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。
-
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
-
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
-
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。
-
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。
-
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
-
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
-
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
-
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
-
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
-
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
-
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
-
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
-
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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