S3C2440驱动简析——I2C驱动（3）

     书接上回，在讨论完i2c设备、i2c适配器等初始化和删除相应驱动的程序后，我们在这个小节把注意力放在file_operations里面的几个函数操作上，先贴上file_operations结构体代码，让我们先看看其包含了哪几个函数。

 

static const struct file_operations i2cdev_fops = { .owner = THIS_MODULE, .llseek = no_llseek, .read = i2cdev_read, .write = i2cdev_write, .unlocked_ioctl = i2cdev_ioctl, .open = i2cdev_open, .release = i2cdev_release, };

 

这个结构体想必大家对其结构都相当清晰啦，由此可见，i2c驱动为用户空间提供的操作函数包括：

1.no_llseek

2.i2cdev_read

3.i2cdev_write

4.i2cdev_ioctl

5.i2cdev_open

6.i2cdev_release

 

下面就对它们逐一进行分析：

1.no_llseek

loff_t no_llseek(struct file *file, loff_t offset, int origin) { return -ESPIPE; }

从结构体i2cdev_fops 的成员名字llseek推测，其作用应该是改变当前I2C器件读写的位置，而现在正如大家所见，驱动程序并未实现这功能。

 

 

2.i2cdev_read

 

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 = 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 %zu 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; }

read函数主要做的事情有：

（1）struct i2c_client *client = file->private_data;

       通过参数传入的file（在上一篇博文有阐述）而找到像对应的一个i2c设备，接下来函数就是对该设备进行操作。

（2）ret = i2c_master_recv(client, tmp, count);

       其中i2c_master_recv函数在i2c-core.c 中实现，为了使本文结构更加清晰，故把跟硬件打交道的代码文件i2c-core.c 放在后面的博文分析。在这里，我们只需简单地理解为i2c_master_recv函数的作用是通过调用上文提及的i2c设备，并从其中提取count 字节的内容存到tmp 缓冲区里。

（3）ret = copy_to_user(buf, tmp, count) ? -EFAULT : ret;

       若无异常情况发生，就会调用咱们非常熟悉的copy_to_user 函数，这个函数在这里就不赘述了，建议还没掌握的朋友查找相关资料，或者我前面的博文都有提及过，这个是基础吖！

 

 

3.i2cdev_write

 

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 = file->private_data; if (count > 8192) count = 8192; tmp = memdup_user(buf, count); if (IS_ERR(tmp)) return PTR_ERR(tmp); pr_debug("i2c-dev: i2c-%d writing %zu bytes./n", iminor(file->f_path.dentry->d_inode), count); ret = i2c_master_send(client, tmp, count); kfree(tmp); return ret; }

write函数又做了什么事情呢？

（1）struct i2c_client *client = file->private_data;

       见上文read函数中的解释。

（2）tmp = memdup_user(buf, count);

       memdup_user 函数在Util.c 中实现，源代码如下

       void *memdup_user(const void __user *src, size_t len) { void *p; /* * Always use GFP_KERNEL, since copy_from_user() can sleep and * cause pagefault, which makes it pointless to use GFP_NOFS * or GFP_ATOMIC. */ p = kmalloc_track_caller(len, GFP_KERNEL); if (!p) return ERR_PTR(-ENOMEM); if (copy_from_user(p, src, len)) { kfree(p); return ERR_PTR(-EFAULT); } return p; }

      函数的作用一目了然，就是把指针src 所指向长度为len 的内容中copy_from_user 到函数返回的指针。

（3）ret = i2c_master_send(client, tmp, count);

       把缓存器tmp 里的内容发送到设备client 。

 

 

4.i2cdev_ioctl

 

static long i2cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { struct i2c_client *client = file->private_data; unsigned long funcs; dev_dbg(&client->adapter->dev, "ioctl, cmd=0x%02x, arg=0x%02lx/n", cmd, 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; case I2C_TENBIT: if (arg) client->flags |= I2C_M_TEN; else client->flags &= ~I2C_M_TEN; return 0; case I2C_PEC: if (arg) client->flags |= I2C_CLIENT_PEC; else client->flags &= ~I2C_CLIENT_PEC; return 0; case I2C_FUNCS: funcs = i2c_get_functionality(client->adapter); return put_user(funcs, (unsigned long __user *)arg); case I2C_RDWR: return i2cdev_ioctl_rdrw(client, arg); case I2C_SMBUS: return i2cdev_ioctl_smbus(client, arg); case I2C_RETRIES: client->adapter->retries = arg; break; case I2C_TIMEOUT: /* For historical reasons, user-space sets the timeout * value in units of 10 ms. */ client->adapter->timeout = msecs_to_jiffies(arg * 10); break; default: /* NOTE: returning a fault code here could cause trouble * in buggy userspace code. Some old kernel bugs returned * zero in this case, and userspace code might accidentally * have depended on that bug. */ return -ENOTTY; } return 0; }

由于i2c适配器的设备节点代表的是整条i2c总线，所以在对其进行具体的文件系统操作之前还必须指明
待访问设备的总线地址。指明地址的操作通过ioctl 系统调用完成的,它最终调用设备方法i2cdev_ioctl。ioctl函数还是采用我们非常熟悉的switch-case逻辑结构，其各种命令对应的操作分析如下

case I2C_SLAVE:
case I2C_SLAVE_FORCE: 设置从设备地址

 

case I2C_TENBIT: 设置7bit 地址 or 10bit 地址

 

case I2C_PEC: 如果I2C_PEC != 0 , to use PEC with SMBus

 

case I2C_FUNCS: 返回控制器算法所支持的传输种类

 

case I2C_RDWR: 执行i2cdev_ioctl_rdrw函数，代码如下

static noinline int i2cdev_ioctl_rdrw(struct i2c_client *client, unsigned long arg) { struct i2c_rdwr_ioctl_data rdwr_arg; struct i2c_msg *rdwr_pa; u8 __user **data_ptrs; int i, res; if (copy_from_user(&rdwr_arg, (struct i2c_rdwr_ioctl_data __user *)arg, sizeof(rdwr_arg))) return -EFAULT; /* Put an arbitrary limit on the number of messages that can * be sent at once */ if (rdwr_arg.nmsgs > I2C_RDRW_IOCTL_MAX_MSGS) return -EINVAL; rdwr_pa = (struct i2c_msg *) kmalloc(rdwr_arg.nmsgs * sizeof(struct i2c_msg), GFP_KERNEL); if (!rdwr_pa) return -ENOMEM; if (copy_from_user(rdwr_pa, rdwr_arg.msgs, rdwr_arg.nmsgs * sizeof(struct i2c_msg))) { kfree(rdwr_pa); return -EFAULT; } data_ptrs = kmalloc(rdwr_arg.nmsgs * sizeof(u8 __user *), GFP_KERNEL); if (data_ptrs == NULL) { kfree(rdwr_pa); return -ENOMEM; } res = 0; for (i = 0; i < rdwr_arg.nmsgs; i++) { /* Limit the size of the message to a sane amount; * and don't let length change either. */ if ((rdwr_pa[i].len > 8192) || (rdwr_pa[i].flags & I2C_M_RECV_LEN)) { res = -EINVAL; break; } data_ptrs[i] = (u8 __user *)rdwr_pa[i].buf; rdwr_pa[i].buf = kmalloc(rdwr_pa[i].len, GFP_KERNEL); if (rdwr_pa[i].buf == NULL) { res = -ENOMEM; break; } if (copy_from_user(rdwr_pa[i].buf, data_ptrs[i], rdwr_pa[i].len)) { ++i; /* Needs to be kfreed too */ res = -EFAULT; break; } } if (res < 0) { int j; for (j = 0; j < i; ++j) kfree(rdwr_pa[j].buf); kfree(data_ptrs); kfree(rdwr_pa); return res; } res = i2c_transfer(client->adapter, rdwr_pa, rdwr_arg.nmsgs); while (i-- > 0) { if (res >= 0 && (rdwr_pa[i].flags & I2C_M_RD)) { if (copy_to_user(data_ptrs[i], rdwr_pa[i].buf, rdwr_pa[i].len)) res = -EFAULT; } kfree(rdwr_pa[i].buf); } kfree(data_ptrs); kfree(rdwr_pa); return res; }

整个函数主要做了以下几件事

（1）copy_from_user(&rdwr_arg,
                              (struct i2c_rdwr_ioctl_data __user *)arg,
                              sizeof(rdwr_arg))

       获得用户传进来的命令，并存放在rdwr_arg。

（2）copy_from_user(rdwr_pa, rdwr_arg.msgs,
                              rdwr_arg.nmsgs * sizeof(struct i2c_msg))

       把指针rdwr_pa指向结构体rdwr_arg的成员msgs。

（3）整个for循环实质就是把要进行传输的每个msg的信息copy到字符串指针data_ptrs当中。

（4）res = i2c_transfer(client->adapter, rdwr_pa, rdwr_arg.nmsgs);

       i2c_transfer ()函数用于进行I2C适配器和I2C设备之间的一组消息交互，i2c_transfer()函数本身不具备驱动适配器物理硬件完成消息交互的能力，它只是寻找到i2c_adapter对应的i2c_algorithm，并使用i2c_algorithm的master_xfer()函数真正驱动硬件流程。至于i2c_algorithm和master_xfer等在接下来的博文介绍。

（5）while循环里把data_ptrs所存的信息统统copy_to_user。

case I2C_SMBUS: 执行i2cdev_ioctl_smbus 函数，主要调用i2c_smbus_xfer，函数在i2c-core.c 里实现。在这里我们先简单理解此函数作用为选择smbus 通信协议。

case I2C_RETRIES: 设置重发次数

case I2C_TIMEOUT: 设置超时时间，设定为 arg * 10 (ms)

呼呼~ioctl函数总算带过了，具体没有深入的地方，均是涉及到i2c-core.c 和数据结构i2c_algorithm的地方，这两个东东留待接下来的博文再来收拾。

5.i2cdev_open

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; i2c_dev = i2c_dev_get_by_minor(minor); if (!i2c_dev) return -ENODEV; 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. */ 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; client->adapter = adap; file->private_data = client; return 0; }

最后两个函数来点轻松的，一眼扫下来，就是一些注册、初始化等工作，唯一要注意地方是这个open函数比之前所介绍的其他驱动多了一点绑定的工作，如：

client->driver = &i2cdev_driver;

client->adapter = adap;
file->private_data = client;

至于其用意，不记得的朋友建议翻看本i2c驱动系列的第一篇博文。

 

6.i2cdev_release

static int i2cdev_release(struct inode *inode, struct file *file) { struct i2c_client *client = file->private_data; i2c_put_adapter(client->adapter); kfree(client); file->private_data = NULL; return 0; }

对照着open函数来看，这个release岂不是小菜一碟？

 

 

 

    到此为止，i2c-dev.c 的驱动代码分析暂告一段落，由于本人水平所限，不能很深刻、到位的写出这份驱动代码的分析，还请各位看客多多提点。内容错漏，请严厉指出！接下来本系列的第4篇博文将讲述i2c-core.c 和数据结构i2c_algorithm，敬请关注！

 

本系列博文链接：

I2C驱动（1）http://blog.youkuaiyun.com/jarvis_xian/archive/2011/05/27/6449939.aspx
I2C驱动（2）http://blog.youkuaiyun.com/jarvis_xian/archive/2011/05/27/6451168.aspx
I2C驱动（3）http://blog.youkuaiyun.com/jarvis_xian/archive/2011/05/28/6452431.aspx
I2C驱动（4）http://blog.youkuaiyun.com/jarvis_xian/archive/2011/05/30/6455697.aspx