Chinaunix首页 | 论坛 | 博客
  • 博客访问: 1942263
  • 博文数量: 185
  • 博客积分: 10707
  • 博客等级: 上将
  • 技术积分: 1777
  • 用 户 组: 普通用户
  • 注册时间: 2008-09-19 17:31
文章分类

全部博文(185)

文章存档

2014年(1)

2012年(6)

2011年(27)

2010年(13)

2009年(75)

2008年(63)

分类: LINUX

2009-03-31 10:11:18

一:前言
I2c是philips提出的外设总线.I2C只有两条线,一条串行数据线:SDA,一条是时钟线SCL.正因为这样,它方便了工程人员的布 线.另外,I2C是一种多主机控制总线.它和USB总线不同,USB是基于master-slave机制,任何设备的通信必须由主机发起才可以.而I2C 是基于multi master机制.一同总线上可允许多个master.关于I2C协议的知识,这里不再赘述.可自行下载spec阅读即可.
二:I2C架构概述
在linux中,I2C驱动架构如下所示:
 
如上图所示,每一条I2C对应一个adapter.在kernel中,每一个adapter提供了一个描述的结构(struct i2c_adapter),也定义了adapter支持的操作(struct i2c_adapter).再通过i2c core层将i2c设备与i2c adapter关联起来.
这个图只是提供了一个大概的框架.在下面的代码分析中,从下至上的来分析这个框架图.以下的代码分析是基于linux 2.6.26.分析的代码基本位于: linux-2.6.26.3/drivers/i2c/位置.
 
三:adapter注册
在kernel中提供了两个adapter注册接口,分别为i2c_add_adapter()和 i2c_add_numbered_adapter().由于在系统中可能存在多个adapter,因为将每一条I2C总线对应一个编号,下文中称为 I2C总线号.这个总线号的PCI中的总线号不同.它和硬件无关,只是软件上便于区分而已.
对于i2c_add_adapter()而言,它使用的是动态总线号,即由系统给其分析一个总线号,而i2c_add_numbered_adapter()则是自己指定总线号,如果这个总线号非法或者是被占用,就会注册失败.
分别来看一下这两个函数的代码:
int i2c_add_adapter(struct i2c_adapter *adapter)
{
    int id, res = 0;
 
retry:
    if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)
        return -ENOMEM;
 
    mutex_lock(&core_lock);
    /* "above" here means "above or equal to", sigh */
    res = idr_get_new_above(&i2c_adapter_idr, adapter,
                __i2c_first_dynamic_bus_num, &id);
    mutex_unlock(&core_lock);
 
    if (res < 0) {
        if (res == -EAGAIN)
            goto retry;
        return res;
    }
 
    adapter->nr = id;
    return i2c_register_adapter(adapter);
}
在这里涉及到一个idr结构.idr结构本来是为了配合page cache中的radix tree而设计的.在这里我们只需要知道,它是一种高效的搜索树,且这个树预先存放了一些内存.避免在内存不够的时候出现问题.所在,在往idr中插入结 构的时候,首先要调用idr_pre_get()为它预留足够的空闲内存,然后再调用idr_get_new_above()将结构插入idr中,该函数 以参数的形式返回一个id.以后凭这个id就可以在idr中找到相对应的结构了.对这个数据结构操作不太理解的可以查阅本站<< linux文件系统之文件的读写>>中有关radix tree的分析.
注意一下idr_get_new_above(&i2c_adapter_idr, adapter,__i2c_first_dynamic_bus_num, &id)的参数的含义,它是将adapter结构插入到i2c_adapter_idr中,存放位置的id必须要大于或者等于 __i2c_first_dynamic_bus_num,
然后将对应的id号存放在adapter->nr中.调用i2c_register_adapter(adapter)对这个adapter进行进一步注册.
 
看一下另外一人注册函数: i2c_add_numbered_adapter( ),如下所示:
int i2c_add_numbered_adapter(struct i2c_adapter *adap)
{
    int id;
    int status;
 
    if (adap->nr & ~MAX_ID_MASK)
        return -EINVAL;
 
retry:
    if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)
        return -ENOMEM;
 
    mutex_lock(&core_lock);
    /* "above" here means "above or equal to", sigh;
     * we need the "equal to" result to force the result
     */
    status = idr_get_new_above(&i2c_adapter_idr, adap, adap->nr, &id);
    if (status == 0 && id != adap->nr) {
        status = -EBUSY;
        idr_remove(&i2c_adapter_idr, id);
    }
    mutex_unlock(&core_lock);
    if (status == -EAGAIN)
        goto retry;
 
    if (status == 0)
        status = i2c_register_adapter(adap);
    return status;
}
对比一下就知道差别了,在这里它已经指定好了adapter->nr了.如果分配的id不和指定的相等,便返回错误.
 
过一步跟踪i2c_register_adapter().代码如下:
static int i2c_register_adapter(struct i2c_adapter *adap)
{
    int res = 0, dummy;
 
    mutex_init(&adap->bus_lock);
    mutex_init(&adap->clist_lock);
    INIT_LIST_HEAD(&adap->clients);
 
    mutex_lock(&core_lock);
 
    /* Add the adapter to the driver core.
     * If the parent pointer is not set up,
     * we add this adapter to the host bus.
     */
    if (adap->dev.parent == NULL) {
        adap->dev.parent = &platform_bus;
        pr_debug("I2C adapter driver [%s] forgot to specify "
             "physical device\n", adap->name);
    }
    sprintf(adap->dev.bus_id, "i2c-%d", adap->nr);
    adap->dev.release = &i2c_adapter_dev_release;
    adap->dev.class = &i2c_adapter_class;
    res = device_register(&adap->dev);
    if (res)
        goto out_list;
 
    dev_dbg(&adap->dev, "adapter [%s] registered\n", adap->name);
 
    /* create pre-declared device nodes for new-style drivers */
    if (adap->nr < __i2c_first_dynamic_bus_num)
        i2c_scan_static_board_info(adap);
 
    /* let legacy drivers scan this bus for matching devices */
    dummy = bus_for_each_drv(&i2c_bus_type, NULL, adap,
                 i2c_do_add_adapter);
 
out_unlock:
    mutex_unlock(&core_lock);
    return res;
 
out_list:
    idr_remove(&i2c_adapter_idr, adap->nr);
    goto out_unlock;
}
首先对adapter和adapter中内嵌的struct device结构进行必须的初始化.之后将adapter内嵌的struct device注册.
在这里注意一下adapter->dev的初始化.它的类别为i2c_adapter_class,如果没有父结点,则将其父结点设为platform_bus.adapter->dev的名字为i2c + 总线号.
测试一下:
[eric@mochow i2c]$ cd /sys/class/i2c-adapter/
[eric@mochow i2c-adapter]$ ls
i2c-0
可以看到,在我的PC上,有一个I2C adapter,看下详细信息:
[eric@mochow i2c-adapter]$ tree
.
`-- i2c-0
    |-- device -> ../../../devices/pci0000:00/0000:00:1f.3/i2c-0
    |-- name
    |-- subsystem -> ../../../class/i2c-adapter
    `-- uevent
 
3 directories, 2 files
可以看到,该adapter是一个PCI设备.
继续往下看:
之后,在注释中看到,有两种类型的driver,一种是new-style drivers,另外一种是legacy drivers
New-style drivers是在2.6近版的kernel加入的.它们最主要的区别是在adapter和i2c driver的匹配上.
 
3.1: new-style 形式的adapter注册
对于第一种,也就是new-style drivers,将相关代码再次列出如下:
    if (adap->nr < __i2c_first_dynamic_bus_num)
        i2c_scan_static_board_info(adap);
如果adap->nr 小于__i2c_first_dynamic_bus_num的话,就会进入到i2c_scan_static_board_info().
结合我们之前分析的adapter的两种注册分式: i2c_add_adapter()所分得的总线号肯会不会小于__i2c_first_dynamic_bus_num.只有i2c_add_numbered_adapter()才有可能满足:
(adap->nr < __i2c_first_dynamic_bus_num)
而且必须要调用i2c_register_board_info()将板子上的I2C设备信息预先注册时才会更改 __i2c_first_dynamic_bus_num的值.在x86上只没有使用i2c_register_board_info()的.因 此,x86平台上的分析可以忽略掉new-style driver的方式.不过,还是详细分析这种情况下.
首先看一下i2c_register_board_info(),如下:
int __init
i2c_register_board_info(int busnum,
    struct i2c_board_info const *info, unsigned len)
{
    int status;
 
    mutex_lock(&__i2c_board_lock);
 
    /* dynamic bus numbers will be assigned after the last static one */
    if (busnum >= __i2c_first_dynamic_bus_num)
        __i2c_first_dynamic_bus_num = busnum + 1;
 
    for (status = 0; len; len--, info++) {
        struct i2c_devinfo  *devinfo;
 
        devinfo = kzalloc(sizeof(*devinfo), GFP_KERNEL);
        if (!devinfo) {
            pr_debug("i2c-core: can't register boardinfo!\n");
            status = -ENOMEM;
            break;
        }
 
        devinfo->busnum = busnum;
        devinfo->board_info = *info;
        list_add_tail(&devinfo->list, &__i2c_board_list);
    }
 
    mutex_unlock(&__i2c_board_lock);
 
    return status;
}
这个函数比较简单, struct i2c_board_info用来表示I2C设备的一些情况,比如所在的总线.名称,地址,中断号等.最后,这些信息会被存放到__i2c_board_list链表.
 
跟踪i2c_scan_static_board_info():代码如下:
static void i2c_scan_static_board_info(struct i2c_adapter *adapter)
{
    struct i2c_devinfo  *devinfo;
 
    mutex_lock(&__i2c_board_lock);
    list_for_each_entry(devinfo, &__i2c_board_list, list) {
        if (devinfo->busnum == adapter->nr
                && !i2c_new_device(adapter,
                        &devinfo->board_info))
            printk(KERN_ERR "i2c-core: can't create i2c%d-%04x\n",
                i2c_adapter_id(adapter),
                devinfo->board_info.addr);
    }
    mutex_unlock(&__i2c_board_lock);
}
该函数遍历挂在__i2c_board_list链表上面的i2c设备的信息,也就是我们在启动的时候指出的i2c设备的信息.
如果指定设备是位于adapter所在的I2C总线上,那么,就调用i2c_new_device().代码如下:
struct i2c_client *
i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)
{
    struct i2c_client   *client;
    int         status;
 
    client = kzalloc(sizeof *client, GFP_KERNEL);
    if (!client)
        return NULL;
 
    client->adapter = adap;
 
    client->dev.platform_data = info->platform_data;
    device_init_wakeup(&client->dev, info->flags & I2C_CLIENT_WAKE);
 
    client->flags = info->flags & ~I2C_CLIENT_WAKE;
    client->addr = info->addr;
    client->irq = info->irq;
 
    strlcpy(client->name, info->type, sizeof(client->name));
 
    /* a new style driver may be bound to this device when we
     * return from this function, or any later moment (e.g. maybe
     * hotplugging will load the driver module).  and the device
     * refcount model is the standard driver model one.
     */
    status = i2c_attach_client(client);
    if (status < 0) {
        kfree(client);
        client = NULL;
    }
    return client;
}
我们又遇到了一个新的结构:struct i2c_client,不要被这个结构吓倒了,其实它就是一个嵌入struct device的I2C设备的封装.它和我们之前遇到的struct usb_device结构的作用是一样的.
首先,在clinet里保存该设备的相关消息.特别的, client->adapter指向了它所在的adapter.
特别的,clinet->name为info->name.也是指定好了的.
一切初始化完成之后,便会调用i2c_attach_client( ).看这个函数的字面意思,是将clinet关联起来.到底怎么样关联呢?继续往下看:
int i2c_attach_client(struct i2c_client *client)
{
    struct i2c_adapter *adapter = client->adapter;
    int res = 0;
 
    //初始化client内嵌的dev结构
    //父结点为所在的adapter,所在bus为i2c_bus_type
    client->dev.parent = &client->adapter->dev;
    client->dev.bus = &i2c_bus_type;
 
    //如果client已经指定了driver,将driver和内嵌的dev关联起来 
    if (client->driver)
        client->dev.driver = &client->driver->driver;
    //指定了driver, 但不是newstyle的
    if (client->driver && !is_newstyle_driver(client->driver)) {
        client->dev.release = i2c_client_release;
        client->dev.uevent_suppress = 1;
    } else
        client->dev.release = i2c_client_dev_release;
 
    //clinet->dev的名称
    snprintf(&client->dev.bus_id[0], sizeof(client->dev.bus_id),
        "%d-%04x", i2c_adapter_id(adapter), client->addr);
    //将内嵌的dev注册
    res = device_register(&client->dev);
    if (res)
        goto out_err;
 
    //将clinet链到adapter->clients中
    mutex_lock(&adapter->clist_lock);
    list_add_tail(&client->list, &adapter->clients);
    mutex_unlock(&adapter->clist_lock);
 
    dev_dbg(&adapter->dev, "client [%s] registered with bus id %s\n",
        client->name, client->dev.bus_id);
    //如果adapter->cleinet_reqister存在,就调用它
    if (adapter->client_register)  {
        if (adapter->client_register(client)) {
            dev_dbg(&adapter->dev, "client_register "
                "failed for client [%s] at 0x%02x\n",
                client->name, client->addr);
        }
    }
 
    return 0;
 
out_err:
    dev_err(&adapter->dev, "Failed to attach i2c client %s at 0x%02x "
        "(%d)\n", client->name, client->addr, res);
    return res;
}
参考上面添加的注释,应该很容易理解这段代码了,就不加详细分析了.这个函数的名字不是i2c_attach_client()么?怎么没看到它的关系过程呢?
这是因为:在代码中设置了client->dev所在的bus为i2c_bus_type .以为只需要有bus为i2c_bus_type的driver注册,就会产生probe了.这个过程呆后面分析i2c driver的时候再来详细分析.
 
3.2: legacy形式的adapter注册
Legacy形式的adapter注册代码片段如下:
    dummy = bus_for_each_drv(&i2c_bus_type, NULL, adap,
                 i2c_do_add_adapter);
这段代码遍历挂在i2c_bus_type上的驱动,然后对每一个驱动和adapter调用i2c_do_add_adapter().
代码如下:
static int i2c_do_add_adapter(struct device_driver *d, void *data)
{
    struct i2c_driver *driver = to_i2c_driver(d);
    struct i2c_adapter *adap = data;
 
    if (driver->attach_adapter) {
        /* We ignore the return code; if it fails, too bad */
        driver->attach_adapter(adap);
    }
    return 0;
}
该函数很简单,就是调用driver的attach_adapter()接口.
到此为止,adapter的注册已经分析完了.
 
四:i2c driver注册
在分析i2c driver的时候,有必要先分析一下i2c架构的初始化
代码如下:
static int __init i2c_init(void)
{
    int retval;
 
    retval = bus_register(&i2c_bus_type);
    if (retval)
        return retval;
    retval = class_register(&i2c_adapter_class);
    if (retval)
        goto bus_err;
    retval = i2c_add_driver(&dummy_driver);
    if (retval)
        goto class_err;
    return 0;
 
class_err:
    class_unregister(&i2c_adapter_class);
bus_err:
    bus_unregister(&i2c_bus_type);
    return retval;
}
subsys_initcall(i2c_init);
很明显,i2c_init()会在系统初始化的时候被调用.
在i2c_init中,先注册了i2c_bus_type的bus,i2c_adapter_class的class.然后再调用i2c_add_driver()注册了一个i2c driver.
 
I2c_bus_type结构如下:
static struct bus_type i2c_bus_type = {
    .name       = "i2c",
    .dev_attrs  = i2c_dev_attrs,
    .match      = i2c_device_match,
    .uevent     = i2c_device_uevent,
    .probe      = i2c_device_probe,
    .remove     = i2c_device_remove,
    .shutdown   = i2c_device_shutdown,
    .suspend    = i2c_device_suspend,
    .resume     = i2c_device_resume,
};
 
这个结构先放在这里吧,以后还会用到里面的信息的.
从上面的初始化函数里也看到了,注册i2c driver的接口为i2c_add_driver().代码如下:
static inline int i2c_add_driver(struct i2c_driver *driver)
{
    return i2c_register_driver(THIS_MODULE, driver);
}
继续跟踪:
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
    int res;
 
    /* new style driver methods can't mix with legacy ones */
    //如果是一个newstyle的driver.但又定义了attach_adapter/detach_adapter.非法
    if (is_newstyle_driver(driver)) {
        if (driver->attach_adapter || driver->detach_adapter
                || driver->detach_client) {
            printk(KERN_WARNING
                    "i2c-core: driver [%s] is confused\n",
                    driver->driver.name);
            return -EINVAL;
        }
    }
 
    /* add the driver to the list of i2c drivers in the driver core */
    //关联到i2c_bus_types
    driver->driver.owner = owner;
    driver->driver.bus = &i2c_bus_type;
 
    /* for new style drivers, when registration returns the driver core
     * will have called probe() for all matching-but-unbound devices.
     */
     //注册内嵌的driver
    res = driver_register(&driver->driver);
    if (res)
        return res;
 
    mutex_lock(&core_lock);
 
    pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);
 
    /* legacy drivers scan i2c busses directly */
    //遍历所有的adapter,对其都调用driver->attach_adapter
    if (driver->attach_adapter) {
        struct i2c_adapter *adapter;
 
        down(&i2c_adapter_class.sem);
        list_for_each_entry(adapter, &i2c_adapter_class.devices,
                    dev.node) {
            driver->attach_adapter(adapter);
        }
        up(&i2c_adapter_class.sem);
    }
 
    mutex_unlock(&core_lock);
    return 0;
}
这里也有两种形式的区分,对于第一种,只需要将内嵌的driver注册就可以了,对于legacy的情况,对每一个adapter都调用driver->attach_adapter().
现在,我们可以将adapter和i2c driver关联起来考虑一下了:
1:如果是news style形式的,在注册adapter的时候,将它上面的i2c 设备转换成了struct client.struct client->dev->bus又指定了和i2c driver同一个bus.因为,它们可以发生probe.
2:如果是legacy形式,就直接找到对应的对象,调用driver->attach_adapter().
 
五: i2c_bus_type的相关操作
I2c_bus_type的操作主要存在于new-style形式的驱动中.接下来分析一下对应的probe过程:
5.1:match过程分析
Match对应的操作函数为i2c_device_match().代码如下
static int i2c_device_match(struct device *dev, struct device_driver *drv)
{
    struct i2c_client   *client = to_i2c_client(dev);
    struct i2c_driver   *driver = to_i2c_driver(drv);
 
    /* make legacy i2c drivers bypass driver model probing entirely;
     * such drivers scan each i2c adapter/bus themselves.
     */
    if (!is_newstyle_driver(driver))
        return 0;
 
    /* match on an id table if there is one */
    if (driver->id_table)
        return i2c_match_id(driver->id_table, client) != NULL;
 
    return 0;
}
如果该驱动不是一个new-style形式的.或者driver没有定义匹配的id_table.都会匹配失败.
继续跟踪进i2c_match_id():
static const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,
                        const struct i2c_client *client)
{
    while (id->name[0]) {
        if (strcmp(client->name, id->name) == 0)
            return id;
        id++;
    }
    return NULL;
}
由此可见.如果client的名字和driver->id_table[]中的名称匹配即为成功.
 
5.2:probe过程分析
Probe对应的函数为: i2c_device_probe()
static int i2c_device_probe(struct device *dev)
{
    struct i2c_client   *client = to_i2c_client(dev);
    struct i2c_driver   *driver = to_i2c_driver(dev->driver);
    const struct i2c_device_id *id;
    int status;
 
    if (!driver->probe)
        return -ENODEV;
    client->driver = driver;
    dev_dbg(dev, "probe\n");
 
    if (driver->id_table)
        id = i2c_match_id(driver->id_table, client);
    else
        id = NULL;
    status = driver->probe(client, id);
    if (status)
        client->driver = NULL;
    return status;
}
这个函数也很简单,就是将probe流程回溯到i2c driver的probe()
 
六:其它的扩展
分析完adapter和i2c driver的注册之后,好像整个架构也差不多了,其它,扩展的东西还有很多.
我们举一个legacy形式的例子,这个例子是在kernel中随便搜索出来的:
在linux-2.6.26.3/drivers/hwmon/ad7418.c中,初始化函数为:
static int __init ad7418_init(void)
{
    return i2c_add_driver(&ad7418_driver);
}
i2c_driver ad7418_driver结构如下:
static struct i2c_driver ad7418_driver = {
    .driver = {
        .name   = "ad7418",
    },
    .attach_adapter = ad7418_attach_adapter,
    .detach_client  = ad7418_detach_client,
};
该结构中没有probe()函数,可以断定是一个legacy形式的驱动.这类驱动注册的时候,会调用driver的attach_adapter函数.在这里也就是ad7418_attach_adapter.
这个函数代码如下:
static int ad7418_attach_adapter(struct i2c_adapter *adapter)
{
    if (!(adapter->class & I2C_CLASS_HWMON))
        return 0;
    return (adapter, &addr_data, ad7418_detect);
}
在这里我们又遇到了一个i2c-core中的函数,i2c_probe().在分析这个函数之前,先来看下addr_data是什么?
#define I2C_CLIENT_MODULE_PARM(var,desc) \
  static unsigned short var[I2C_CLIENT_MAX_OPTS] = I2C_CLIENT_DEFAULTS; \
  static unsigned int var##_num; \
  module_param_array(var, short, &var##_num, 0); \
  MODULE_PARM_DESC(var,desc)
 
#define I2C_CLIENT_MODULE_PARM_FORCE(name)              \
I2C_CLIENT_MODULE_PARM(force_##name,                    \
               "List of adapter,address pairs which are "    \
               "unquestionably assumed to contain a `"       \
               # name "' chip")
 
 
#define I2C_CLIENT_INSMOD_COMMON                    \
I2C_CLIENT_MODULE_PARM(probe, "List of adapter,address pairs to scan "  \
               "additionally");                  \
I2C_CLIENT_MODULE_PARM(ignore, "List of adapter,address pairs not to "  \
               "scan");                      \
static const struct i2c_client_address_data addr_data = {       \
    .normal_i2c = normal_i2c,                   \
    .probe      = probe,                    \
    .ignore     = ignore,                   \
    .forces     = forces,                   \
}
 
#define I2C_CLIENT_FORCE_TEXT \
    "List of adapter,address pairs to boldly assume to be present"
由此可知道,addr_data中的三个成员都是模块参数.在加载模块的时候可以用参数的方式对其赋值.三个模块参数为别为probe,ignore,force.另外需要指出的是normal_i2c不能以模块参数的方式对其赋值,只能在驱动内部静态指定.
从模块参数的模述看来, probe是指"List of adapter,address pairs to scan additionally"
Ignore是指"List of adapter,address pairs not to scan "
Force是指"List of adapter,address pairs to boldly assume to be present"
事实上,它们里面的数据都是成对出现的.前面一部份表示所在的总线号,ANY_I2C_BUS表示任一总线.后一部份表示设备的地址.
现在可以来跟踪i2c_probe()的代码了.如下:
int i2c_probe(struct i2c_adapter *adapter,
          const struct i2c_client_address_data *address_data,
          int (*found_proc) (struct i2c_adapter *, int, int))
{
    int i, err;
    int adap_id = i2c_adapter_id(adapter);
 
    /* Force entries are done first, and are not affected by ignore
       entries */
       //先扫描force里面的信息,注意它是一个二级指针.ignore里的信息对它是无效的
    if (address_data->forces) {
        const unsigned short * const *forces = address_data->forces;
        int kind;
 
        for (kind = 0; forces[kind]; kind++) {
            for (i = 0; forces[kind][i] != I2C_CLIENT_END;
                 i += 2) {
                if (forces[kind][i] == adap_id
                 || forces[kind][i] == ANY_I2C_BUS) {
                    dev_dbg(&adapter->dev, "found force "
                        "parameter for adapter %d, "
                        "addr 0x%02x, kind %d\n",
                        adap_id, forces[kind][i + 1],
                        kind);
                    err = i2c_probe_address(adapter,
                        forces[kind][i + 1],
                        kind, found_proc);
                    if (err)
                        return err;
                }
            }
        }
    }
 
    /* Stop here if we can't use SMBUS_QUICK */
    //如果adapter不支持quick.不能够遍历这个adapter上面的设备
    if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_QUICK)) {
        if (address_data->probe[0] == I2C_CLIENT_END
         && address_data->normal_i2c[0] == I2C_CLIENT_END)
            return 0;
 
        dev_warn(&adapter->dev, "SMBus Quick command not supported, "
             "can't probe for chips\n");
        return -1;
    }
 
    /* Probe entries are done second, and are not affected by ignore
       entries either */
      //遍历probe上面的信息.ignore上的信息也对它是没有影响的
    for (i = 0; address_data->probe[i] != I2C_CLIENT_END; i += 2) {
        if (address_data->probe[i] == adap_id
         || address_data->probe[i] == ANY_I2C_BUS) {
            dev_dbg(&adapter->dev, "found probe parameter for "
                "adapter %d, addr 0x%02x\n", adap_id,
                address_data->probe[i + 1]);
            err = i2c_probe_address(adapter,
                        address_data->probe[i + 1],
                        -1, found_proc);
            if (err)
                return err;
        }
    }
 
    /* Normal entries are done last, unless shadowed by an ignore entry */
    //最后遍历normal_i2c上面的信息.它上面的信息不能在ignore中.
    for (i = 0; address_data->normal_i2c[i] != I2C_CLIENT_END; i += 1) {
        int j, ignore;
 
        ignore = 0;
        for (j = 0; address_data->ignore[j] != I2C_CLIENT_END;
             j += 2) {
            if ((address_data->ignore[j] == adap_id ||
                 address_data->ignore[j] == ANY_I2C_BUS)
             && address_data->ignore[j + 1]
                == address_data->normal_i2c[i]) {
                dev_dbg(&adapter->dev, "found ignore "
                    "parameter for adapter %d, "
                    "addr 0x%02x\n", adap_id,
                    address_data->ignore[j + 1]);
                ignore = 1;
                break;
            }
        }
        if (ignore)
            continue;
 
        dev_dbg(&adapter->dev, "found normal entry for adapter %d, "
            "addr 0x%02x\n", adap_id,
            address_data->normal_i2c[i]);
        err = i2c_probe_address(adapter, address_data->normal_i2c[i],
                    -1, found_proc);
        if (err)
            return err;
    }
 
    return 0;
}
这段代码很简单,结合代码上面添加的注释应该很好理解.如果匹配成功,则会调用i2c_probe_address ().这个函数代码如下:
static int i2c_probe_address(struct i2c_adapter *adapter, int addr, int kind,
                 int (*found_proc) (struct i2c_adapter *, int, int))
{
    int err;
 
    /* Make sure the address is valid */
    //地址小于0x03或者大于0x77都是不合法的
    if (addr < 0x03 || addr > 0x77) {
        dev_warn(&adapter->dev, "Invalid probe address 0x%02x\n",
             addr);
        return -EINVAL;
    }
 
    /* Skip if already in use */
    //adapter上已经有这个设备了
    if (i2c_check_addr(adapter, addr))
        return 0;
 
    /* Make sure there is something at this address, unless forced */
    //如果kind小于0.检查adapter上是否有这个设备
    if (kind < 0) {
        if (i2c_smbus_xfer(adapter, addr, 0, 0, 0,
                   I2C_SMBUS_QUICK, NULL) < 0)
            return 0;
 
        /* prevent 24RF08 corruption */
        if ((addr & ~0x0f) == 0x50)
            i2c_smbus_xfer(adapter, addr, 0, 0, 0,
                       I2C_SMBUS_QUICK, NULL);
    }
 
    /* Finally call the custom detection function */
    //调用回调函数
    err = found_proc(adapter, addr, kind);
    /* -ENODEV can be returned if there is a chip at the given address
       but it isn't supported by this chip driver. We catch it here as
       this isn't an error. */
    if (err == -ENODEV)
        err = 0;
 
    if (err)
        dev_warn(&adapter->dev, "Client creation failed at 0x%x (%d)\n",
             addr, err);
    return err;
}
首先,对传入的参数进行一系列的合法性检查.另外,如果该adapter上已经有了这个地址的设备了.也会返回失败.所有adapter下面的 设备都是以adapter->dev为父结点的.因此只需要遍历adapter->dev下面的子设备就可以得到当前地址是不是被占用了.
如果kind < 0.还得要adapter检查该总线是否有这个地址的设备.方法是向这个地址发送一个Read的Quick请求.如果该地址有应答,则说明这个地址上有这个设备.另外还有一种情况是在24RF08设备的特例.
如果adapter上确实有这个设备,就会调用驱动调用时的回调函数.
 
在上面涉及到了IIC的传输方式,有疑问的可以参考intel ICH5手册的有关smbus部份.
跟踪i2c_smbus_xfer().代码如下:
s32 i2c_smbus_xfer(struct i2c_adapter * adapter, u16 addr, unsigned short flags,
                   char read_write, u8 command, int size,
                   union i2c_smbus_data * data)
{
    s32 res;
 
    flags &= I2C_M_TEN | I2C_CLIENT_PEC;
 
    if (adapter->algo->smbus_xfer) {
        mutex_lock(&adapter->bus_lock);
        res = adapter->algo->smbus_xfer(adapter,addr,flags,read_write,
                                        command,size,data);
        mutex_unlock(&adapter->bus_lock);
    } else
        res = i2c_smbus_xfer_emulated(adapter,addr,flags,read_write,
                                          command,size,data);
 
    return res;
}
如果adapter有smbus_xfer()函数,则直接调用它发送,否则,也就是在adapter不支持smbus协议的情况下,调用i2c_smbus_xfer_emulated()继续处理.
跟进i2c_smbus_xfer_emulated().代码如下:
static s32 i2c_smbus_xfer_emulated(struct i2c_adapter * adapter, u16 addr,
                                   unsigned short flags,
                                   char read_write, u8 command, int size,
                                   union i2c_smbus_data * data)
{
    /* So we need to generate a series of msgs. In the case of writing, we
      need to use only one message; when reading, we need two. We initialize
      most things with sane defaults, to keep the code below somewhat
      simpler. */
     //写操作只会进行一次交互,而读操作,有时会有两次操作.
     //因为有时候读操作要先写command,再从总线上读数据
     //在这里为了代码的简洁.使用了两个缓存区,将两种情况统一起来.
    unsigned char msgbuf0[I2C_SMBUS_BLOCK_MAX+3];
    unsigned char msgbuf1[I2C_SMBUS_BLOCK_MAX+2];
    //一般来说,读操作要交互两次.例外的情况我们在下面会接着分析
    int num = read_write == I2C_SMBUS_READ?2:1;
    //与设备交互的数据,一般在msg[0]存放写入设备的信息,在msb[1]里存放接收到的
    //信息.不过也有例外的
    //msg[2]的初始化,默认发送缓存区占一个字节,无接收缓存
    struct i2c_msg msg[2] = { { addr, flags, 1, msgbuf0 },
                              { addr, flags | I2C_M_RD, 0, msgbuf1 }
                            };
    int i;
    u8 partial_pec = 0;
 
    //将要发送的信息copy到发送缓存区的第一字节
    msgbuf0[0] = command;
    switch(size) {
        //quick类型的,其它并不传输有效数据,只是将地址写到总线上,等待应答即可
        //所以将发送缓存区长度置为0 .再根据读/写操作,调整msg[0]的标志位
        //这类传输只需要一次总线交互
    case I2C_SMBUS_QUICK:
        msg[0].len = 0;
        /* Special case: The read/write field is used as data */
        msg[0].flags = flags | (read_write==I2C_SMBUS_READ)?I2C_M_RD:0;
        num = 1;
        break;
    case I2C_SMBUS_BYTE:
        //BYTE类型指一次写和读只有一个字节.这种情况下,读和写都只会交互一次
        //这种类型的读有例外,它读取出来的数据不是放在msg[1]中的,而是存放在msg[0]
        if (read_write == I2C_SMBUS_READ) {
            /* Special case: only a read! */
            msg[0].flags = I2C_M_RD | flags;
            num = 1;
        }
        break;
    case I2C_SMBUS_BYTE_DATA:
        //Byte_Data是指命令+数据的传输形式.在这种情况下,写只需要一次交互,读却要两次
        //第一次将command写到总线上,第二次要转换方向.要将设备地址和read标志写入总线.
        //应回答之后再进行read操作
        //写操作占两字节,分别是command+data.读操作的有效数据只有一个字节
        //交互次数用初始化值就可以了
        if (read_write == I2C_SMBUS_READ)
            msg[1].len = 1;
        else {
            msg[0].len = 2;
            msgbuf0[1] = data->byte;
        }
        break;
    case I2C_SMBUS_WORD_DATA:
        //Word_Data是指命令+双字节的形式.这种情况跟Byte_Data的情况类似
        //两者相比只是交互的数据大小不同
        if (read_write == I2C_SMBUS_READ)
            msg[1].len = 2;
        else {
            msg[0].len=3;
            msgbuf0[1] = data->word & 0xff;
            msgbuf0[2] = data->word >> 8;
        }
        break;
    case I2C_SMBUS_PROC_CALL:
        //Proc_Call的方式与write 的Word_Data相似,只不过写完Word_Data之后,要等待它的应答
        //应该它需要交互两次,一次写一次读
        num = 2; /* Special case */
        read_write = I2C_SMBUS_READ;
        msg[0].len = 3;
        msg[1].len = 2;
        msgbuf0[1] = data->word & 0xff;
        msgbuf0[2] = data->word >> 8;
        break;
    case I2C_SMBUS_BLOCK_DATA:
        //Block_Data:指command+N段数据的情况.
        //如果是读操作,它首先要写command到总线,然后再读N段数据.要写的command已经
        //放在msg[0]了.现在只需要将msg[1]的标志置I2C_M_RECV_LEN位,msg[1]有效长度为1字节.因为
        //adapter驱动会处理好的.现在现在还不知道要传多少段数据.
 
        //对于写的情况:msg[1]照例不需要.将要写的数据全部都放到msb[0]中.相应的也要更新
        //msg[0]中的缓存区长度
        if (read_write == I2C_SMBUS_READ) {
            msg[1].flags |= I2C_M_RECV_LEN;
            msg[1].len = 1; /* block length will be added by
                       the underlying bus driver */
        } else {
            //data->block[0]表示后面有多少段数据.总长度要加2是因为command+count+N段数据
            msg[0].len = data->block[0] + 2;
            if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 2) {
                dev_err(&adapter->dev, "smbus_access called with "
                       "invalid block write size (%d)\n",
                       data->block[0]);
                return -1;
            }
            for (i = 1; i < msg[0].len; i++)
                msgbuf0[i] = data->block[i-1];
        }
        break;
    case I2C_SMBUS_BLOCK_PROC_CALL:
        //Proc_Call:表示写完Block_Data之后,要等它的应答消息它和Block_Data相比,只是多了一部份应答而已
        num = 2; /* Another special case */
        read_write = I2C_SMBUS_READ;
        if (data->block[0] > I2C_SMBUS_BLOCK_MAX) {
            dev_err(&adapter->dev, "%s called with invalid "
                "block proc call size (%d)\n", __func__,
                data->block[0]);
            return -1;
        }
        msg[0].len = data->block[0] + 2;
        for (i = 1; i < msg[0].len; i++)
            msgbuf0[i] = data->block[i-1];
        msg[1].flags |= I2C_M_RECV_LEN;
        msg[1].len = 1; /* block length will be added by
                   the underlying bus driver */
        break;
    case I2C_SMBUS_I2C_BLOCK_DATA:
        //I2c Block_Data与Block_Data相似,只不过read的时候,数据长度是预先定义好了的.另外
        //与Block_Data相比,中间不需要传输Count字段.(Count表示数据段数目)
        if (read_write == I2C_SMBUS_READ) {
            msg[1].len = data->block[0];
        } else {
            msg[0].len = data->block[0] + 1;
            if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 1) {
                dev_err(&adapter->dev, "i2c_smbus_xfer_emulated called with "
                       "invalid block write size (%d)\n",
                       data->block[0]);
                return -1;
            }
            for (i = 1; i <= data->block[0]; i++)
                msgbuf0[i] = data->block[i];
        }
        break;
    default:
        dev_err(&adapter->dev, "smbus_access called with invalid size (%d)\n",
               size);
        return -1;
    }
 
    //如果启用了PEC.Quick和I2c Block_Data是不支持PEC的
    i = ((flags & I2C_CLIENT_PEC) && size != I2C_SMBUS_QUICK
                      && size != I2C_SMBUS_I2C_BLOCK_DATA);
    if (i) {
        /* Compute PEC if first message is a write */
        //如果第一个操作是写操作
        if (!(msg[0].flags & I2C_M_RD)) {
            //如果只是写操作
            if (num == 1) /* Write only */
                //如果只有写操作,写缓存区要扩充一个字节,用来存放计算出来的PEC
                i2c_smbus_add_pec(&msg[0]);
            else /* Write followed by read */
                //如果后面还有读操作,先计算前面写部份的PEC(注意这种情况下不需要
                //扩充写缓存区,因为不需要发送PEC.只会接收到PEC)
                partial_pec = i2c_smbus_msg_pec(0, &msg[0]);
        }
        /* Ask for PEC if last message is a read */
        //如果最后一次是读消息.还要接收到来自slave的PEC.所以接收缓存区要扩充一个字节
        if (msg[num-1].flags & I2C_M_RD)
            msg[num-1].len++;
    }
 
    if (i2c_transfer(adapter, msg, num) < 0)
        return -1;
 
    /* Check PEC if last message is a read */
    //操作完了之后,如果最后一个操作是PEC的读操作.检验后面的PEC是否正确
    if (i && (msg[num-1].flags & I2C_M_RD)) {
        if (i2c_smbus_check_pec(partial_pec, &msg[num-1]) < 0)
            return -1;
    }
 
    //操作完了,现在可以将数据放到data部份返回了.
    if (read_write == I2C_SMBUS_READ)
        switch(size) {
            case I2C_SMBUS_BYTE:
                data->byte = msgbuf0[0];
                break;
            case I2C_SMBUS_BYTE_DATA:
                data->byte = msgbuf1[0];
                break;
            case I2C_SMBUS_WORD_DATA:
            case I2C_SMBUS_PROC_CALL:
                data->word = msgbuf1[0] | (msgbuf1[1] << 8);
                break;
            case I2C_SMBUS_I2C_BLOCK_DATA:
                for (i = 0; i < data->block[0]; i++)
                    data->block[i+1] = msgbuf1[i];
                break;
            case I2C_SMBUS_BLOCK_DATA:
            case I2C_SMBUS_BLOCK_PROC_CALL:
                for (i = 0; i < msgbuf1[0] + 1; i++)
                    data->block[i] = msgbuf1[i];
                break;
        }
    return 0;
}
在这个函数添上了很详细的注释,配和intel的datasheet,应该很容易看懂.在上面的交互过程中,调用了子函数i2c_transfer().它的代码如下所示:
int i2c_transfer(struct i2c_adapter * adap, struct i2c_msg *msgs, int num)
{
    int ret;
 
    if (adap->algo->master_xfer) {
#ifdef DEBUG
        for (ret = 0; ret < num; ret++) {
            dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
                "len=%d%s\n", ret, (msgs[ret].flags & I2C_M_RD)
                ? 'R' : 'W', msgs[ret].addr, msgs[ret].len,
                (msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
        }
#endif
 
        if (in_atomic() || irqs_disabled()) {
            ret = mutex_trylock(&adap->bus_lock);
            if (!ret)
                /* I2C activity is ongoing. */
                return -EAGAIN;
        } else {
            mutex_lock_nested(&adap->bus_lock, adap->level);
        }
 
        ret = adap->algo->master_xfer(adap,msgs,num);
        mutex_unlock(&adap->bus_lock);
 
        return ret;
    } else {
        dev_dbg(&adap->dev, "I2C level transfers not supported\n");
        return -ENOSYS;
    }
}
因为在这里的同步用的是mutex.首先判断判断是否充许睡眠,如果不允许,尝试获锁.如果获锁失败,则返回,这样的操作是避免进入睡眠,我们在后面也可以看到,实际的传输工作交给了adap->algo->master_xfer()完成.
 
在这里,我们终于把i2c_probe_address()的执行分析完了,经过这个分析,我们也知道了数据是怎么样传输的.我们接着 i2c_probe()往下看.如果i2c_probe_address()成功.说明总线上确实有这样的设备.那么就会调用驱动中的回调函数.在 ad7148的驱动中,如下所示:
return i2c_probe(adapter, &addr_data, ad7418_detect);
也就是说,要调用的回调函数是ad7418_detect().这个函数中我们只分析和i2c框架相关的部份.代码片段如下所示:
static int ad7418_detect(struct i2c_adapter *adapter, int address, int kind)
{
    struct i2c_client *client;
    ……
    ……
client->addr = address;
    client->adapter = adapter;
    client->driver = &ad7418_driver;
 
    i2c_set_clientdata(client, data);
    ……
    ……
if ((err = i2c_attach_client(client)))
        goto exit_free;
    ……
    ……
}
结合上面关于new-style形式的驱动分析.发现这里走的是同一个套路,即初始化了client.然后调用 i2c_attach_client().后面的流程就跟上面分析的一样了.只不过,不相同的是,这里clinet已经指定了驱动为 ad7418_driver.应该在注册clinet->dev之后,就不会走bus->match和bus->probe的流程了.
 
七:i2c dev节点操作
现在来分析上面架构图中的i2c-dev.c中的部份.这个部份为用户空间提供了操作adapter的接口.这部份代码其实对应就晃一个模块.它的初始化函数为:
module_init(i2c_dev_init);
i2c_dev_init()代码如下:
static int __init i2c_dev_init(void)
{
    int res;
 
    printk(KERN_INFO "i2c /dev entries driver\n");
 
    res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);
    if (res)
        goto out;
 
    i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");
    if (IS_ERR(i2c_dev_class))
        goto out_unreg_chrdev;
 
    res = i2c_add_driver(&i2cdev_driver);
    if (res)
        goto out_unreg_class;
 
    return 0;
 
out_unreg_class:
    class_destroy(i2c_dev_class);
out_unreg_chrdev:
    unregister_chrdev(I2C_MAJOR, "i2c");
out:
    printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
    return res;
}
首先为主册了一个主设备号为I2C_MAJOR(89),操作集为i2cdev_fops的字符设备.然后注册了一个名为”i2c-dev”的class.之后再注册了一个i2c的driver.如下所示:
res = i2c_add_driver(&i2cdev_driver);
    if (res)
        goto out_unreg_class;
i2cdev_driver定义如下:
static struct i2c_driver i2cdev_driver = {
    .driver = {
        .name   = "dev_driver",
    },
    .id     = I2C_DRIVERID_I2CDEV,
    .attach_adapter = i2cdev_attach_adapter,
    .detach_adapter = i2cdev_detach_adapter,
    .detach_client  = i2cdev_detach_client,
};
也就是说,当它注册或者有新的adapter注册后,就会它的attach_adapter()函数.该函数代码如下:
static int i2cdev_attach_adapter(struct i2c_adapter *adap)
{
    struct i2c_dev *i2c_dev;
    int res;
 
    i2c_dev = get_free_i2c_dev(adap);
    if (IS_ERR(i2c_dev))
        return PTR_ERR(i2c_dev);
 
    /* register this i2c device with the driver core */
    i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
                     MKDEV(I2C_MAJOR, adap->nr),
                     "i2c-%d", adap->nr);
    if (IS_ERR(i2c_dev->dev)) {
        res = PTR_ERR(i2c_dev->dev);
        goto error;
    }
    res = device_create_file(i2c_dev->dev, &dev_attr_name);
    if (res)
        goto error_destroy;
 
    pr_debug("i2c-dev: adapter [%s] registered as minor %d\n",
         adap->name, adap->nr);
    return 0;
error_destroy:
    device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));
error:
    return_i2c_dev(i2c_dev);
    return res;
}
这个函数也很简单,首先调用get_free_i2c_dev()分配并初始化了一个struct i2c_dev结构,使i2c_dev->adap指向操作的adapter.之后,该i2c_dev会被链入链表i2c_dev_list中.再 分别以I2C_MAJOR, adap->nr为主次设备号创建了一个device.如果此时系统配置了udev或者是hotplug,那么就么在/dev下自动创建相关的设备节点了.
刚才我们说过,所有主设备号为I2C_MAJOR的设备节点的操作函数是i2cdev_fops.它的定义如下所示:
static const struct file_operations i2cdev_fops = {
    .owner      = THIS_MODULE,
    .llseek     = no_llseek,
    .read       = i2cdev_read,
    .write      = i2cdev_write,
    .ioctl      = i2cdev_ioctl,
    .open       = i2cdev_open,
    .release    = i2cdev_release,
};
 
7.1:i2c dev的open操作
Open操作对应的函数为i2cdev_open().代码如下:
 
 static int i2cdev_open(struct inode *inode, struct file *file)
{
    unsigned int minor = iminor(inode);
    struct i2c_client *client;
    struct i2c_adapter *adap;
    struct i2c_dev *i2c_dev;
 
    //以次设备号从i2c_dev_list链表中取得i2c_dev
    i2c_dev = i2c_dev_get_by_minor(minor);
    if (!i2c_dev)
        return -ENODEV;
 
    //以apapter的总线号从i2c_adapter_idr中找到adapter
    adap = i2c_get_adapter(i2c_dev->adap->nr);
    if (!adap)
        return -ENODEV;
 
    /* This creates an anonymous i2c_client, which may later be
     * pointed to some address using I2C_SLAVE or I2C_SLAVE_FORCE.
     *
     * This client is ** NEVER REGISTERED ** with the driver model
     * or I2C core code!!  It just holds private copies of addressing
     * information and maybe a PEC flag.
     */
     //分配并初始化一个i2c_client结构
    client = kzalloc(sizeof(*client), GFP_KERNEL);
    if (!client) {
        i2c_put_adapter(adap);
        return -ENOMEM;
    }
    snprintf(client->name, I2C_NAME_SIZE, "i2c-dev %d", adap->nr);
    client->driver = &i2cdev_driver;
 
    //clinet->adapter指向操作的adapter
    client->adapter = adap;
    //关联到file
    file->private_data = client;
 
    return 0;
}
注意这里分配并初始化了一个struct i2c_client结构.但是没有注册这个clinet.此外,这个函数中还有一个比较奇怪的操作.不是在前面已经将i2c_dev->adap 指向要操作的adapter么?为什么还要以adapter->nr为关键字从i2c_adapter_idr去找这个操作的adapter呢?注 意了,调用i2c_get_adapter()从总线号nr找到操作的adapter的时候,还会增加module的引用计数.这样可以防止模块意外被释 放掉.也许有人会有这样的疑问,那 i2c_dev->adap->nr操作,如果i2c_dev->adap被释放掉的话,不是一样会引起系统崩溃么?这里因为,在 i2cdev_attach_adapter()间接的增加了一次adapter的一次引用计数.如下:
tatic int i2cdev_attach_adapter(struct i2c_adapter *adap)
{
......
i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
                     MKDEV(I2C_MAJOR, adap->nr),
                     "i2c-%d", adap->nr);
......
}
看到了么,i2c_dev内嵌的device是以adap->dev为父结点,在device_create()中会增次adap->dev的一次引用计数.
好了,open()操作到此就完成了.
 
7.2:read操作
Read操作对应的操作函数如下示:
static ssize_t i2cdev_read (struct file *file, char __user *buf, size_t count,
                            loff_t *offset)
{
    char *tmp;
    int ret;
 
    struct i2c_client *client = (struct i2c_client *)file->private_data;
 
    if (count > 8192)
        count = 8192;
 
    tmp = kmalloc(count,GFP_KERNEL);
    if (tmp==NULL)
        return -ENOMEM;
 
    pr_debug("i2c-dev: i2c-%d reading %zd bytes.\n",
        iminor(file->f_path.dentry->d_inode), count);
 
    ret = i2c_master_recv(client,tmp,count);
    if (ret >= 0)
        ret = copy_to_user(buf,tmp,count)?-EFAULT:ret;
    kfree(tmp);
    return ret;
}
首先从file结构中取得struct i2c_clinet.然后在kernel同分配相同长度的缓存区,随之调用i2c_master_recv()从设备中读取数据.再将读取出来的数据copy到用户空间中.
I2c_master_recv()代码如下:
int i2c_master_recv(struct i2c_client *client, char *buf ,int count)
{
    struct i2c_adapter *adap=client->adapter;
    struct i2c_msg msg;
    int ret;
 
    msg.addr = client->addr;
    msg.flags = client->flags & I2C_M_TEN;
    msg.flags |= I2C_M_RD;
    msg.len = count;
    msg.buf = buf;
 
    ret = i2c_transfer(adap, &msg, 1);
 
    /* If everything went ok (i.e. 1 msg transmitted), return #bytes
       transmitted, else error code. */
    return (ret == 1) ? count : ret;
}
看完前面的代码之后,这个函数应该很简单了,就是为读操作初始化了一个i2c_msg.然后调用i2c_tanster().代码中的 client->flags & I2C_M_TEN表示adapter是否采用10位寻址的方式.在这里就不再详细分析了.
另外,有人可能看出了一个问题.这里clinet->addr是从哪来的呢?对,在read之前应该还要有一步操作来设置 clinet->addr的值.这个过程是ioctl的操作.ioctl可以设置PEC标志,重试次数,超时时间,和发送接收数据等,我们在这里只 看一下clinet->addr的设置.代码片段如下示:
static int i2cdev_ioctl(struct inode *inode, struct file *file,
        unsigned int cmd, unsigned long arg)
{
    ......
    ......
    switch ( cmd ) {
    case I2C_SLAVE:
    case I2C_SLAVE_FORCE:
        /* NOTE:  devices set up to work with "new style" drivers
         * can't use I2C_SLAVE, even when the device node is not
         * bound to a driver.  Only I2C_SLAVE_FORCE will work.
         *
         * Setting the PEC flag here won't affect kernel drivers,
         * which will be using the i2c_client node registered with
         * the driver model core.  Likewise, when that client has
         * the PEC flag already set, the i2c-dev driver won't see
         * (or use) this setting.
         */
        if ((arg > 0x3ff) ||
            (((client->flags & I2C_M_TEN) == 0) && arg > 0x7f))
            return -EINVAL;
        if (cmd == I2C_SLAVE && i2cdev_check_addr(client->adapter, arg))
            return -EBUSY;
        /* REVISIT: address could become busy later */
        client->addr = arg;
        return 0;
    ......
    ......
}
由此可见,调用I2C_SLAVE或者I2C_SLAVE_FORCE的Ioctl就会设置clinet->addr.另外,注释中也说 得很清楚了.如果是I2C_SLAVE的话,还会调用其所长i2cdev_check_addr().进行地址检查,如果adapter已经关联到这个地 址的设备,就会检查失败.
 
7.2:write操作
Write操作如下所示:
static ssize_t i2cdev_write (struct file *file, const char __user *buf, size_t count,
                             loff_t *offset)
{
    int ret;
    char *tmp;
    struct i2c_client *client = (struct i2c_client *)file->private_data;
 
    if (count > 8192)
        count = 8192;
 
    tmp = kmalloc(count,GFP_KERNEL);
    if (tmp==NULL)
        return -ENOMEM;
    if (copy_from_user(tmp,buf,count)) {
        kfree(tmp);
        return -EFAULT;
    }
 
    pr_debug("i2c-dev: i2c-%d writing %zd bytes.\n",
        iminor(file->f_path.dentry->d_inode), count);
 
    ret = i2c_master_send(client,tmp,count);
    kfree(tmp);
    return ret;
}
该操作比较简单,就是将用户空间的数据发送到i2c 设备.
 
八:小结
在本节中,分析了i2c的框架设计.这个框架大体上沿用了Linux的设备驱动框架,不过之中又做了很多变通.在之后的分析中,会分别举一个adapter和i2c device的例子来详细描述一下有关i2c driver的设计.
 
阅读(3547) | 评论(0) | 转发(8) |
给主人留下些什么吧!~~