创建 I2C 设备驱动程序结构设置

Question

我正在编写一个使用 I2C 与主机通信的设备驱动程序。

下面是我想学习和理解的代码。如果我对下面的代码的理解是错误的，请帮助我。 “//”是我自己的评论&我对代码的理解。

// declare there will be a member struct inside the class example_state. This member is pointing to i2c_client.
struct example_state {
struct i2c_client *client; 
};

static int example_probe(struct i2c_client *client, const struct i2c_device_id *id{

// declare this to be a local struct inside the example_probe
struct example_state *state;

// get "struct device" to be pointed client's member name dev
// Question: is "struct device *dev" part of "struct i2c_client"?
// if "struct device *dev" imported from somewhere that why there is no need to allocate memory?
struct device *dev = &client->dev;

// allocate a memory space for "struct example_state"
// at this point "struct example_state" is still empty/NULL
// **Question:** but what is the relationship of this local "struct example_state"
// with the one declared before entering struct example_state function?
state = kzalloc(sizeof(struct example_state), GFP_KERNEL); 
if (state == NULL) {
    dev_err(dev, "failed to create our state\n");
    return -ENOMEM;
}

    // after memory allocated set the "struct i2c_client" point to "struct example_state"'s member namely "client".
state->client = client; 

   // set the our device I2C driver information to the host.
   // Question: Where to we set our device driver data?
i2c_set_clientdata(client, state); 

/* rest of the initialisation goes here. */

dev_info(dev, "example client created\n");

return 0;
}

static int __devexit example_remove(struct i2c_client *client){
  // get the loaded value from host, i guess is like unregister
  // my device driver information from the host when i exited.
struct example_state *state = i2c_get_clientdata(client);

kfree(state);
return 0;
}

static struct i2c_device_id example_idtable[] = {
{ "example", 0 },
{ }
};

Answer 1

我建议您不要研究 Linux 内核的 i2c 支持，而应该再看看 K&R 2 相反：

// declare there will be a member struct inside the class example_state. This member is pointing to i2c_client.
struct example_state {
struct i2c_client *client; 
};

C 没有类。 Linux 内核的 sysfs 抽象引入了内核自己关于什么是“类”的想法 - 请参阅 drivers/base/class.c - 但它根本没有任何关系到 C++、Java、C# 或您熟悉的任何其他语言中的类。

static int example_probe(struct i2c_client *client, const struct i2c_device_id *id{

// declare this to be a local struct inside the example_probe
struct example_state *state;

这将 state 声明为指向 struct example_state 内存的指针。 （我通常会在这里使用“对象”这个词，但我不想抵消上一段中的“C 没有类”的言论。它是一块大小为或者大于保存 struct example_state 的所有成员所需的数量，并且编译器知道检查操作的类型一致性......）无论如何

。 ，这行代码中只留出了足够的内存指针——不是结构本身。

// get "struct device" to be pointed client's member name dev
// Question: is "struct device *dev" part of "struct i2c_client"?
// if "struct device *dev" imported from somewhere that why there is no need to allocate memory?
struct device *dev = &client->dev;

此行中的 struct device *dev 为指针分配了足够的内存，并告诉编译器该指针仅指向 struct device 对象。 =&client->dev 查找 client 参数以查找 dev 成员并创建一个别名 以便在此例程中更轻松地使用。

// allocate a memory space for "struct example_state"
// at this point "struct example_state" is still empty/NULL
// **Question:** but what is the relationship of this local "struct example_state"
// with the one declared before entering struct example_state function?
state = kzalloc(sizeof(struct example_state), GFP_KERNEL); 
if (state == NULL) {
    dev_err(dev, "failed to create our state\n");
    return -ENOMEM;
}

之前的 struct example_state { .. } 是一个类型声明。它仅向编译器指示存储在带有 struct example_state 标签的结构中的成员的名称、大小和类型。它不分配任何内存。这里的 kzalloc() 调用确实分配内存，即我们的 struct example_state 对象的大小，并且它请求正常的 GFP_KERNEL< /code> 使用内存分配池。有关不同可用内存“池”的详细信息，请参阅 include/linux/gfp.h。根据传递给内核内存分配器的标志，内存分配可能仅在内核被迫执行某种“垃圾收集”之后发生——将脏内存缓冲区写回磁盘，丢弃页面缓存、交换进程，甚至可能调用 OOM 内存杀手来分配内存。当内核绝对不能在此时休眠时，使用 GFP_ATOMIC 池——非常适合在持有自旋锁或在中断上下文中使用。

虽然可以同时学习 C 语言和内核内部原理，但这是一个非常不允许犯错误的环境。在普通用户空间程序中跟踪流浪指针会使进程出现段错误，但在内核中跟踪流浪指针会使机器出现恐慌或损坏内存、文件系统等。