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
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。
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：
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：
#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。
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）
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:
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：
/*
* 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：
/*
* 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：/*
* 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。
有关驱动的移植可以参考：