linux gpio模拟i2c的使用用GPIO模拟I2C总线

linux gpio模拟i2c的使用用GPIO模拟I2C总线 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。[html] view plaincopyprint? 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 plaincopyprint? 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 plaincopyprint? 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 plaincopyprint? 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 plaincopyprint?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 plaincopyprint? 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 plaincopyprint?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 plaincopyprint?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 plaincopyprint? 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 plaincopyprint?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 plaincopyprint? 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 plaincopyprint?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 plaincopyprint?# #define I2C_BOARD_INFO(dev_type, dev_addr) \ # .type = dev_type, .addr = (dev_addr) [html] view plaincopyprint?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 plaincopyprint? 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 plaincopyprint?# 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 plaincopyprint? 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。