Linux I2C子系统分析-I2C总线驱动 1

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。

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