16-AT24XX EEPROM的I2C设备驱动实例
15.6 AT24XX EEPROM的I2C设备驱动实例
drivers/i2c/chips/at24.c文件支持大多数I2C接口的EEPROM，正如我们之前所述，一个具体的I2C设备个驱动由两部分组成，一部分是i2c_driver，用于将设备挂接于I2C总线，一类是设备本身的驱动。对于EEPROM而言，设备本身的驱动以bin_attribute二进制sysfs结点形式呈现。代码清单15.26给出了该驱动的框架。
代码清单15.26 AT24XX EEPROM驱动
1 / bin_attribute部分 /
2
3 static ssize_t at24_bin_read(struct kobject kobj, struct bin_attribute attr,
4 char *buf, loff_t off, size_t count)
5 {
6 struct at24_data *at24;
7 ssize_t retval = 0;
8
9 at24 = dev_get_drvdata(container_of(kobj, struct device, kobj));
10
11 ...
12
13 while (count) {
14 ssize_t status;
15
16 status = at24_eeprom_read(at24, buf, off, count);
17 ...
18 }
19
20 return retval;
21 }
22
23 static ssize_t at24_bin_write(struct kobject kobj, struct bin_attribute attr,
24 char *buf, loff_t off, size_t count)
25 {
26 struct at24_data *at24;
27 ssize_t retval = 0;
28
29 at24 = dev_get_drvdata(container_of(kobj, struct device, kobj));
30
31 ...
32
33 while (count) {
34 ssize_t status;
35
36 status = at24 eeprom write(at24, buf, off, count);
37 ...
38 }
39
40 ...
41 }
42
43 / i2c_driver部分 /
44
45 static const struct i2c_device_id at24_ids[] = {
46 { "24c00", AT24_DEVICE_MAGIC(128 / 8, AT24_FLAG_TAKE8ADDR) },
47 { "24c01", AT24_DEVICE_MAGIC(1024 / 8, 0) },
48 ...
49 { "at24", 0 },
50 { / END OF LIST / }
51 };
52 MODULE_DEVICE_TABLE(i2c, at24_ids);
53
54 static int at24_probe(struct i2c_client client, const struct i2c_device_id id)
55 {
56 ...
57 / 以sysfs二进制结点的形式呈现eeprom数据 /
58 at24->bin.attr.name = "eeprom";
59 at24->bin.attr.mode = chip.flags & AT24_FLAG_IRUGO ? S_IRUGO : S_IRUSR;
60 at24->bin.read = at24_bin_read;
61 at24->bin.size = chip.byte_len;
62 ...
63 at24->bin.write = at24_bin_write;
64
65 ...
66 err = sysfs create bin_ file(&client->dev.kobj, &at24->bin);
67 if (err)
68 goto err_clients;
69
70 i2c_set_clientdata(client, at24);
71
72 ...
73 }
74
75 static int __devexit at24_remove(struct i2c_client *client)
76 {
77 struct at24_data *at24;
78 int i;
79
80 at24 = i2c_get_clientdata(client);
81 sysfs remove bin_ file(&client->dev.kobj, &at24->bin);
82
83 for (i = 1; i < at24->num_addresses; i++)
84 i2c_unregister_device(at24->client[i]);
85
86 ...
87 }
88
89 static struct i2c_driver at24_driver = {
90 .driver = {
91 .name = "at24",
92 .owner = THIS_MODULE,
93 },
94 .probe = at24_probe,
95 .remove = __devexit_p(at24_remove),
96 .id_table = at24_ids,
97 };
98
99 static int __init at24_init(void)
100 {
101 io_limit = rounddown_pow_of_two(io_limit);
102 return i2c_add_driver(&at24_driver);
103 }
104 module_init(at24_init);
105
106 static void __exit at24_exit(void)
107 {
108 i2c_del_driver(&at24_driver);
109 }
110 module_exit(at24_exit);
上述代码中的1～40行对应EEPROM驱动本身的读写实现即bin_attribute驱动，之后的一部分是i2c_driver，两者在i2c_driver的probe()、remove()函数中建立关联。i2c_driver的probe()函数中初始化并通过第66行的sysfs_create_bin_file()注册了二进制sysfs结点，而remove()函数则通过第81行的sysfs_remove_bin_file()注销了sysfs结点。
第16行调用的at24_eeprom_read()和第36行调用的at24_eeprom_write()通过i2cmsg和i2c transfer完成实际的数据传输。
drivers/i2c/chips/at24.c不依赖于具体的CPU和I2C控制器硬件特性，因此，如果某一电路板包含该外设，只需要在板文件中添加对应的i2c_board_info，如对于LDD6410在arch/arm/machs3c6410/ mach-ldd6410.c中添加的信息为：
static struct i2c_board_info i2c_devs0[] __initdata = {
{ I2C_BOARD_INFO("24c02", 0x50), },
};
此后，我们在LDD6410上透过/sys/class/i2c-adapter/i2c-0/0-0050/eeprom文件结点即可操作连接的EEPROM。