事务链表头)
    memset(x, 0, sizeof x); //
    if (n_tx) {
        x[0].len = n_tx;
        spi_message_add_tail(&x[0], &message);  //添加spi传输到spi消息传输链表
    }
    if (n_rx) {
        x[1].len = n_rx;
        spi_message_add_tail(&x[1], &message);  //添加spi传输到spi消息传输链表
    }
    if (!mutex_trylock(&lock)) {    //尝试上锁 失败
        local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);    //则分配local_buf内存
        if (!local_buf)
            return -ENOMEM;
    } else
        local_buf = buf;    //指向默认分配好内存的缓冲区(spi_init中初始化的)
    memcpy(local_buf, txbuf, n_tx); //发送缓冲区的内容复制到local_buf中
    x[0].tx_buf = local_buf;        //发送的spi传输结构体
    x[1].rx_buf = local_buf + n_tx; //接收的spi传输结构体
    status = spi_sync(spi, &message);   //spi同步传输--发送数据
    if (status == 0)
        memcpy(rxbuf, x[1].rx_buf, n_rx);   //接收返回的数据复制到rxbuf中
    if (x[0].tx_buf == buf)
        mutex_unlock(&lock);    //解锁
    else
        kfree(local_buf);   //释放内存
    return status;
}
EXPORT_SYMBOL_GPL(spi_write_then_read);

4.spi写8位后读8位数据 spi_w8r8 

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); //写8位读8位
    return (status < 0) ? status : result;
}

5.spi写8位数据后读16位数据 spi_w8r16

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);  //写8位读16位
    return (status < 0) ? status : result;
}

这里的API是内核空间使用的接口,设备驱动程序调用这些API直接操作spi的读写操作,来完成任务