Linux Kernel 设备驱动之I2C之host描述

对于I2C传输来说，其由三部分组成，一个是I2C控制端，二是控制端对外提供的总线，三是总线上的设备。
所以，存在三个方面的抽象，控制端一般叫host或者adapter等，其结构如下：
/*
 * i2c_adapter is the structure used to identify a physical i2c bus along
 * with the access algorithms necessary to access it.
 */
struct i2c_adapter {
 struct module *owner;
 unsigned int class;    /* classes to allow probing for */
 const struct i2c_algorithm *algo; /* the algorithm to access the bus */
 void *algo_data;这两个是I2C传输数据的关键。
 /* data fields that are valid for all devices */
 const struct i2c_lock_operations *lock_ops;
 struct rt_mutex bus_lock;
 struct rt_mutex mux_lock;
 int timeout;   /* in jiffies */
 int retries;
 struct device dev;  /* the adapter device */
 int nr;
 char name[48];
 struct completion dev_released;
 struct mutex userspace_clients_lock;
 struct list_head userspace_clients;
 struct i2c_bus_recovery_info *bus_recovery_info;
 const struct i2c_adapter_quirks *quirks;
};
对于adapter来说，要么静态定义，要么动态分配，在简单初始化后，调用函数
i2c_add_adapter()或者函数i2c_add_numbered_adapter()把adapter添加到
i2c驱动的框架层。
这两个函数都是调用i2c_register_adapter()完成adapter的注册。
 

static int i2c_register_adapter(struct i2c_adapter *adap)
{
 int res = -EINVAL;
 /* Can't register until after driver model init */
 if (WARN_ON(!is_registered)) {
  res = -EAGAIN;
  goto out_list;
 }
 /* Sanity checks */
 if (WARN(!adap->name[0], "i2c adapter has no name"))
  goto out_list;
 if (!adap->algo) {
  pr_err("adapter '%s': no algo supplied!\n", adap->name);
  goto out_list;
 }
 if (!adap->lock_ops)
  adap->lock_ops = &i2c_adapter_lock_ops;
 rt_mutex_init(&adap->bus_lock);
 rt_mutex_init(&adap->mux_lock);
 mutex_init(&adap->userspace_clients_lock);
 INIT_LIST_HEAD(&adap->userspace_clients);
 /* Set default timeout to 1 second if not already set */
 if (adap->timeout == 0)
  adap->timeout = HZ;
 dev_set_name(&adap->dev, "i2c-%d", adap->nr);
 adap->dev.bus = &i2c_bus_type;设备模型中，设备总线类型
 adap->dev.type = &i2c_adapter_type;设备类型
 res = device_register(&adap->dev);设备模型中设备注册
 if (res) {
  pr_err("adapter '%s': can't register device (%d)\n", adap->name, res);
  goto out_list;
 }
 dev_dbg(&adap->dev, "adapter [%s] registered\n", adap->name);
//PM支持回调
 pm_runtime_no_callbacks(&adap->dev);
 pm_suspend_ignore_children(&adap->dev, true);
 pm_runtime_enable(&adap->dev);
#ifdef CONFIG_I2C_COMPAT
 res = class_compat_create_link(i2c_adapter_compat_class, &adap->dev,
           adap->dev.parent);
 if (res)
  dev_warn(&adap->dev,
    "Failed to create compatibility class link\n");
#endif
 i2c_init_recovery(adap);
 /* create pre-declared device nodes */
 of_i2c_register_devices(adap);
 i2c_acpi_register_devices(adap);
 i2c_acpi_install_space_handler(adap);
 if (adap->nr < __i2c_first_dynamic_bus_num)
  i2c_scan_static_board_info(adap);
 /* Notify drivers */
 mutex_lock(&core_lock);
 bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
 mutex_unlock(&core_lock);这一步是对总线上设备探测，这里的回调函数是__process_new_adapter
 return 0;
out_list:
 mutex_lock(&core_lock);
 idr_remove(&i2c_adapter_idr, adap->nr);
 mutex_unlock(&core_lock);
 return res;
}
在添加总线时，我们看看是否有设备在总线上，如果有则调用设备的驱动，对设备初始化。这里并不一定对设备初始化，
也可以提前做些事情。
static int __process_new_adapter(struct device_driver *d, void *data)
{
 return i2c_do_add_adapter(to_i2c_driver(d), data);
}
static int i2c_do_add_adapter(struct i2c_driver *driver,设备驱动
         struct i2c_adapter *adap)host对象
{
 /* Detect supported devices on that bus, and instantiate them */
 i2c_detect(adap, driver);
 /* Let legacy drivers scan this bus for matching devices */
 if (driver->attach_adapter) {
  dev_warn(&adap->dev, "%s: attach_adapter method is deprecated\n",
    driver->driver.name);
  dev_warn(&adap->dev,
    "Please use another way to instantiate your i2c_client\n");
  /* We ignore the return code; if it fails, too bad */
  driver->attach_adapter(adap);
 }
 return 0;
}