Linux I2C子系统分析-I2C总线驱动

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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static int __init i2c_gpio_init(void)

{

int ret;

ret = platform_driver_register(&i2c_gpio_driver);

if (ret)

printk(KERN_ERR "i2c-gpio: probe failed: %d\n", ret);

return ret;

}

module_init(i2c_gpio_init);

static void __exit i2c_gpio_exit(void)

{

platform_driver_unregister(&i2c_gpio_driver);

}

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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static struct platform_driver i2c_gpio_driver = {

.driver = {

.name = "i2c-gpio",

.owner = THIS_MODULE,

},

.probe = i2c_gpio_probe,

.remove = __devexit_p(i2c_gpio_remove),

};

小提示：是不是我们应该注册一个平台设备，以和这个驱动匹配，那先来注册这个平台设备。

先定义这个平台设备结构，至于怎么注册平台设备我想大家都应该知道吧。

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#if defined(CONFIG_I2C_GPIO) | \

defined(CONFIG_I2C_GPIO_MODULE)

static struct i2c_gpio_platform_data i2c_gpio_adapter_data = {

.sda_pin = PINID_GPMI_D05,

.scl_pin = PINID_GPMI_D04,

.udelay = 5, //100KHz

.timeout = 100,

.sda_is_open_drain = 1,

.scl_is_open_drain = 1,

};

static struct platform_device i2c_gpio = {

.name = "i2c-gpio",

.id = 0,

.dev = {

.platform_data = &i2c_gpio_adapter_data,

.release = mxs_nop_release,

},

};

#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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struct i2c_gpio_platform_data {

unsigned int sda_pin;

unsigned int scl_pin;

int udelay;

int timeout;

unsigned int sda_is_open_drain:1;

unsigned int scl_is_open_drain:1;

unsigned int scl_is_output_only:1;

};

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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static int __devinit i2c_gpio_probe(struct platform_device *pdev)

{

struct i2c_gpio_platform_data *pdata;

struct i2c_algo_bit_data *bit_data;

struct i2c_adapter *adap;

int ret;

pdata = pdev->dev.platform_data;

if (!pdata)

return -ENXIO;

ret = -ENOMEM;

adap = kzalloc(sizeof(struct i2c_adapter), GFP_KERNEL);

if (!adap)

goto err_alloc_adap;

bit_data = kzalloc(sizeof(struct i2c_algo_bit_data), GFP_KERNEL);

if (!bit_data)

goto err_alloc_bit_data;

ret = gpio_request(pdata->sda_pin, "sda");

if (ret)

goto err_request_sda;

ret = gpio_request(pdata->scl_pin, "scl");

if (ret)

goto err_request_scl;

if (pdata->sda_is_open_drain) {

gpio_direction_output(pdata->sda_pin, 1);

bit_data->setsda = i2c_gpio_setsda_val;

} else {

gpio_direction_input(pdata->sda_pin);

bit_data->setsda = i2c_gpio_setsda_dir;

}

if (pdata->scl_is_open_drain || pdata->scl_is_output_only) {

gpio_direction_output(pdata->scl_pin, 1);

bit_data->setscl = i2c_gpio_setscl_val;

} else {

gpio_direction_input(pdata->scl_pin);

bit_data->setscl = i2c_gpio_setscl_dir;

}

if (!pdata->scl_is_output_only)

bit_data->getscl = i2c_gpio_getscl;

bit_data->getsda = i2c_gpio_getsda;

if (pdata->udelay)

bit_data->udelay = pdata->udelay;

else if (pdata->scl_is_output_only)

bit_data->udelay = 50; /* 10 kHz */

else

bit_data->udelay = 5; /* 100 kHz */

if (pdata->timeout)

bit_data->timeout = pdata->timeout;

else

bit_data->timeout = HZ / 10; /* 100 ms */

bit_data->data = pdata;

adap->owner = THIS_MODULE;

snprintf(adap->name, sizeof(adap->name), "i2c-gpio%d", pdev->id);

adap->algo_data = bit_data;

adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;

adap->dev.parent = &pdev->dev;

/*

* If "dev->id" is negative we consider it as zero.

* The reason to do so is to avoid sysfs names that only make

* sense when there are multiple adapters.

*/

adap->nr = (pdev->id != -1) ? pdev->id : 0;

ret = i2c_bit_add_numbered_bus(adap);

if (ret)

goto err_add_bus;

platform_set_drvdata(pdev, adap);

dev_info(&pdev->dev, "using pins %u (SDA) and %u (SCL%s)\n",

pdata->sda_pin, pdata->scl_pin,

pdata->scl_is_output_only

? ", no clock stretching" : "");

return 0;

err_add_bus:

gpio_free(pdata->scl_pin);

err_request_scl:

gpio_free(pdata->sda_pin);

err_request_sda:

kfree(bit_data);

err_alloc_bit_data:

kfree(adap);

err_alloc_adap:

return ret;

}

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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struct i2c_adapter {

struct module *owner;

unsigned int id;

unsigned int class; /* classes to allow probing for */

const struct i2c_algorithm *algo; /* the algorithm to access the bus */

void *algo_data;

/* data fields that are valid for all devices */

u8 level; /* nesting level for lockdep */

struct mutex bus_lock;

int timeout; /* in jiffies */

int retries;

struct device dev; /* the adapter device */

int nr;

char name[48];

struct completion dev_released;

};

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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struct i2c_algo_bit_data {

void *data; /* private data for lowlevel routines */

void (*setsda) (void *data, int state);

void (*setscl) (void *data, int state);

int (*getsda) (void *data);

int (*getscl) (void *data);

/* local settings */

int udelay; /* half clock cycle time in us,

minimum 2 us for fast-mode I2C,

minimum 5 us for standard-mode I2C and SMBus,

maximum 50 us for SMBus */

int timeout; /* in jiffies */

};

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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int i2c_bit_add_numbered_bus(struct i2c_adapter *adap)

{

int err;

err = i2c_bit_prepare_bus(adap);

if (err)

return err;

return i2c_add_numbered_adapter(adap);

}

先看i2c_bit_prepare_bus函数

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static int i2c_bit_prepare_bus(struct i2c_adapter *adap)

{

struct i2c_algo_bit_data *bit_adap = adap->algo_data;

if (bit_test) {

int ret = test_bus(bit_adap, adap->name);

if (ret < 0)

return -ENODEV;

}

/* register new adapter to i2c module... */

adap->algo = &i2c_bit_algo;

adap->retries = 3;

return 0;

}

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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static const struct i2c_algorithm i2c_bit_algo = {

.master_xfer = bit_xfer,

.functionality = bit_func,

};

先看这个结构类型在哪里定义的include/linux/i2c.h

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struct i2c_algorithm {

/* If an adapter algorithm can't do I2C-level access, set master_xfer

to NULL. If an adapter algorithm can do SMBus access, set

smbus_xfer. If set to NULL, the SMBus protocol is simulated

using common I2C messages */

/* master_xfer should return the number of messages successfully

processed, or a negative value on error */

int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,

int num);

int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,

unsigned short flags, char read_write,

u8 command, int size, union i2c_smbus_data *data);

/* To determine what the adapter supports */

u32 (*functionality) (struct i2c_adapter *);

};

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。

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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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static void i2c_start(struct i2c_algo_bit_data *adap)

{

/* assert: scl, sda are high */

setsda(adap, 0);

udelay(adap->udelay);

scllo(adap);

}

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。

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#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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struct i2c_msg {

__u16 addr; /* slave address */

__u16 flags;

#define I2C_M_TEN 0x0010 /* this is a ten bit chip address */

#define I2C_M_RD 0x0001 /* read data, from slave to master */

#define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_RECV_LEN 0x0400 /* length will be first received byte */

__u16 len; /* msg length */

__u8 *buf; /* pointer to msg data */

};

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。

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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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static int try_address(struct i2c_adapter *i2c_adap,

unsigned char addr, int retries)

{

struct i2c_algo_bit_data *adap = i2c_adap->algo_data;

int i, ret = 0;

for (i = 0; i <= retries; i++) {

ret = i2c_outb(i2c_adap, addr);

if (ret == 1 || i == retries)

break;

bit_dbg(3, &i2c_adap->dev, "emitting stop condition\n");

i2c_stop(adap);

udelay(adap->udelay);

yield();

bit_dbg(3, &i2c_adap->dev, "emitting start condition\n");

i2c_start(adap);

}

if (i && ret)

bit_dbg(1, &i2c_adap->dev, "Used %d tries to %s client at "

"0x%02x: %s\n", i + 1,

addr & 1 ? "read from" : "write to", addr >> 1,

ret == 1 ? "success" : "failed, timeout?");

return ret;

}

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

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static int i2c_outb(struct i2c_adapter *i2c_adap, unsigned char c)

{

int i;

int sb;

int ack;

struct i2c_algo_bit_data *adap = i2c_adap->algo_data;

/* assert: scl is low */

for (i = 7; i >= 0; i--) {

sb = (c >> i) & 1;

setsda(adap, sb);

udelay((adap->udelay + 1) / 2);

if (sclhi(adap) < 0) { /* timed out */

bit_dbg(1, &i2c_adap->dev, "i2c_outb: 0x%02x, "

"timeout at bit #%d\n", (int)c, i);

return -ETIMEDOUT;

}

/* FIXME do arbitration here:

* if (sb && !getsda(adap)) -> ouch! Get out of here.

*

* Report a unique code, so higher level code can retry

* the whole (combined) message and *NOT* issue STOP.

*/

scllo(adap);

}

sdahi(adap);

if (sclhi(adap) < 0) { /* timeout */

bit_dbg(1, &i2c_adap->dev, "i2c_outb: 0x%02x, "

"timeout at ack\n", (int)c);

return -ETIMEDOUT;

}

/* read ack: SDA should be pulled down by slave, or it may

* NAK (usually to report problems with the data we wrote).

*/

ack = !getsda(adap); /* ack: sda is pulled low -> success */

bit_dbg(2, &i2c_adap->dev, "i2c_outb: 0x%02x %s\n", (int)c,

ack ? "A" : "NA");

scllo(adap);

return ack;

/* assert: scl is low (sda undef) */

}

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

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static int readbytes(struct i2c_adapter *i2c_adap, struct i2c_msg *msg)

{

int inval;

int rdcount = 0; /* counts bytes read */

unsigned char *temp = msg->buf;

int count = msg->len;

const unsigned flags = msg->flags;

while (count > 0) {

inval = i2c_inb(i2c_adap);

if (inval >= 0) {

*temp = inval;

rdcount++;

} else { /* read timed out */

break;

}

temp++;

count--;

/* Some SMBus transactions require that we receive the

transaction length as the first read byte. */

if (rdcount == 1 && (flags & I2C_M_RECV_LEN)) {

if (inval <= 0 || inval > I2C_SMBUS_BLOCK_MAX) {

if (!(flags & I2C_M_NO_RD_ACK))

acknak(i2c_adap, 0);

dev_err(&i2c_adap->dev, "readbytes: invalid "

"block length (%d)\n", inval);

return -EREMOTEIO;

}

/* The original count value accounts for the extra

bytes, that is, either 1 for a regular transaction,

or 2 for a PEC transaction. */

count += inval;

msg->len += inval;

}

bit_dbg(2, &i2c_adap->dev, "readbytes: 0x%02x %s\n",

inval,

(flags & I2C_M_NO_RD_ACK)

? "(no ack/nak)"

: (count ? "A" : "NA"));

if (!(flags & I2C_M_NO_RD_ACK)) {

inval = acknak(i2c_adap, count);

if (inval < 0)

return inval;

}

}

return rdcount;

}

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。

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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

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static int sendbytes(struct i2c_adapter *i2c_adap, struct i2c_msg *msg)

{

const unsigned char *temp = msg->buf;

int count = msg->len;

unsigned short nak_ok = msg->flags & I2C_M_IGNORE_NAK;

int retval;

int wrcount = 0;

while (count > 0) {

retval = i2c_outb(i2c_adap, *temp);

/* OK/ACK; or ignored NAK */

if ((retval > 0) || (nak_ok && (retval == 0))) {

count--;

temp++;

wrcount++;

/* A slave NAKing the master means the slave didn't like

* something about the data it saw. For example, maybe

* the SMBus PEC was wrong.

*/

} else if (retval == 0) {

dev_err(&i2c_adap->dev, "sendbytes: NAK bailout.\n");

return -EIO;

/* Timeout; or (someday) lost arbitration

*

* FIXME Lost ARB implies retrying the transaction from

* the first message, after the "winning" master issues

* its STOP. As a rule, upper layer code has no reason

* to know or care about this ... it is *NOT* an error.

*/

} else {

dev_err(&i2c_adap->dev, "sendbytes: error %d\n",

retval);

return retval;

}

}

return wrcount;

}

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。

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static void i2c_stop(struct i2c_algo_bit_data *adap)

{

/* assert: scl is low */

sdalo(adap);

sclhi(adap);

setsda(adap, 1);

udelay(adap->udelay);

}

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。

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int i2c_add_numbered_adapter(struct i2c_adapter *adap)

{

int id;

int status;

if (adap->nr & ~MAX_ID_MASK)

return -EINVAL;

retry:

if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)

return -ENOMEM;

mutex_lock(&core_lock);

/* "above" here means "above or equal to", sigh;

* we need the "equal to" result to force the result

*/

status = idr_get_new_above(&i2c_adapter_idr, adap, adap->nr, &id);

if (status == 0 && id != adap->nr) {

status = -EBUSY;

idr_remove(&i2c_adapter_idr, id);

}

mutex_unlock(&core_lock);

if (status == -EAGAIN)

goto retry;

if (status == 0)

status = i2c_register_adapter(adap);

return status;

}

最重要的是这句i2c_register_adapter，注册这条I2C总线，进去看看

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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

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adap->dev.bus = &i2c_bus_type;

adap->dev.type = &i2c_adapter_type;

res = device_register(&adap->dev);

这个设备的总线类型为i2c_bus_type

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struct bus_type i2c_bus_type = {

.name = "i2c",

.match = i2c_device_match,

.probe = i2c_device_probe,

.remove = i2c_device_remove,

.shutdown = i2c_device_shutdown,

.suspend = i2c_device_suspend,

.resume = i2c_device_resume,

};

看一下它的match函数

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static int i2c_device_match(struct device *dev, struct device_driver *drv)

{

struct i2c_client *client = i2c_verify_client(dev);

struct i2c_driver *driver;

if (!client)

return 0;

driver = to_i2c_driver(drv);

/* match on an id table if there is one */

if (driver->id_table)

return i2c_match_id(driver->id_table, client) != NULL;

return 0;

}

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。

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static const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,

const struct i2c_client *client)

{

while (id->name[0]) {

if (strcmp(client->name, id->name) == 0)

return id;

id++;

}

return NULL;

}

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

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static void i2c_scan_static_board_info(struct i2c_adapter *adapter)

{

struct i2c_devinfo *devinfo;

down_read(&__i2c_board_lock);

list_for_each_entry(devinfo, &__i2c_board_list, list) {

if (devinfo->busnum == adapter->nr

&& !i2c_new_device(adapter,

&devinfo->board_info))

dev_err(&adapter->dev,

"Can't create device at 0x%02x\n",

devinfo->board_info.addr);

}

up_read(&__i2c_board_lock);

}

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。

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struct i2c_board_info {

char type[I2C_NAME_SIZE];

unsigned short flags;

unsigned short addr;

void *platform_data;

struct dev_archdata *archdata;

int irq;

};

定义这样一个信息呢一般使用一个宏I2C_BOARD_INFO

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#define I2C_BOARD_INFO(dev_type, dev_addr) \

.type = dev_type, .addr = (dev_addr)

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。

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int __init

i2c_register_board_info(int busnum,

struct i2c_board_info const *info, unsigned len)

{

int status;

down_write(&__i2c_board_lock);

/* dynamic bus numbers will be assigned after the last static one */

if (busnum >= __i2c_first_dynamic_bus_num)

__i2c_first_dynamic_bus_num = busnum + 1;

for (status = 0; len; len--, info++) {

struct i2c_devinfo *devinfo;

devinfo = kzalloc(sizeof(*devinfo), GFP_KERNEL);

if (!devinfo) {

pr_debug("i2c-core: can't register boardinfo!\n");

status = -ENOMEM;

break;

}

devinfo->busnum = busnum;

devinfo->board_info = *info;

list_add_tail(&devinfo->list, &__i2c_board_list);

}

up_write(&__i2c_board_lock);

return status;

}

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。

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struct i2c_devinfo {

struct list_head list;

int busnum;

struct i2c_board_info board_info;

};

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

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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

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static int i2c_do_add_adapter(struct device_driver *d, void *data)

{

struct i2c_driver *driver = to_i2c_driver(d);

struct i2c_adapter *adap = data;

/* Detect supported devices on that bus, and instantiate them */

i2c_detect(adap, driver);

/* Let legacy drivers scan this bus for matching devices */

if (driver->attach_adapter) {

/* We ignore the return code; if it fails, too bad */

driver->attach_adapter(adap);

}

return 0;

}

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。