Linux I2C子系统分析之（一） ----- 用GPIO模拟I2C总线

- 日理万妓 2022-08-22 13:22 600阅读 0赞

原文地址：[http://blog.csdn.net/tommy\_wxie/article/details/7408328][http_blog.csdn.net_tommy_wxie_article_details_7408328]

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. \};

这个结构专门用于数据传输相关的addr为I2C设备地址，flags为一些标志位，len为数据的长度，buf为数据。这里宏定义的一些标志还是需要了解一下。  
I2C\_M\_TEN表示10位设备地址  
I2C\_M\_RD读标志  
I2C\_M\_NOSTART无起始信号标志  
I2C\_M\_IGNORE\_NAK忽略应答信号标志  
回到for，这里的num代表有几个struct i2c\_msg，进入for语句，接下来是个if语句，判断这个设备是否定义了I2C\_M\_NOSTART标志，这个标志主要用于写操作时，不必重新发送起始信号和设备地址，但是对于读操作就不同了，要调用i2c\_repstart这个函数去重新发送起始信号，调用bit\_doAddress函数去重新构造设备地址字节，来看这个函数。  
static int bit\_doAddress(struct i2c\_adapter \*i2c\_adap, struct i2c\_msg \*msg)  
\{  
unsigned short flags = msg->flags;  
unsigned short nak\_ok = msg->flags & I2C\_M\_IGNORE\_NAK;  
struct i2c\_algo\_bit\_data \*adap = i2c\_adap->algo\_data;

unsigned char addr;  
    int ret, retries;  
    
    retries = nak_ok ? 0 : i2c_adap->retries;  
    
    if (flags & I2C_M_TEN) {  
        /* a ten bit address */  
        addr = 0xf0 | ((msg->addr >> 7) & 0x03);  
        bit_dbg(2, &i2c_adap->dev, "addr0: %d\n", addr);  
        /* try extended address code...*/  
        ret = try_address(i2c_adap, addr, retries);  
        if ((ret != 1) && !nak_ok)  {  
            dev_err(&i2c_adap->dev,  
                "died at extended address code\n");  
            return -EREMOTEIO;  
        }  
        /* the remaining 8 bit address */  
        ret = i2c_outb(i2c_adap, msg->addr & 0x7f);  
        if ((ret != 1) && !nak_ok) {  
            /* the chip did not ack / xmission error occurred */  
            dev_err(&i2c_adap->dev, "died at 2nd address code\n");  
            return -EREMOTEIO;  
        }  
        if (flags & I2C_M_RD) {  
            bit_dbg(3, &i2c_adap->dev, "emitting repeated "  
                "start condition\n");  
            i2c_repstart(adap);  
            /* okay, now switch into reading mode */  
            addr |= 0x01;  
            ret = try_address(i2c_adap, addr, retries);  
            if ((ret != 1) && !nak_ok) {  
                dev_err(&i2c_adap->dev,  
                    "died at repeated address code\n");  
                return -EREMOTEIO;  
            }  
        }  
    } else {        /* normal 7bit address  */  
        addr = msg->addr << 1;  
        if (flags & I2C_M_RD)  
            addr |= 1;  
        if (flags & I2C_M_REV_DIR_ADDR)  
            addr ^= 1;  
        ret = try_address(i2c_adap, addr, retries);  
        if ((ret != 1) && !nak_ok)  
            return -ENXIO;  
    }  
    
    return 0;

\}  
这里先做了一个判断，10位设备地址和7位设备地址分别做不同的处理，通常一条I2C总线上不会挂那么多I2C设备，所以10位地址不常用，直接看对7位地址的处理。struct i2c\_msg中addr中是真正的设备地址，而这里发送的addr高7位才是设备地址，最低位为读写位，如果为读，最低位为1，如果为写，最低位为0。所以要将struct i2c\_msg中addr向左移1位，如果定义了I2C\_M\_RD标志，就将addr或上1，前面就说过，这个标志就代表读，如果是写，这里就不用处理，因为最低位本身就是0。最后调用try\_address函数将这个地址字节发送出去。  
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1. static int try\_address(struct i2c\_adapter \*i2c\_adap,  
2. unsigned char addr, int retries)  
3. \{  
4. struct i2c\_algo\_bit\_data \*adap = i2c\_adap->algo\_data;  
5. int i, ret = 0;  
6.  
7. for (i = 0; i <= retries; i++) \{  
8. ret = i2c\_outb(i2c\_adap, addr);  
9. if (ret == 1 || i == retries)  
10. break;  
11. bit\_dbg(3, &i2c\_adap->dev, “emitting stop condition\\n”);  
12. i2c\_stop(adap);  
13. udelay(adap->udelay);  
14. yield();  
15. bit\_dbg(3, &i2c\_adap->dev, “emitting start condition\\n”);  
16. i2c\_start(adap);  
17. \}  
18. if (i && ret)  
19. bit\_dbg(1, &i2c\_adap->dev, “Used %d tries to %s client at ”  
20. “0x%02x: %s\\n”, i + 1,  
21. addr & 1 ? “read from” : “write to”, addr >> 1,  
22. ret == 1 ? “success” : “failed, timeout?”);  
23. return ret;  
24. \}

最主要的就是调用i2c\_outb发送一个字节，retries为重复次数，看前面adap->retries= 3;  
如果发送失败，也就是设备没有给出应答信号，那就发送停止信号，发送起始信号，再发送这个地址字节，这就叫retries。来看这个具体的i2c\_outb函数  
\[html\] view plain copy print?  
1. static int i2c\_outb(struct i2c\_adapter \*i2c\_adap, unsigned char c)  
2. \{  
3. int i;  
4. int sb;  
5. int ack;  
6. struct i2c\_algo\_bit\_data \*adap = i2c\_adap->algo\_data;  
7.  
8. /\* assert: scl is low \*/  
9. for (i = 7; i >= 0; i–) \{  
10. sb = (c >> i) & 1;  
11. setsda(adap, sb);  
12. udelay((adap->udelay + 1) / 2);  
13. if (sclhi(adap) < 0) \{ /\* timed out \*/  
14. bit\_dbg(1, &i2c\_adap->dev, “i2c\_outb: 0x%02x, ”  
15. “timeout at bit \#%d\\n”, (int)c, i);  
16. return -ETIMEDOUT;  
17. \}  
18. /\* FIXME do arbitration here:  
19. \* if (sb && !getsda(adap)) -> ouch! Get out of here.  
20. \*  
21. \* Report a unique code, so higher level code can retry  
22. \* the whole (combined) message and *NOT* issue STOP.  
23. \*/  
24. scllo(adap);  
25. \}  
26. sdahi(adap);  
27. if (sclhi(adap) < 0) \{ /\* timeout \*/  
28. bit\_dbg(1, &i2c\_adap->dev, “i2c\_outb: 0x%02x, ”  
29. “timeout at ack\\n”, (int)c);  
30. return -ETIMEDOUT;  
31. \}  
32.  
33. /\* read ack: SDA should be pulled down by slave, or it may  
34. \* NAK (usually to report problems with the data we wrote).  
35. \*/  
36. ack = !getsda(adap); /\* ack: sda is pulled low -> success \*/  
37. bit\_dbg(2, &i2c\_adap->dev, “i2c\_outb: 0x%02x %s\\n”, (int)c,  
38. ack ? “A” : “NA”);  
39.  
40. scllo(adap);  
41. return ack;  
42. /\* assert: scl is low (sda undef) \*/  
43. \}  
这个函数有两个参数，一个是structi2c\_adapter代表I2C主机，一个是发送的字节数据。那么I2C是怎样将一个字节数据发送出去的呢，那再来看看协议。

首先是发送字节数据的最高位，在时钟为高电平期间将一位数据发送出去，最后是发送字节数据的最低位。发送完成之后，我们需要一个ACK信号，要不然我怎么知道发送成功没有，ACK信号就是在第九个时钟周期时数据线为低，所以在一个字节数据传送完成后，还要将数据线拉高，我们看程序中就是这一句sdahi(adap);等待这个ACK信号的到来，这样一个字节数据就发送完成。  
回到bit\_xfer函数中，前面只是将设备地址字节发送出去了，那么接下来就是该发送数据了。  
注意：这里的数据包括操作设备的基地址  
如果是读则调用readbytes函数去读，如果是写则调用sendbytes去写，先看readbytes函数  
\[html\] view plain copy print?  
1. static int readbytes(struct i2c\_adapter \*i2c\_adap, struct i2c\_msg \*msg)  
2. \{  
3. int inval;  
4. int rdcount = 0; /\* counts bytes read \*/  
5. unsigned char \*temp = msg->buf;  
6. int count = msg->len;  
7. const unsigned flags = msg->flags;  
8.  
9. while (count > 0) \{  
10. inval = i2c\_inb(i2c\_adap);  
11. if (inval >= 0) \{  
12. \*temp = inval;  
13. rdcount++;  
14. \} else \{ /\* read timed out \*/  
15. break;  
16. \}  
17.  
18. temp++;  
19. count–;  
20.  
21. /\* Some SMBus transactions require that we receive the  
22. transaction length as the first read byte. \*/  
23. if (rdcount == 1 && (flags & I2C\_M\_RECV\_LEN)) \{  
24. if (inval <= 0 || inval > I2C\_SMBUS\_BLOCK\_MAX) \{  
25. if (!(flags & I2C\_M\_NO\_RD\_ACK))  
26. acknak(i2c\_adap, 0);  
27. dev\_err(&i2c\_adap->dev, “readbytes: invalid ”  
28. “block length (%d)\\n”, inval);  
29. return -EREMOTEIO;  
30. \}  
31. /\* The original count value accounts for the extra  
32. bytes, that is, either 1 for a regular transaction,  
33. or 2 for a PEC transaction. \*/  
34. count += inval;  
35. msg->len += inval;  
36. \}  
37.  
38. bit\_dbg(2, &i2c\_adap->dev, “readbytes: 0x%02x %s\\n”,  
39. inval,  
40. (flags & I2C\_M\_NO\_RD\_ACK)  
41. ? “(no ack/nak)”  
42. : (count ? “A” : “NA”));  
43.  
44. if (!(flags & I2C\_M\_NO\_RD\_ACK)) \{  
45. inval = acknak(i2c\_adap, count);  
46. if (inval < 0)  
47. return inval;  
48. \}  
49. \}  
50. return rdcount;  
51. \}  
其中一个大的while循环，调用i2c\_inb去读一个字节，count为数据的长度，单位为多少个字节，  
那就来看i2c\_inb函数。  
static int i2c\_inb(struct i2c\_adapter \*i2c\_adap)  
\{  
/\* read byte via i2c port, without start/stop sequence \*/  
/\* acknowledge is sent in i2c\_read. \*/  
int i;  
unsigned char indata = 0;  
struct i2c\_algo\_bit\_data \*adap = i2c\_adap->algo\_data;

/* assert: scl is low */  
    sdahi(adap);  
    for (i = 0; i < 8; i++) {  
        if (sclhi(adap) < 0) { /* timeout */  
            bit_dbg(1, &i2c_adap->dev, "i2c_inb: timeout at bit "  
                "#%d\n", 7 - i);  
            return -ETIMEDOUT;  
        }  
        indata *= 2;  
        if (getsda(adap))  
            indata |= 0x01;  
        setscl(adap, 0);  
        udelay(i == 7 ? adap->udelay / 2 : adap->udelay);  
    }  
    /* assert: scl is low */  
    return indata;

\}  
再来看sendbytes函数  
\[html\] view plain copy print?  
1. static int sendbytes(struct i2c\_adapter \*i2c\_adap, struct i2c\_msg \*msg)  
2. \{  
3. const unsigned char \*temp = msg->buf;  
4. int count = msg->len;  
5. unsigned short nak\_ok = msg->flags & I2C\_M\_IGNORE\_NAK;  
6. int retval;  
7. int wrcount = 0;  
8.  
9. while (count > 0) \{  
10. retval = i2c\_outb(i2c\_adap, \*temp);  
11.  
12. /\* OK/ACK; or ignored NAK \*/  
13. if ((retval > 0) || (nak\_ok && (retval == 0))) \{  
14. count–;  
15. temp++;  
16. wrcount++;  
17.  
18. /\* A slave NAKing the master means the slave didn’t like  
19. \* something about the data it saw. For example, maybe  
20. \* the SMBus PEC was wrong.  
21. \*/  
22. \} else if (retval == 0) \{  
23. dev\_err(&i2c\_adap->dev, “sendbytes: NAK bailout.\\n”);  
24. return -EIO;  
25.  
26. /\* Timeout; or (someday) lost arbitration  
27. \*  
28. \* FIXME Lost ARB implies retrying the transaction from  
29. \* the first message, after the “winning” master issues  
30. \* its STOP. As a rule, upper layer code has no reason  
31. \* to know or care about this … it is *NOT* an error.  
32. \*/  
33. \} else \{  
34. dev\_err(&i2c\_adap->dev, “sendbytes: error %d\\n”,  
35. retval);  
36. return retval;  
37. \}  
38. \}  
39. return wrcount;  
40. \}  
也是一个大的while循环，同发送地址字节一样，也是调用i2c\_outb去发送一个字节，count也是数据长度，由于i2c\_outb函数在前面发送设备地址那里已经介绍了，这里也就不贴出来了。  
还是回到bit\_xfer函数，数据传输完成后，调用i2c\_stop函数发送停止信号。我们看停止信号函数怎么去实现的。  
\[html\] view plain copy print?  
1. static void i2c\_stop(struct i2c\_algo\_bit\_data \*adap)  
2. \{  
3. /\* assert: scl is low \*/  
4. sdalo(adap);  
5. sclhi(adap);  
6. setsda(adap, 1);  
7. udelay(adap->udelay);  
8. \}

看前面发送起始信号的那张图，停止信号就是在时钟为高电平期间，数据线从低到高的跳变。我们看程序是先将数据线拉低，将时钟线拉高，最后将数据拉高，这样就够成了一个停止信号。  
还是回到i2c\_bit\_add\_numbered\_bus这个函数中来，看另外一个函数调用i2c\_add\_numbered\_adapter。  
\[html\] view plain copy print?  
1. int i2c\_add\_numbered\_adapter(struct i2c\_adapter \*adap)  
2. \{  
3. int id;  
4. int status;  
5.  
6. if (adap->nr & ~MAX\_ID\_MASK)  
7. return -EINVAL;  
8.  
9. retry:  
10. if (idr\_pre\_get(&i2c\_adapter\_idr, GFP\_KERNEL) == 0)  
11. return -ENOMEM;  
12.  
13. mutex\_lock(&core\_lock);  
14. /\* “above” here means “above or equal to”, sigh;  
15. \* we need the “equal to” result to force the result  
16. \*/  
17. status = idr\_get\_new\_above(&i2c\_adapter\_idr, adap, adap->nr, &id);  
18. if (status == 0 && id != adap->nr) \{  
19. status = -EBUSY;  
20. idr\_remove(&i2c\_adapter\_idr, id);  
21. \}  
22. mutex\_unlock(&core\_lock);  
23. if (status == -EAGAIN)  
24. goto retry;  
25.  
26. if (status == 0)  
27. status = i2c\_register\_adapter(adap);  
28. return status;  
29. \}  
最重要的是这句i2c\_register\_adapter，注册这条I2C总线，进去看看  
static int i2c\_register\_adapter(struct i2c\_adapter \*adap)  
\{  
int res = 0, dummy;

/* Can't register until after driver model init */  
    if (unlikely(WARN_ON(!i2c_bus_type.p))) {  
        res = -EAGAIN;  
        goto out_list;  
    }  
    
    mutex_init(&adap->bus_lock);  
    
    /* Set default timeout to 1 second if not already set */  
    if (adap->timeout == 0)  
        adap->timeout = HZ;  
    
    dev_set_name(&adap->dev, "i2c-%d", adap->nr);  
    adap->dev.bus = &i2c_bus_type;  
    adap->dev.type = &i2c_adapter_type;  
    res = device_register(&adap->dev);  
    if (res)  
        goto out_list;  
    
    dev_dbg(&adap->dev, "adapter [%s] registered\n", adap->name);

# ifdef CONFIG\_I2C\_COMPAT #

res = class_compat_create_link(i2c_adapter_compat_class, &adap->dev,  
                       adap->dev.parent);  
    if (res)  
        dev_warn(&adap->dev,  
             "Failed to create compatibility class link\n");

# endif #

/* create pre-declared device nodes */  
    if (adap->nr < __i2c_first_dynamic_bus_num)  
        i2c_scan_static_board_info(adap);  
    
    /* Notify drivers */  
    mutex_lock(&core_lock);  
    dummy = bus_for_each_drv(&i2c_bus_type, NULL, adap,  
                 i2c_do_add_adapter);  
    mutex_unlock(&core_lock);  
    
    return 0;

out\_list:  
mutex\_lock(&core\_lock);  
idr\_remove(&i2c\_adapter\_idr, adap->nr);  
mutex\_unlock(&core\_lock);  
return res;  
\}  
看内核代码有时就会这样，会陷入内核代码的汪洋大海中，而拔不出来，直接后果是最后都忘记看这段代码的目的，丧失继续看下去的信心。所以为了避免这样情况出现，所以最好在开始看代码的时候要明确目标，我通过这段代码到底要了解什么东西，主干要抓住，其它枝叶就不要看了。  
在这里我认为主要的有  
1.注册这个I2C总线设备  
\[html\] view plain copy print?  
1. adap->dev.bus = &i2c\_bus\_type;  
2. adap->dev.type = &i2c\_adapter\_type;  
3. res = device\_register(&adap->dev);  
这个设备的总线类型为i2c\_bus\_type  
\[html\] view plain copy print?  
1. struct bus\_type i2c\_bus\_type = \{  
2. .name = “i2c”,  
3. .match = i2c\_device\_match,  
4. .probe = i2c\_device\_probe,  
5. .remove = i2c\_device\_remove,  
6. .shutdown = i2c\_device\_shutdown,  
7. .suspend = i2c\_device\_suspend,  
8. .resume = i2c\_device\_resume,  
9. \};

看一下它的match函数  
\[html\] view plain copy print?  
1. static int i2c\_device\_match(struct device \*dev, struct device\_driver \*drv)  
2. \{  
3. struct i2c\_client \*client = i2c\_verify\_client(dev);  
4. struct i2c\_driver \*driver;  
5.  
6. if (!client)  
7. return 0;  
8.  
9. driver = to\_i2c\_driver(drv);  
10. /\* match on an id table if there is one \*/  
11. if (driver->id\_table)  
12. return i2c\_match\_id(driver->id\_table, client) != NULL;  
13.  
14. return 0;  
15. \}

这个match函数主要用来匹配我们的I2C设备和I2C驱动的，如果匹配成功，最后会调用驱动的probe函数，来看它如何匹配的。  
\[html\] view plain copy print?  
1. static const struct i2c\_device\_id \*i2c\_match\_id(const struct i2c\_device\_id \*id,  
2. const struct i2c\_client \*client)  
3. \{  
4. while (id->name\[0\]) \{  
5. if (strcmp(client->name, id->name) == 0)  
6. return id;  
7. id++;  
8. \}  
9. return NULL;  
0. \}

就是判断I2C设备的name字段和驱动中id\_table中定义的name字段是否相等。  
2.往这条总线上添加设备  
\[html\] view plain copy print?  
1. static void i2c\_scan\_static\_board\_info(struct i2c\_adapter \*adapter)  
2. \{  
3. struct i2c\_devinfo \*devinfo;  
4.  
5. down\_read(&\_\_i2c\_board\_lock);  
6. list\_for\_each\_entry(devinfo, &\_\_i2c\_board\_list, list) \{  
7. if (devinfo->busnum == adapter->nr  
8. && !i2c\_new\_device(adapter,  
9. &devinfo->board\_info))  
10. dev\_err(&adapter->dev,  
11. “Can’t create device at 0x%02x\\n”,  
12. devinfo->board\_info.addr);  
13. \}  
14. up\_read(&\_\_i2c\_board\_lock);  
15. \}  
遍历\_\_i2c\_board\_list这条链表，看下面的if语句，首先要让struct i2c\_devinfo结构中的busnum等于struct i2c\_adapter中的nr，我们前面也说了，这个nr就是i2c总线的总线号，这里可以理解为是在往这条总线上添加设备。所以，如果我们要向I2C注册一个I2C设备的话，直接向\_\_i2c\_board\_list添加一个设备信息就可以了，先来看这个设备信息结构是怎么定义的。  
\[html\] view plain copy print?  
1. struct i2c\_board\_info \{  
2. char type\[I2C\_NAME\_SIZE\];  
3. unsigned short flags;  
4. unsigned short addr;  
5. void \*platform\_data;  
6. struct dev\_archdata \*archdata;  
7. int irq;  
8. \};  
定义这样一个信息呢一般使用一个宏I2C\_BOARD\_INFO  
\[html\] view plain copy print?

# \#define I2C\_BOARD\_INFO(dev\_type, dev\_addr) \\ #

# .type = dev\_type, .addr = (dev\_addr) #

\[html\] view plain copy print?  
dev\_type为设备的名字，前面也说了，这个name一定要和I2C驱动相同。addr为设备的地址。

定义了这样一组信息之后呢，接下来当然是往链表添加这些信息了。

\[html\] view plain copy print?  
1. int \_\_init  
2. i2c\_register\_board\_info(int busnum,  
3. struct i2c\_board\_info const \*info, unsigned len)  
4. \{  
5. int status;  
6.  
7. down\_write(&\_\_i2c\_board\_lock);  
8.  
9. /\* dynamic bus numbers will be assigned after the last static one \*/  
10. if (busnum >= \_\_i2c\_first\_dynamic\_bus\_num)  
11. \_\_i2c\_first\_dynamic\_bus\_num = busnum + 1;  
12.  
13. for (status = 0; len; len–, info++) \{  
14. struct i2c\_devinfo \*devinfo;  
15.  
16. devinfo = kzalloc(sizeof(\*devinfo), GFP\_KERNEL);  
17. if (!devinfo) \{  
18. pr\_debug(“i2c-core: can’t register boardinfo!\\n”);  
19. status = -ENOMEM;  
20. break;  
21. \}  
22.  
23. devinfo->busnum = busnum;  
24. devinfo->board\_info = \*info;  
25. list\_add\_tail(&devinfo->list, &\_\_i2c\_board\_list);  
26. \}  
27.  
28. up\_write(&\_\_i2c\_board\_lock);  
29.  
30. return status;  
31. \}

第一个参数呢需要注意，它是I2C总线号，一定要和具体的I2C总线对应。我们看又定义了这样一个结构struct i2c\_devinfo。

最后是调用list\_add\_tail往\_\_i2c\_board\_list这条链表添加设备信息。  
然后是i2c\_new\_device  
\[html\] view plain copy print?

# struct i2c\_client \* #

# i2c\_new\_device(struct i2c\_adapter \*adap, struct i2c\_board\_info const \*info) #

# \{ #

# struct i2c\_client \*client; #

# int status; #

#  #

# /*为I2C设备申请内存*/ #

# client = kzalloc(sizeof \*client, GFP\_KERNEL); #

# if (!client) #

# return NULL; #

#  #

# /*指定I2C设备的总线*/ #

# client->adapter = adap; #

#  #

# client->dev.platform\_data = info->platform\_data; #

#  #

# if (info->archdata) #

# client->dev.archdata = \*info->archdata; #

#  #

# client->flags = info->flags; #

# client->addr = info->addr; /*I2C设备地址*/ #

# client->irq = info->irq; #

#  #

# strlcpy(client->name, info->type, sizeof(client->name)); #

#  #

# /*检查这个地址有没有被设备占用*/ #

# /\* Check for address business \*/ #

# status = i2c\_check\_addr(adap, client->addr); #

# if (status) #

# goto out\_err; #

#  #

# client->dev.parent = &client->adapter->dev; /*指定设备的父设备*/ #

# client->dev.bus = &i2c\_bus\_type; /*指定设备的总线类型*/ #

# client->dev.type = &i2c\_client\_type; #

#  #

# dev\_set\_name(&client->dev, “%d-%04x”, i2c\_adapter\_id(adap), #

# client->addr); #

# status = device\_register(&client->dev); /*注册设备*/ #

# if (status) #

# goto out\_err; #

#  #

# dev\_dbg(&adap->dev, “client \[%s\] registered with bus id %s\\n”, #

# client->name, dev\_name(&client->dev)); #

#  #

# return client; #

#  #

# out\_err: #

# dev\_err(&adap->dev, “Failed to register i2c client %s at 0x%02x “ #

# “(%d)\\n”, client->name, client->addr, status); #

# kfree(client); #

# return NULL; #

这个函数的功能是新建一个I2C设备并注册它，在I2C子系统中，I2C设备使用结构structi2c\_client描述，那么首先要申请内存空间，I2C设备的主机是谁，必须知道挂载到哪条总线上的，然后就是一些赋值操作，最后就是注册设备，那么这个设备就实实在在的挂在到这条总线上了，这也是新的I2C设备注册方式。  
3.i2c\_do\_add\_adapter  
你看说着说着就跑远了  
\[html\] view plain copy print?  
1. static int i2c\_do\_add\_adapter(struct device\_driver \*d, void \*data)  
2. \{  
3. struct i2c\_driver \*driver = to\_i2c\_driver(d);  
4. struct i2c\_adapter \*adap = data;  
5.  
6. /\* Detect supported devices on that bus, and instantiate them \*/  
7. i2c\_detect(adap, driver);  
8.  
9. /\* Let legacy drivers scan this bus for matching devices \*/  
10. if (driver->attach\_adapter) \{  
11. /\* We ignore the return code; if it fails, too bad \*/  
12. driver->attach\_adapter(adap);  
13. \}  
14. return 0;  
15. \}

前面通过i2c\_scan\_static\_board\_info往I2C总线上添加设备是新的方式，而这里调用每个I2C设备驱动的attach\_adapter函数，然后在attach\_adapter函数中去实现设备的注册，这是老的方式，i2c-dev.c中就是采用的这种方式。至此，总线这块就看完了。

[http_blog.csdn.net_tommy_wxie_article_details_7408328]: http://blog.csdn.net/tommy_wxie/article/details/7408328