linux gpio模拟i2c的使用/用GPIO模拟I2C总线-2

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. };