可以接上面I2C驱动

本文将介绍Linux中AT24C02驱动。AT24C02是一种EEPROM，使用I2C接口来访问。

在开发板中，使用I2C控制器0和AT24C02连接，这里就不给出原理图了，如需要，可以搜索TQ2440开发板的原理图。

目标平台：TQ2440

CPU：s3c2440

内核版本：2.6.32

本文所有的代码均位于内核源码：linux/drivers/misc/eeprom/at24.c中。

1. 模块注册和注销

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static int __init at24_init(void)

{

/* 将io_limit向下圆整到最近的2的幂*/

io_limit = rounddown_pow_of_two(io_limit);

return i2c_add_driver(&at24_driver); /* i2c 驱动注册*/

}

module_init(at24_init);

static void __exit at24_exit(void)

{

i2c_del_driver(&at24_driver);

}

module_exit(at24_exit);

MODULE_DESCRIPTION("Driver for most I2C EEPROMs");

MODULE_AUTHOR("David Brownell and Wolfram Sang");

MODULE_LICENSE("GPL");

注册函数很简单。io_limit为写入时允许一次写入的最大字节，该参数为驱动模块参数，可由用户设置，默认值为128字节。

首先对io_limit向下圆整到最近的2的幂，接着直接调用了i2c_add_driver来注册一个i2c驱动。

注销函数更简单。注销之前注册的i2c驱动。

2. 设备驱动绑定

熟悉I2C驱动架构的可能会知道I2C驱动的match函数，该函数将使用id表(struct
i2c_device_id)和i2c设备（struct i2c_client）进行匹配，判断是否有name字段相同，如果相同则匹配完成，即可完成设备和驱动的绑定，接着便会调用驱动提供的probe方法。我们来看下驱动提供的id表。

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static struct i2c_driver at24_driver = {

.driver = {

.name = "at24",

.owner = THIS_MODULE,

},

.probe = at24_probe,

.remove = __devexit_p(at24_remove),

.id_table = at24_ids,

};

驱动提供的id为at24_ids，如下：

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static const struct i2c_device_id at24_ids[] = {

/* needs 8 addresses as A0-A2 are ignored */

{ "24c00", AT24_DEVICE_MAGIC(128 / 8, AT24_FLAG_TAKE8ADDR) },

/* old variants can't be handled with this generic entry! */

{ "24c01", AT24_DEVICE_MAGIC(1024 / 8, 0) },

{ "24c02", AT24_DEVICE_MAGIC(2048 / 8, 0) },

/* spd is a 24c02 in memory DIMMs */

{ "spd", AT24_DEVICE_MAGIC(2048 / 8,

AT24_FLAG_READONLY | AT24_FLAG_IRUGO) },

{ "24c04", AT24_DEVICE_MAGIC(4096 / 8, 0) },

/* 24rf08 quirk is handled at i2c-core */

{ "24c08", AT24_DEVICE_MAGIC(8192 / 8, 0) },

{ "24c16", AT24_DEVICE_MAGIC(16384 / 8, 0) },

{ "24c32", AT24_DEVICE_MAGIC(32768 / 8, AT24_FLAG_ADDR16) },

{ "24c64", AT24_DEVICE_MAGIC(65536 / 8, AT24_FLAG_ADDR16) },

{ "24c128", AT24_DEVICE_MAGIC(131072 / 8, AT24_FLAG_ADDR16) },

{ "24c256", AT24_DEVICE_MAGIC(262144 / 8, AT24_FLAG_ADDR16) },

{ "24c512", AT24_DEVICE_MAGIC(524288 / 8, AT24_FLAG_ADDR16) },

{ "24c1024", AT24_DEVICE_MAGIC(1048576 / 8, AT24_FLAG_ADDR16) },

{ "at24", 0 },

{ /* END OF LIST */ }

};

结构体成员的第一个参数即为name，表示设备的名字。第二个参数，在该驱动中，为一个幻术（magic），通过AT24_DEVICE_MAGIC宏计算。

宏第一个参数为eeprom的大小，第二参数为一些标志位。我们看下这个宏：

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#define AT24_SIZE_BYTELEN 5

#define AT24_SIZE_FLAGS 8

/* create non-zero magic value for given eeprom parameters */

#define AT24_DEVICE_MAGIC(_len, _flags) \

((1 << AT24_SIZE_FLAGS | (_flags)) \

<< AT24_SIZE_BYTELEN | ilog2(_len))

在这个表中，针对这里讲解的24c02，其大小为256字节，标志位为空。

3.probe函数

当i2c总线完成设备驱动绑定后，就会调用probe方法了。具体看下这个函数。

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static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)

{

struct at24_platform_data chip;

bool writable;

bool use_smbus = false;

struct at24_data *at24;

int err;

unsigned i, num_addresses;

kernel_ulong_t magic;

/* 获取板级设备信息*/

if (client->dev.platform_data) {

chip = *(struct at24_platform_data *)client->dev.platform_data;

} else {

/* 没有板级设备信息，也没有driver_data，直接出错*/

if (!id->driver_data) {

err = -ENODEV;

goto err_out;

}

magic = id->driver_data;

chip.byte_len = BIT(magic & AT24_BITMASK(AT24_SIZE_BYTELEN));

magic >>= AT24_SIZE_BYTELEN;

chip.flags = magic & AT24_BITMASK(AT24_SIZE_FLAGS);

/*

* This is slow, but we can't know all eeproms, so we better

* play safe. Specifying custom eeprom-types via platform_data

* is recommended anyhow.

*/

chip.page_size = 1;

chip.setup = NULL;

chip.context = NULL;

}

/* 检查参数，

byte_len和page_size必须为2的幂，不是则打印警告*/

if (!is_power_of_2(chip.byte_len))

dev_warn(&client->dev,

"byte_len looks suspicious (no power of 2)!\n");

if (!is_power_of_2(chip.page_size))

dev_warn(&client->dev,

"page_size looks suspicious (no power of 2)!\n");

/* Use I2C operations unless we're stuck with SMBus extensions. */

/* 检查是否支持I2C协议，

如果不支持，则检查是否使用SMBUS协议*/

if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {

/* 不支持I2C协议，但是使用16位地址，出错*/

if (chip.flags & AT24_FLAG_ADDR16) {

err = -EPFNOSUPPORT;

goto err_out;

}

/* 不支持I2C协议，使用8位地址，

但是不支持I2C_FUNC_SMBUS_READ_I2C_BLOCK，出错*/

if (!i2c_check_functionality(client->adapter,

I2C_FUNC_SMBUS_READ_I2C_BLOCK)) {

err = -EPFNOSUPPORT;

goto err_out;

}

use_smbus = true; /*使用 SMBUS协议*/

}

/*是否使用8个地址，根据id表，

目前只有AT24C00使用8个地址，其他都为1个*/

if (chip.flags & AT24_FLAG_TAKE8ADDR)

num_addresses = 8;

else

/* 24C02需要1个地址，24C04为2个，以此类推*/

num_addresses = DIV_ROUND_UP(chip.byte_len,

(chip.flags & AT24_FLAG_ADDR16) ? 65536 : 256);

/* 分配struct at24_data，同时根据地址个数分配struct i2c_client*/

at24 = kzalloc(sizeof(struct at24_data) +

num_addresses * sizeof(struct i2c_client *), GFP_KERNEL);

if (!at24) {

err = -ENOMEM;

goto err_out;

}

/* 初始化struct at24_data*/

mutex_init(&at24->lock);

at24->use_smbus = use_smbus;

at24->chip = chip;

at24->num_addresses = num_addresses;

/*

* Export the EEPROM bytes through sysfs, since that's convenient.

* By default, only root should see the data (maybe passwords etc)

*/

/* 设置bin_attribute字段，二进制文件名为eeprom，

通过它即可读写设备 */

at24->bin.attr.name = "eeprom";

at24->bin.attr.mode = chip.flags & AT24_FLAG_IRUGO ? S_IRUGO : S_IRUSR;

at24->bin.read = at24_bin_read;

at24->bin.size = chip.byte_len;

at24->macc.read = at24_macc_read; /*** 先忽略***/

/* 判断设备是否可写*/

writable = !(chip.flags & AT24_FLAG_READONLY);

if (writable) {

if (!use_smbus || i2c_check_functionality(client->adapter,

I2C_FUNC_SMBUS_WRITE_I2C_BLOCK)) {

unsigned write_max = chip.page_size;

at24->macc.write = at24_macc_write; /*** 先忽略***/

at24->bin.write = at24_bin_write; /* 写函数*/

at24->bin.attr.mode |= S_IWUSR; /* 文件拥有者可写*/

if (write_max > io_limit) /* 一次最多写io_limit个字节*/

write_max = io_limit;

/* 如果使用smbus，对write_max检查*/

if (use_smbus && write_max > I2C_SMBUS_BLOCK_MAX)

write_max = I2C_SMBUS_BLOCK_MAX;

at24->write_max = write_max;

/* buffer (data + address at the beginning) */

/* 分配写缓冲区，多余两个字节用于保存寄存器地址*/

at24->writebuf = kmalloc(write_max + 2, GFP_KERNEL);

if (!at24->writebuf) {

err = -ENOMEM;

goto err_struct;

}

} else {

dev_warn(&client->dev,

"cannot write due to controller restrictions.");

}

}

at24->client[0] = client; /* 保存i2c设备client*/

/* use dummy devices for multiple-address chips */

/* 为其余设备地址注册一个dummy设备*/

for (i = 1; i < num_addresses; i++) {

at24->client[i] = i2c_new_dummy(client->adapter,

client->addr + i); /* 设备地址每次加1 */

if (!at24->client[i]) {

dev_err(&client->dev, "address 0x%02x unavailable\n",

client->addr + i);

err = -EADDRINUSE;

goto err_clients;

}

}

/* 创建二进制属性*/

err = sysfs_create_bin_file(&client->dev.kobj, &at24->bin);

if (err)

goto err_clients;

i2c_set_clientdata(client, at24); /* 保存驱动数据*/

/* 打印设备信息*/

dev_info(&client->dev, "%zu byte %s EEPROM %s\n",

at24->bin.size, client->name,

writable ? "(writable)" : "(read-only)");

dev_dbg(&client->dev,

"page_size %d, num_addresses %d, write_max %d%s\n",

chip.page_size, num_addresses,

at24->write_max,

use_smbus ? ", use_smbus" : "");

/* export data to kernel code */

if (chip.setup)

chip.setup(&at24->macc, chip.context);

return 0;

err_clients:

for (i = 1; i < num_addresses; i++)

if (at24->client[i])

i2c_unregister_device(at24->client[i]);

kfree(at24->writebuf);

err_struct:

kfree(at24);

}

驱动首先获取板级设备信息（client->dev.platform_data），我们假设驱动移植时，添加了该板级设备信息。

判断是使用I2C协议还是SMBus协议。在这里，I2C adpater使用I2C协议。

然后，判断设备需要多少个i2c设备地址。

这里补充下：根据at24c02的datasheet，设备地址的第1位到第3位，将根据不同的设备来进行设置。

例如，如果是at24c04，则设备地址的第1位将用来表示寄存器地址，因为内存大小为512字节，而寄存器地址只有8位（256字节），

需要额外的一位用来表示512字节，因此使用了设备地址当中的一位来实现此目的。具体的请看datasheet。

这里使用at24c02，num_addresses将为1。

接着分配struct at24_data和struct i2c_client指针数组空间。

然后对struct at24_data进行了初始化工作。

接着，对二进制属性进行了配置。名字为eeprom，同时配置了其读方法（at24_bin_read），如果设备可写，还将配置其写方法（at24_bin_write）。

接下来很重要的一步，如果设备使用多个地址，则需要为所有地址（除了第一个地址）分配一个dummy device，这样这些地址就不会被其他的I2C设备占用了。

最后，向sys文件系统注册了二进制属性文件，通过该二进制文件，用户即可访问该设备。

注意：驱动使用了struct memory_accessor的东东，对这个东东不是太了解，所以先忽略，这个东西不影响驱动整体的架构。

4.设备访问方法

从第3结的分析可知，驱动并没有注册任何字符设备或者杂项设备，只是向sys文件系统注册了一个二进制属性文件。因此要访问设备，必须通过该文件的读写函数来。

读写函数在probe函数中指定为at24_bin_write和at24_bin_read，我们来分别看下。

4.1 写函数（at24_bin_write）

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static ssize_t at24_bin_write(struct kobject *kobj, struct bin_attribute *attr,

char *buf, loff_t off, size_t count)

{

struct at24_data *at24;

/* 通过kobj获取device，再获取driver_data */

at24 = dev_get_drvdata(container_of(kobj, struct device, kobj));

return at24_write(at24, buf, off, count);

}

该函数首先通过kobj获取了struct device的指针，再获取了at24。

接着直接调用了at24_write。如下:

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tatic ssize_t at24_write(struct at24_data *at24, const char *buf, loff_t off,

size_t count)

{

ssize_t retval = 0;

if (unlikely(!count))

return count;

/*

* Write data to chip, protecting against concurrent updates

* from this host, but not from other I2C masters.

*/

/* 访问设备前，加锁*/

mutex_lock(&at24->lock);

while (count) {

ssize_t status;

status = at24_eeprom_write(at24, buf, off, count);

if (status <= 0) {

if (retval == 0)

retval = status;

break;

}

buf += status;

off += status;

count -= status;

retval += status;

}

mutex_unlock(&at24->lock);

return retval;

}

该函数不复杂。在访问设备前，首先加锁互斥体，以防止竞态。然后根据count来调用at24_eeprom_write函数将数据写入设备。

写入成功后，更新偏移量等信息，如果还需要写入，则再次调用at24_eeprom_write函数。

看下at24_eeprom_write函数：

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/*

* Note that if the hardware write-protect pin is pulled high, the whole

* chip is normally write protected. But there are plenty of product

* variants here, including OTP fuses and partial chip protect.

*

* We only use page mode writes; the alternative is sloooow. This routine

* writes at most one page.

*/

static ssize_t at24_eeprom_write(struct at24_data *at24, const char *buf,

unsigned offset, size_t count)

{

struct i2c_client *client;

struct i2c_msg msg;

ssize_t status;

unsigned long timeout, write_time;

unsigned next_page;

/* Get corresponding I2C address and adjust offset */

client = at24_translate_offset(at24, &offset);

/* write_max is at most a page */

/* 检查写入的字节数*/

if (count > at24->write_max)

count = at24->write_max;

/* Never roll over backwards, to the start of this page */

/* 写入不会超过下一页的边界*/

next_page = roundup(offset + 1, at24->chip.page_size);

/* 根据页大小调整count*/

if (offset + count > next_page)

count = next_page - offset;

/* If we'll use I2C calls for I/O, set up the message */

/* 使用I2C协议，需要填充msg*/

if (!at24->use_smbus) {

int i = 0;

msg.addr = client->addr; /*设备地址*/

msg.flags = 0;

/* msg.buf is u8 and casts will mask the values */

/* 使用writebuf作为发送缓冲区 */

msg.buf = at24->writebuf;

/* 根据是8位还是16位地址，msg.buf的前一(两)个字节

为设备内部的寄存器地址*/

if (at24->chip.flags & AT24_FLAG_ADDR16)

msg.buf[i++] = offset >> 8; /* 16位地址，先写高位地址*/

msg.buf[i++] = offset;

/* 复制需要发送的数据 */

memcpy(&msg.buf[i], buf, count);

msg.len = i + count; /* 发送长度为数据长度加上地址长度*/

}

/*

* Writes fail if the previous one didn't complete yet. We may

* loop a few times until this one succeeds, waiting at least

* long enough for one entire page write to work.

*/

timeout = jiffies + msecs_to_jiffies(write_timeout);

do {

write_time = jiffies;

if (at24->use_smbus) {

/* 使用SMBus协议发送*/

status = i2c_smbus_write_i2c_block_data(client,

offset, count, buf);

if (status == 0)

status = count;

} else {

/* 使用I2C协议发送*/

status = i2c_transfer(client->adapter, &msg, 1);

if (status == 1)

status = count;

}

dev_dbg(&client->dev, "write %zu@%d --> %zd (%ld)\n",

count, offset, status, jiffies);

if (status == count)

return count; /* 已全部写入，返回*/

/* REVISIT: at HZ=100, this is sloooow */

msleep(1);

} while (time_before(write_time, timeout)); /* 使用timeout */

return -ETIMEDOUT; /* 超时，返回错误*/

}

该函数首先调用了at24_translate_offset函数，来获取地址对应的client：

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/*

* This routine supports chips which consume multiple I2C addresses. It

* computes the addressing information to be used for a given r/w request.

* Assumes that sanity checks for offset happened at sysfs-layer.

*/

static struct i2c_client *at24_translate_offset(struct at24_data *at24,

unsigned *offset)

{

unsigned i;

/* 有多个I2C设备地址，根据offset获取该地址对应的client*/

if (at24->chip.flags & AT24_FLAG_ADDR16) {

i = *offset >> 16;

*offset &= 0xffff;

} else {

i = *offset >> 8;

*offset &= 0xff;

}

return at24->client[i];

}

然后，对写入的字节数（count）进行了调整。

随后，如果使用I2C协议，则要组建msg用于发送。

最后，根据使用I2C还是SMBus协议，调用相应的发送函数来发送数据。

注意的是，这里使用了超时，超时时间write_timeout为驱动模块参数，可由用户设置，默认为25ms。如果发送超时了，while循环将终止。

至此，at24c02的写入过程就结束了。

4.2 读函数（at24_bin_read）

写函数和读函数非常相似，只是在使用I2C协议时，组建的msg有所不同。同样读函数也使用了超时。
因此，这里仅仅给出代码：

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/*

* This routine supports chips which consume multiple I2C addresses. It

* computes the addressing information to be used for a given r/w request.

* Assumes that sanity checks for offset happened at sysfs-layer.

*/

static struct i2c_client *at24_translate_offset(struct at24_data *at24,

unsigned *offset)

{

unsigned i;

/* 有多个I2C设备地址，根据offset获取该地址对应的client*/

if (at24->chip.flags & AT24_FLAG_ADDR16) {

i = *offset >> 16;

*offset &= 0xffff;

} else {

i = *offset >> 8;

*offset &= 0xff;

}

return at24->client[i];

}

static ssize_t at24_eeprom_read(struct at24_data *at24, char *buf,

unsigned offset, size_t count)

{

struct i2c_msg msg[2];

u8 msgbuf[2];

struct i2c_client *client;

unsigned long timeout, read_time;

int status, i;

memset(msg, 0, sizeof(msg));

/*

* REVISIT some multi-address chips don't rollover page reads to

* the next slave address, so we may need to truncate the count.

* Those chips might need another quirk flag.

*

* If the real hardware used four adjacent 24c02 chips and that

* were misconfigured as one 24c08, that would be a similar effect:

* one "eeprom" file not four, but larger reads would fail when

* they crossed certain pages.

*/

/*

* Slave address and byte offset derive from the offset. Always

* set the byte address; on a multi-master board, another master

* may have changed the chip's "current" address pointer.

*/

client = at24_translate_offset(at24, &offset);

if (count > io_limit)

count = io_limit;

if (at24->use_smbus) {

/* Smaller eeproms can work given some SMBus extension calls */

if (count > I2C_SMBUS_BLOCK_MAX)

count = I2C_SMBUS_BLOCK_MAX;

} else {

/* 使用I2C协议，需要填充msg*/

/*

* When we have a better choice than SMBus calls, use a

* combined I2C message. Write address; then read up to

* io_limit data bytes. Note that read page rollover helps us

* here (unlike writes). msgbuf is u8 and will cast to our

* needs.

*/

i = 0;

if (at24->chip.flags & AT24_FLAG_ADDR16)

msgbuf[i++] = offset >> 8;

msgbuf[i++] = offset;

msg[0].addr = client->addr;

msg[0].buf = msgbuf;

msg[0].len = i;

msg[1].addr = client->addr;

msg[1].flags = I2C_M_RD; /* 读模式*/

msg[1].buf = buf;

msg[1].len = count;

}

/*

* Reads fail if the previous write didn't complete yet. We may

* loop a few times until this one succeeds, waiting at least

* long enough for one entire page write to work.

*/

timeout = jiffies + msecs_to_jiffies(write_timeout);

do {

read_time = jiffies;

if (at24->use_smbus) {

status = i2c_smbus_read_i2c_block_data(client, offset,

count, buf);

} else {

status = i2c_transfer(client->adapter, msg, 2);

if (status == 2)

status = count;

}

dev_dbg(&client->dev, "read %zu@%d --> %d (%ld)\n",

count, offset, status, jiffies);

if (status == count)

return count;

/* REVISIT: at HZ=100, this is sloooow */

msleep(1);

} while (time_before(read_time, timeout)); /* 使用timeout */

return -ETIMEDOUT;

}

static ssize_t at24_read(struct at24_data *at24,

char *buf, loff_t off, size_t count)

{

ssize_t retval = 0;

if (unlikely(!count))

return count;

/*

* Read data from chip, protecting against concurrent updates

* from this host, but not from other I2C masters.

*/

/* 访问设备前，加锁*/

mutex_lock(&at24->lock);

while (count) {

ssize_t status;

status = at24_eeprom_read(at24, buf, off, count);

if (status <= 0) {

if (retval == 0)

retval = status;

break;

}

buf += status;

off += status;

count -= status;

retval += status;

}

mutex_unlock(&at24->lock);

return retval;

}

static ssize_t at24_bin_read(struct kobject *kobj, struct bin_attribute *attr,

char *buf, loff_t off, size_t count)

{

struct at24_data *at24;

/* 通过kobj获取device，再获取driver_data */

at24 = dev_get_drvdata(container_of(kobj, struct device, kobj));

return at24_read(at24, buf, off, count);

}

5. 总结

本文主要对at24c02的驱动架构进行了分析。该驱动基于i2c总线架构，提供了id表来帮助设备驱动的绑定，该驱动支持AT24CXX等多个系列，不仅仅是at24c02。

其次，该驱动并没有注册任何字符设备或者杂项设备，而是通过sys文件系统的二进制属性文件来对设备进行访问。此外，驱动同时支持I2C协议和SMBus协议来访问设备。

转自：http://blog.csdn.net/yj4231/article/details/18145973