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linux gpio模拟i2c的使用/用GPIO模拟I2C总线

dwlinux_gs 2017-04-18

展开全文

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

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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

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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

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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。

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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。

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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

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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

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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

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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。

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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

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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。

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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

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# #define I2C_BOARD_INFO(dev_type, dev_addr) \
# .type = dev_type, .addr = (dev_addr)

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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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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

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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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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。

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