I2C体系结构
linux的I2C体系结构分为3个组成部分:
(1) I2C核心。提供I2C总线驱动和设备驱动的注册和注销方法。
(2) I2C总线驱动。对适配器端的实现。
(3) I2C设备驱动。设备端的实现。
整个体系架构如图:
I2C设备在sysfs文件系统中显示在sys/bus/i2c目录,例如:
理解i2c体系结构,首先要理解i2c_driver、i2c_client、i2c_adapter和i2c_algorithm这四个数据结构。这四个数据结构均在i2c.h在定义:
struct i2c_driver { unsigned int class; /* Notifies the driver that a new bus has appeared. You should avoid * using this, it will be removed in a near future. */ int (*attach_adapter)(struct i2c_adapter *) __deprecated; /* Standard driver model interfaces */ int (*probe)(struct i2c_client *, const struct i2c_device_id *); int (*remove)(struct i2c_client *); /* driver model interfaces that don‘t relate to enumeration */ void (*shutdown)(struct i2c_client *); int (*suspend)(struct i2c_client *, pm_message_t mesg); int (*resume)(struct i2c_client *); /* Alert callback, for example for the SMBus alert protocol. * The format and meaning of the data value depends on the protocol. * For the SMBus alert protocol, there is a single bit of data passed * as the alert response‘s low bit ("event flag"). */ void (*alert)(struct i2c_client *, unsigned int data); /* a ioctl like command that can be used to perform specific functions * with the device. */ int (*command)(struct i2c_client *client, unsigned int cmd, void *arg); struct device_driver driver; const struct i2c_device_id *id_table; /* Device detection callback for automatic device creation */ int (*detect)(struct i2c_client *, struct i2c_board_info *); const unsigned short *address_list; struct list_head clients; };
struct i2c_client { unsigned short flags; /* div., see below */ unsigned short addr; /* chip address - NOTE: 7bit */ /* addresses are stored in the */ /* _LOWER_ 7 bits */ char name[I2C_NAME_SIZE]; struct i2c_adapter *adapter; /* the adapter we sit on */ struct device dev; /* the device structure */ int irq; /* irq issued by device */ struct list_head detected; };
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; /* data fields that are valid for all devices */ struct rt_mutex bus_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; };
struct i2c_algorithm { /* If an adapter algorithm can‘t do I2C-level access, set master_xfer to NULL. If an adapter algorithm can do SMBus access, set smbus_xfer. If set to NULL, the SMBus protocol is simulated using common I2C messages */ /* master_xfer should return the number of messages successfully processed, or a negative value on error */ int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs, int num); int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr, unsigned short flags, char read_write, u8 command, int size, union i2c_smbus_data *data); /* To determine what the adapter supports */ u32 (*functionality) (struct i2c_adapter *); };
1. i2c_adapter对应物理上一个适配器,而i2c_algorithm对应一套通信方法。一个i2c适配器需要i2c_algorithm中提供的通信函数来控制适配器上产生特定的访问周期。i2c_algorithm中关键函数master_xfer用于产生i2c访问周期需要的信号,以i2c_msg为单位。
2. i2c_driver对应一套驱动方法,辅助作用的数据结构,不对应任何物理实体。i2c_client对应于真实的物理设备,每个i2c设备都需要一个i2c_client来描述。i2c_driver的attach_adapter函数运行时将i2c_driver和i2c_client关联起来。
3. i2c_adapter和i2c_client的关系与i2c硬件体系中适配器和设备的关系一致,即i2c_client依附于i2c_adapter。
I2C核心
i2c核心中提供了一组不依赖于硬件平台的接口函数,具体可以查看dirvers/i2c/i2c-core.c文件。主要是增加删除i2c_driver和增加删除i2c_adapter,i2c传输发送接收等。主要功能函数必须看代码理解,不太需要我们自己去改。
I2C总线驱动
适配器的加载主要是初始化i2c适配器所使用的硬件资源,如申请I/O地址、中断后等,通过i2c_add_adapter添加i2c_adapter数据结构。卸载工作则与之相反。
i2c适配器通信方法,我们需要根据特定的适配器实现i2c_algorithm中的master_xfer方法,master_xfer方法用于传输i2c消息,内核源码中有很多可以参考的实例,这里摘取一个:
static int i2c_xfer(struct i2c_adapter *i2c_adap, struct i2c_msg *msgs, int num) { struct saa7164_i2c *bus = i2c_adap->algo_data; struct saa7164_dev *dev = bus->dev; int i, retval = 0; dprintk(DBGLVL_I2C, "%s(num = %d)\n", __func__, num); for (i = 0 ; i < num; i++) { dprintk(DBGLVL_I2C, "%s(num = %d) addr = 0x%02x len = 0x%x\n", __func__, num, msgs[i].addr, msgs[i].len); if (msgs[i].flags & I2C_M_RD) { /* Unsupported - Yet*/ printk(KERN_ERR "%s() Unsupported - Yet\n", __func__); continue; } else if (i + 1 < num && (msgs[i + 1].flags & I2C_M_RD) && msgs[i].addr == msgs[i + 1].addr) { /* write then read from same address */ retval = saa7164_api_i2c_read(bus, msgs[i].addr, msgs[i].len, msgs[i].buf, msgs[i+1].len, msgs[i+1].buf ); i++; if (retval < 0) goto err; } else { /* write */ retval = saa7164_api_i2c_write(bus, msgs[i].addr, msgs[i].len, msgs[i].buf); } if (retval < 0) goto err; } return num; err: return retval; }
i2c设备驱动需要使用i2c_driver和i2c_client数据结构并填充其中的成员函数。
i2c驱动的注册加载,仍然从源码中摘取一段,看看:
static struct i2c_driver adnp_i2c_driver = { .driver = { .name = "gpio-adnp", .owner = THIS_MODULE, .of_match_table = adnp_of_match, }, .probe = adnp_i2c_probe, .remove = adnp_i2c_remove, .id_table = adnp_i2c_id, };
int saa7164_i2c_register(struct saa7164_i2c *bus) { struct saa7164_dev *dev = bus->dev; dprintk(DBGLVL_I2C, "%s(bus = %d)\n", __func__, bus->nr); bus->i2c_adap = saa7164_i2c_adap_template; bus->i2c_client = saa7164_i2c_client_template; bus->i2c_adap.dev.parent = &dev->pci->dev; strlcpy(bus->i2c_adap.name, bus->dev->name, sizeof(bus->i2c_adap.name)); bus->i2c_adap.algo_data = bus; i2c_set_adapdata(&bus->i2c_adap, bus); i2c_add_adapter(&bus->i2c_adap); bus->i2c_client.adapter = &bus->i2c_adap; if (0 != bus->i2c_rc) printk(KERN_ERR "%s: i2c bus %d register FAILED\n", dev->name, bus->nr); return bus->i2c_rc; }
总得说来,我们需要完成i2c适配器的硬件驱动、探测等,根据特定的传输规则实现algorithm中的master_xfer方法,实现设备与驱动的接口,attach_adapter。最后实现i2c设备驱动的文件操作接口。
这些理论还是通过实例会理解的更深刻一些。i2c的实例很普遍,还是通过实例学习好一些。改日通过一个显示屏的驱动实例再来过一遍吧。。
原文地址:http://blog.csdn.net/njufeng/article/details/24902435