linux gpio模拟i2c的使用/用GPIO模拟I2C总线-2 .

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在drivers/i2c/busses下包含各种I2C总线驱动，如S3C2440的I2C总线驱动i2c-s3c2410.c，使用GPIO模拟I2C总线的驱动i2c-gpio.c，这里只分析i2c-gpio.c。

i2c-gpio.c它是gpio模拟I2C总线的驱动，总线也是个设备，在这里将总线当作平台设备处理，那驱动当然是平台设备驱动，看它的驱动注册和注销函数。

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1. static int __init i2c_gpio_init(void)
2. {
3. int ret;
4.
5. ret = platform_driver_register(&i2c_gpio_driver);
6. if (ret)
7. printk(KERN_ERR "i2c-gpio: probe failed: %d\n", ret);
8.
9. return ret;
10. }
11. module_init(i2c_gpio_init);
12.
13. static void __exit i2c_gpio_exit(void)
14. {
15. platform_driver_unregister(&i2c_gpio_driver);
16. }
17. module_exit(i2c_gpio_exit);

没有什么好说的，它的初始化和注销函数就是注册和注销一个平台设备驱动，直接看它的platform_driver结构i2c_gpio_driver

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1. static struct platform_driver i2c_gpio_driver = {
2. .driver = {
3. .name = "i2c-gpio",
4. .owner = THIS_MODULE,
5. },
6. .probe = i2c_gpio_probe,
7. .remove = __devexit_p(i2c_gpio_remove),
8. };

平台驱动设备放在arch/arm/mach-xxxx/board-xxx.c中

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1. #if defined(CONFIG_I2C_GPIO) | \
2. defined(CONFIG_I2C_GPIO_MODULE)
3. static struct i2c_gpio_platform_data i2c_gpio_adapter_data = {
4. .sda_pin = PINID_GPMI_D05,
5. .scl_pin = PINID_GPMI_D04,
6. .udelay = 5, //100KHz
7. .timeout = 100,
8. .sda_is_open_drain = 1,
9. .scl_is_open_drain = 1,
10. };
11.
12. static struct platform_device i2c_gpio = {
13. .name = "i2c-gpio",
14. .id = 0,
15. .dev = {
16. .platform_data = &i2c_gpio_adapter_data,
17. .release = mxs_nop_release,
18. },
19. };
20. #endif

在这里struct platform_device结构中的name字段要和struct platform_driver中driver字段中name字段要相同，因为平台总线就是通过这个来判断设备和驱动是否匹配的。注意这里的id将它赋值了0，至于到底有什么用，后面再来细看。这个结构里面还包含一个最重要的数据i2c_gpio_adapter_data，它struct i2c_gpio_platform_data结构类型变量，这个结构体类型定义在include/linux/i2c-gpio.h中。

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1. struct i2c_gpio_platform_data {
2. unsigned int sda_pin;
3. unsigned int scl_pin;
4. int udelay;
5. int timeout;
6. unsigned int sda_is_open_drain:1;
7. unsigned int scl_is_open_drain:1;
8. unsigned int scl_is_output_only:1;
9. };

这个结构体主要描述gpio模拟i2c总线，sda_pin和scl_pin表示使用哪两个IO管脚来模拟I2C总线，udelay和timeout分别为它的时钟频率和超时时间，sda_is_open_drain和scl_is_open_drain表示sda、scl这两个管脚是否是开漏(opendrain)电路，如果是设置为1，scl_is_output_only表示scl这个管脚是否只是作为输出，如果是设置为1。

回到驱动中，看其中最重要的i2c_gpio_probe。

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1. static int __devinit i2c_gpio_probe(struct platform_device *pdev)
2. {
3. struct i2c_gpio_platform_data *pdata;
4. struct i2c_algo_bit_data *bit_data;
5. struct i2c_adapter *adap;
6. int ret;
7.
8. pdata = pdev->dev.platform_data;
9. if (!pdata)
10. return -ENXIO;
11.
12. ret = -ENOMEM;
13. adap = kzalloc(sizeof(struct i2c_adapter), GFP_KERNEL);
14. if (!adap)
15. goto err_alloc_adap;
16. bit_data = kzalloc(sizeof(struct i2c_algo_bit_data), GFP_KERNEL);
17. if (!bit_data)
18. goto err_alloc_bit_data;
19.
20. ret = gpio_request(pdata->sda_pin, "sda");
21. if (ret)
22. goto err_request_sda;
23. ret = gpio_request(pdata->scl_pin, "scl");
24. if (ret)
25. goto err_request_scl;
26.
27. if (pdata->sda_is_open_drain) {
28. gpio_direction_output(pdata->sda_pin, 1);
29. bit_data->setsda = i2c_gpio_setsda_val;
30. } else {
31. gpio_direction_input(pdata->sda_pin);
32. bit_data->setsda = i2c_gpio_setsda_dir;
33. }
34.
35. if (pdata->scl_is_open_drain || pdata->scl_is_output_only) {
36. gpio_direction_output(pdata->scl_pin, 1);
37. bit_data->setscl = i2c_gpio_setscl_val;
38. } else {
39. gpio_direction_input(pdata->scl_pin);
40. bit_data->setscl = i2c_gpio_setscl_dir;
41. }
42.
43. if (!pdata->scl_is_output_only)
44. bit_data->getscl = i2c_gpio_getscl;
45. bit_data->getsda = i2c_gpio_getsda;
46.
47. if (pdata->udelay)
48. bit_data->udelay = pdata->udelay;
49. else if (pdata->scl_is_output_only)
50. bit_data->udelay = 50; /* 10 kHz */
51. else
52. bit_data->udelay = 5; /* 100 kHz */
53.
54. if (pdata->timeout)
55. bit_data->timeout = pdata->timeout;
56. else
57. bit_data->timeout = HZ / 10; /* 100 ms */
58.
59. bit_data->data = pdata;
60.
61. adap->owner = THIS_MODULE;
62. snprintf(adap->name, sizeof(adap->name), "i2c-gpio%d", pdev->id);
63. adap->algo_data = bit_data;
64. adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;
65. adap->dev.parent = &pdev->dev;
66.
67. /*
68. * If "dev->id" is negative we consider it as zero.
69. * The reason to do so is to avoid sysfs names that only make
70. * sense when there are multiple adapters.
71. */
72. adap->nr = (pdev->id != -1) ? pdev->id : 0;
73. ret = i2c_bit_add_numbered_bus(adap);
74. if (ret)
75. goto err_add_bus;
76.
77. platform_set_drvdata(pdev, adap);
78.
79. dev_info(&pdev->dev, "using pins %u (SDA) and %u (SCL%s)\n",
80. pdata->sda_pin, pdata->scl_pin,
81. pdata->scl_is_output_only
82. ? ", no clock stretching" : "");
83.
84. return 0;
85.
86. err_add_bus:
87. gpio_free(pdata->scl_pin);
88. err_request_scl:
89. gpio_free(pdata->sda_pin);
90. err_request_sda:
91. kfree(bit_data);
92. err_alloc_bit_data:
93. kfree(adap);
94. err_alloc_adap:
95. return ret;
96. }

从这句开始pdata= pdev->dev.platform_data;这不正是我们在平台设备结构中定义的数据吗。然后是使用kzalloc申请两段内存空间，一个是为结构struct i2c_adapter申请的，另一个是为结构structi2c_algo_bit_data申请的。

struct i2c_adapter结构定义在include/linux/i2c.h中

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1. struct i2c_adapter {
2. struct module *owner;
3. unsigned int id;
4. unsigned int class; /* classes to allow probing for */
5. const struct i2c_algorithm *algo; /* the algorithm to access the bus */
6. void *algo_data;
7.
8. /* data fields that are valid for all devices */
9. u8 level; /* nesting level for lockdep */
10. struct mutex bus_lock;
11.
12. int timeout; /* in jiffies */
13. int retries;
14. struct device dev; /* the adapter device */
15.
16. int nr;
17. char name[48];
18. struct completion dev_released;
19. };

在I2C子系统中，I2C适配器使用结构struct i2c_adapter描述，代表一条实际的I2C总线。

struct i2c_algo_bit_data结构定义在include/linux/i2c-algo-bit.h中

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1. struct i2c_algo_bit_data {
2. void *data; /* private data for lowlevel routines */
3. void (*setsda) (void *data, int state);
4. void (*setscl) (void *data, int state);
5. int (*getsda) (void *data);
6. int (*getscl) (void *data);
7.
8. /* local settings */
9. int udelay; /* half clock cycle time in us,
10. minimum 2 us for fast-mode I2C,
11. minimum 5 us for standard-mode I2C and SMBus,
12. maximum 50 us for SMBus */
13. int timeout; /* in jiffies */
14. };

这个结构主要用来定义对GPIO管脚的一些操作，还是回到probe中

接下来使用gpio_request去申请这个两个GPIO管脚，申请的目的是为了防止重复使用管脚。然后是根据struct i2c_gpio_platform_data结构中定义的后面三个数据对struct i2c_algo_bit_data结构中的函数指针做一些赋值操作。接下来是I2C时钟频率和超时设置，如果在struct i2c_gpio_platform_data结构中定义了值，那么就采用定义的值，否则就采用默认的值。然后是对struct i2c_adapter结构的一些赋值操作，比如指定它的父设备为这里的平台设备，前面在平台设备中定义了一个id，这里用到了，赋给了struct i2c_adapter中的nr成员，这个值表示总线号，这里的总线号和硬件无关，只是在软件上的区分。然后到了最后的主角i2c_bit_add_numbered_bus，这个函数定义在drivers/i2c/algos/i2c-algo-bit.c中

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1. int i2c_bit_add_numbered_bus(struct i2c_adapter *adap)
2. {
3. int err;
4.
5. err = i2c_bit_prepare_bus(adap);
6. if (err)
7. return err;
8.
9. return i2c_add_numbered_adapter(adap);
0. }

先看i2c_bit_prepare_bus函数

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1. static int i2c_bit_prepare_bus(struct i2c_adapter *adap)
2. {
3. struct i2c_algo_bit_data *bit_adap = adap->algo_data;
4.
5. if (bit_test) {
6. int ret = test_bus(bit_adap, adap->name);
7. if (ret < 0)
8. return -ENODEV;
9. }
10.
11. /* register new adapter to i2c module... */
12. adap->algo = &i2c_bit_algo;
13. adap->retries = 3;
14.
15. return 0;
16. }

bit_test为模块参数，这里不管它，看这样一句adap->algo= &i2c_bit_algo;

来看这个结构定义

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1. static const struct i2c_algorithm i2c_bit_algo = {
2. .master_xfer = bit_xfer,
3. .functionality = bit_func,
4. };

先看这个结构类型在哪里定义的include/linux/i2c.h

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1. struct i2c_algorithm {
2. /* If an adapter algorithm can't do I2C-level access, set master_xfer
3. to NULL. If an adapter algorithm can do SMBus access, set
4. smbus_xfer. If set to NULL, the SMBus protocol is simulated
5. using common I2C messages */
6. /* master_xfer should return the number of messages successfully
7. processed, or a negative value on error */
8. int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
9. int num);
10. int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
11. unsigned short flags, char read_write,
12. u8 command, int size, union i2c_smbus_data *data);
13.
14. /* To determine what the adapter supports */
15. u32 (*functionality) (struct i2c_adapter *);
16. };

其实也没什么，就三个函数指针外加一长串注释

这个结构的master_xfer指针为主机的数据传输，具体来看bit_xfer这个函数，这个函数和I2C协议相关，I2C协议规定要先发送起始信号，才能开始进行数据的传输，最后数据传输完成后发送停止信号，看接下来代码对I2C协议要熟悉，所以这里的关键点是I2C协议。

static int bit_xfer(struct i2c_adapter *i2c_adap,
struct i2c_msg msgs[], int num)
{
struct i2c_msg *pmsg;
struct i2c_algo_bit_data *adap = i2c_adap->algo_data;
int i, ret;
unsigned short nak_ok;
bit_dbg(3, &i2c_adap->dev, "emitting start condition\n");
/*发送起始信号*/
i2c_start(adap);
for (i = 0; i < num; i++) {
pmsg = &msgs[i];
nak_ok = pmsg->flags & I2C_M_IGNORE_NAK;
if (!(pmsg->flags & I2C_M_NOSTART)) {
if (i) {
bit_dbg(3, &i2c_adap->dev, "emitting "
"repeated start condition\n");
i2c_repstart(adap);
}
ret = bit_doAddress(i2c_adap, pmsg);
if ((ret != 0) && !nak_ok) {
bit_dbg(1, &i2c_adap->dev, "NAK from "
"device addr 0x%02x msg #%d\n",
msgs[i].addr, i);
goto bailout;
}
}
if (pmsg->flags & I2C_M_RD) {
/* read bytes into buffer*/
ret = readbytes(i2c_adap, pmsg);
if (ret >= 1)
bit_dbg(2, &i2c_adap->dev, "read %d byte%s\n",
ret, ret == 1 ? "" : "s");
if (ret < pmsg->len) {
if (ret >= 0)
ret = -EREMOTEIO;
goto bailout;
}
} else {
/* write bytes from buffer */
ret = sendbytes(i2c_adap, pmsg);
if (ret >= 1)
bit_dbg(2, &i2c_adap->dev, "wrote %d byte%s\n",
ret, ret == 1 ? "" : "s");
if (ret < pmsg->len) {
if (ret >= 0)
ret = -EREMOTEIO;
goto bailout;
}
}
}
ret = i;
bailout:
bit_dbg(3, &i2c_adap->dev, "emitting stop condition\n");
i2c_stop(adap);
return ret;
}

1.发送起始信号

i2c_start(adap);

看这个函数前，先看I2C协议怎么定义起始信号的

起始信号就是在SCL为高电平期间，SDA从高到低的跳变，再来看代码是怎么实现的

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1. static void i2c_start(struct i2c_algo_bit_data *adap)
2. {
3. /* assert: scl, sda are high */
4. setsda(adap, 0);
5. udelay(adap->udelay);
6. scllo(adap);
7. }

这些setsda和setscl这些都是使用的总线的函数，在这里是使用的i2c-gpio.c中定义的函数，还记得那一系列判断赋值吗。

#define setsda(adap, val) adap->setsda(adap->data, val)
#define setscl(adap, val) adap->setscl(adap->data, val)
#define getsda(adap) adap->getsda(adap->data)
#define getscl(adap) adap->getscl(adap->data)

2.往下是个大的for循环

到了这里又不得不说这个struct i2c_msg结构，这个结构定义在include/linux/i2c.h中

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1. struct i2c_msg {
2. __u16 addr; /* slave address */
3. __u16 flags;
4. #define I2C_M_TEN 0x0010 /* this is a ten bit chip address */
5. #define I2C_M_RD 0x0001 /* read data, from slave to master */
6. #define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_PROTOCOL_MANGLING */
7. #define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */
8. #define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */
9. #define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */
10. #define I2C_M_RECV_LEN 0x0400 /* length will be first received byte */
11. __u16 len; /* msg length */
12. __u8 *buf; /* pointer to msg data */
13. };

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