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上篇分析了Linux Kernel中的I2C驱动框架，本篇举一个具体的I2C设备驱动（eeprom）来对I2C设备驱动有个实际的认识。
s3c24xx系列集成了一个基于I2C的eeprom设备at24cxx系列。at24cxx系列芯片包含at24c01, at24c02, at24c04, at24c08, at24c16 等，其中xx代表芯片可寻址范围，如01代表1kB，02代表2kB，如此类推。at24xx系列芯片还支持多地址寻址功能，即，支持多个地址芯片。
直接分析代码：drivers/misc/eeprom/at24.c
1，驱动注册：
static const struct i2c_device_id at24_ids[] = { // at24设备驱动程序支持at24cxx全系列驱动
/* 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 */ }
};
static struct i2c_driver at24_driver = {
.driver = {
.name = "at24",
.acpi_match_table = ACPI_PTR(at24_acpi_ids),
},
.probe = at24_probe, // i2c_bus_type match()之后，调用 i2c_driver的probe()
.remove = at24_remove,
.id_table = at24_ids, // 驱动支持设备id列表
};
static int __init at24_init(void)
{
if (!io_limit) {
pr_err("at24: io_limit must not be 0!\n");
return -EINVAL;
}
io_limit = rounddown_pow_of_two(io_limit);
return i2c_add_driver(&at24_driver); // 注册at24cxx系列i2c设备驱动
}
2，设备探测probe
（1）at24cxx设备平台数据结构
struct at24_platform_data {
u32 byte_len; /* size (sum of all addr) */ // 设备支持的容量
u16 page_size; /* for writes */ // 设备支持一次读取或写入的大小
u8 flags; // 设备特性，如下
#define AT24_FLAG_ADDR16 0x80 /* address pointer is 16 bit */
#define AT24_FLAG_READONLY 0x40 /* sysfs-entry will be read-only */
#define AT24_FLAG_IRUGO 0x20 /* sysfs-entry will be world-readable */
#define AT24_FLAG_TAKE8ADDR 0x10 /* take always 8 addresses (24c00) */
void (*setup)(struct memory_accessor *, void *context);
void *context;
};
（2）核心数据结构 struct at24_data
struct at24_data {
struct at24_platform_data chip; // 芯片平台数据
struct memory_accessor macc;
int use_smbus;
int use_smbus_write;
/*
* Lock protects against activities from other Linux tasks,
* but not from changes by other I2C masters.
*/
struct mutex lock;
struct bin_attribute bin; // 通过sysfs提供给用户的接口
u8 *writebuf;
unsigned write_max;
unsigned num_addresses; // 芯片支持的地址数量
/*
* Some chips tie up multiple I2C addresses; dummy devices reserve
* them for us, and we'll use them with SMBus calls.
*/
struct i2c_client *client[]; // 多地址芯片实例
};
static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)
{
struct at24_platform_data chip;
kernel_ulong_t magic = 0;
bool writable;
int use_smbus = 0;
int use_smbus_write = 0;
struct at24_data *at24;
int err;
unsigned i, num_addresses;
// 获取eeprom设备平台相关数据，如
if (client->dev.platform_data) {
// 设备树或者平台相关代码已经初始化好平台相关数据
chip = *(struct at24_platform_data *)client->dev.platform_data;
} else {
// 没有预先配置好，从驱动支持列表的 magic 中解析平台相关数据，如容量，特性等
if (id) {
magic = id->driver_data;
} else {
const struct acpi_device_id *aid;
aid = acpi_match_device(at24_acpi_ids, &client->dev);
if (aid)
magic = aid->driver_data;
}
if (!magic)
return -ENODEV;
chip.byte_len = BIT(magic & AT24_BITMASK(AT24_SIZE_BYTELEN)); // 从magic解析eeprom设备容量
magic >>= AT24_SIZE_BYTELEN;
chip.flags = magic & AT24_BITMASK(AT24_SIZE_FLAGS); // 从magic解析eeprom设备特性
/*
* 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;
/* update chipdata if OF is present */
at24_get_ofdata(client, &chip);
chip.setup = NULL;
chip.context = NULL;
}
if (!is_power_of_2(chip.byte_len))
dev_warn(&client->dev,
"byte_len looks suspicious (no power of 2)!\n");
if (!chip.page_size) {
dev_err(&client->dev, "page_size must not be 0!\n");
return -EINVAL;
}
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. */
if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
if (chip.flags & AT24_FLAG_ADDR16)
return -EPFNOSUPPORT;
if (i2c_check_functionality(client->adapter,
I2C_FUNC_SMBUS_READ_I2C_BLOCK)) {
use_smbus = I2C_SMBUS_I2C_BLOCK_DATA;
} else if (i2c_check_functionality(client->adapter,
I2C_FUNC_SMBUS_READ_WORD_DATA)) {
use_smbus = I2C_SMBUS_WORD_DATA;
} else if (i2c_check_functionality(client->adapter,
I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
use_smbus = I2C_SMBUS_BYTE_DATA;
} else {
return -EPFNOSUPPORT;
}
}
/* Use I2C operations unless we're stuck with SMBus extensions. */
if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
if (i2c_check_functionality(client->adapter,
I2C_FUNC_SMBUS_WRITE_I2C_BLOCK)) {
use_smbus_write = I2C_SMBUS_I2C_BLOCK_DATA;
} else if (i2c_check_functionality(client->adapter,
I2C_FUNC_SMBUS_WRITE_BYTE_DATA)) {
use_smbus_write = I2C_SMBUS_BYTE_DATA;
chip.page_size = 1;
}
}
if (chip.flags & AT24_FLAG_TAKE8ADDR) // 明确指定支持芯片支持8地址
num_addresses = 8;
else
num_addresses = DIV_ROUND_UP(chip.byte_len,
(chip.flags & AT24_FLAG_ADDR16) ? 65536 : 256); // 根据芯片容量计算支持的地址数量
at24 = devm_kzalloc(&client->dev, sizeof(struct at24_data) +
num_addresses * sizeof(struct i2c_client *), GFP_KERNEL); // 分配核心结构对象 at24_data
if (!at24)
return -ENOMEM;
mutex_init(&at24->lock);
at24->use_smbus = use_smbus;
at24->use_smbus_write = use_smbus_write;
at24->chip = chip; // 初始化芯片平台相关数据
at24->num_addresses = num_addresses; // 初始化芯片支持的地址数量
/*
* Export the EEPROM bytes through sysfs, since that's convenient.
* By default, only root should see the data (maybe passwords etc)
*/
// sysfs接口初始化
sysfs_bin_attr_init(&at24->bin);
at24->bin.attr.name = "eeprom";
at24->bin.attr.mode = chip.flags & AT24_FLAG_IRUGO ? S_IRUGO : S_IRUSR;
at24->bin.read = at24_bin_read; // sysfs读取接口
at24->bin.size = chip.byte_len; // 芯片容量，即，sysfs文件大小
at24->macc.read = at24_macc_read;
writable = !(chip.flags & AT24_FLAG_READONLY); // 设备特性是否可写
if (writable) {
if (!use_smbus || use_smbus_write) {
unsigned write_max = chip.page_size;
at24->macc.write = at24_macc_write;
at24->bin.write = at24_bin_write; // sysfs写入接口
at24->bin.attr.mode |= S_IWUSR;
if (write_max > io_limit)
write_max = io_limit;
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) */
at24->writebuf = devm_kzalloc(&client->dev,
write_max + 2, GFP_KERNEL); // 分配写入buffer
if (!at24->writebuf)
return -ENOMEM;
} else {
dev_warn(&client->dev,
"cannot write due to controller restrictions.");
}
}
// 对于支持多地址芯片，每个地址范围分配一个i2c_client实例
at24->client[0] = client; // 默认地址范围client
/* use dummy devices for multiple-address chips */
for (i = 1; i < num_addresses; i++) {
at24->client[i] = i2c_new_dummy(client->adapter,
client->addr + i); // 多地址，其他地址范围每个分配一个dummy client
if (!at24->client[i]) {
dev_err(&client->dev, "address 0x%02x unavailable\n",
client->addr + i);
err = -EADDRINUSE;
goto err_clients;
}
}
err = sysfs_create_bin_file(&client->dev.kobj, &at24->bin); // 创建sysfs文件，给用户提供访问接口
if (err)
goto err_clients;
i2c_set_clientdata(client, at24); // 方便通过 client 获取 at24 核心私有结构
dev_info(&client->dev, "%zu byte %s EEPROM, %s, %u bytes/write\n",
at24->bin.size, client->name,
writable ? "writable" : "read-only", at24->write_max);
if (use_smbus == I2C_SMBUS_WORD_DATA ||
use_smbus == I2C_SMBUS_BYTE_DATA) {
dev_notice(&client->dev, "Falling back to %s reads, "
"performance will suffer\n", use_smbus ==
I2C_SMBUS_WORD_DATA ? "word" : "byte");
}
/* 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]);
return err;
}
3，sysfs读取操作
at24cxx系列i2c芯片读取协议：先发送写命令，写入要访问eeprom的目标地址（8位或者16位，由芯片平台数据flag决定），再发送读数据命令，读取目标地址数据。因此，读取操作需要两个消息，一个写入消息，一个读取消息。
static ssize_t at24_bin_read(struct file *filp, struct kobject *kobj,
struct bin_attribute *attr,
char *buf, loff_t off, size_t count)
{
struct at24_data *at24;
at24 = dev_get_drvdata(container_of(kobj, struct device, kobj));
return at24_read(at24, buf, off, count);
}
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.
*/
mutex_lock(&at24->lock);
while (count) {
ssize_t status;
status = at24_eeprom_read(at24, buf, off, count); // 实际读取操作，返回读取的字节数
if (status <= 0) {
if (retval == 0)
retval = status;
break;
}
buf += status; // 空闲buffer首地址更新
off += status; // 要读取的eeprom地址更新
count -= status; // 空闲buffer大小更新
retval += status; // 累计读取的大小更新
}
mutex_unlock(&at24->lock);
return retval;
}
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); // 多地址芯片，计算目标地址落在哪个client，以及更新client地址范围内偏移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 {
/*
* 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; // 如果是16位目标地址，先发送高8位
msgbuf[i++] = offset; // 16位目标地址的低8位，或者8位目标地址
// msg[0]是写命令，写入eeprom的目标地址
msg[0].addr = client->addr; // client->addr是eeprom的I2C地址，通常是0x50
msg[0].buf = msgbuf;
msg[0].len = i;
// msg[1]是读命令，读取目标地址的数据
msg[1].addr = client->addr;
msg[1].flags = I2C_M_RD;
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_or_emulated(client, offset,
count, buf);
} else {
status = i2c_transfer(client->adapter, msg, 2); // 调用i2c框架传输接口，执行实际数据传输操作
if (status == 2) // i2c_transfer返回执行成功的msg个数
status = count;
}
dev_dbg(&client->dev, "read %zu@%d --> %d (%ld)\n",
count, offset, status, jiffies);
if (status == count)
return count; // 返回实际读取的数据长度
/* REVISIT: at HZ=100, this is sloooow */
msleep(1);
} while (time_before(read_time, timeout));
return -ETIMEDOUT;
}
4，sysfs写入操作
at24cxx系列i2c芯片写入协议：先发送写命令，写入要访问eeprom的目标地址（8位或者16位，由芯片平台数据flag决定），再发送写数据命令，读取目标地址数据。因此，写入操作只需要一个写入消息就可以了。
static ssize_t at24_bin_write(struct file *filp, struct kobject *kobj,
struct bin_attribute *attr,
char *buf, loff_t off, size_t count)
{
struct at24_data *at24;
at24 = dev_get_drvdata(container_of(kobj, struct device, kobj));
return at24_write(at24, buf, off, count);
}
static 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.
*/
mutex_lock(&at24->lock);
while (count) {
ssize_t status;
status = at24_eeprom_write(at24, buf, off, count); // 实际写入操作，返回写入的字节数
if (status <= 0) {
if (retval == 0)
retval = status;
break;
}
// 同读取操作类似
buf += status;
off += status;
count -= status;
retval += status;
}
mutex_unlock(&at24->lock);
return retval;
}
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 = 0;
unsigned long timeout, write_time;
unsigned next_page;
/* Get corresponding I2C address and adjust offset */
client = at24_translate_offset(at24, &offset); // 多地址芯片，计算目标地址落在哪个client，以及更新client地址范围内偏移offset
/* write_max is at most a page */
if (count > at24->write_max)
count = at24->write_max;
/* Never roll over backwards, to the start of this page */
next_page = roundup(offset + 1, at24->chip.page_size);
if (offset + count > next_page)
count = next_page - offset; // 保证每次写入不会跨page
/* If we'll use I2C calls for I/O, set up the message */
if (!at24->use_smbus) {
int i = 0;
msg.addr = client->addr;
msg.flags = 0;
/* msg.buf is u8 and casts will mask the values */
msg.buf = at24->writebuf;
if (at24->chip.flags & AT24_FLAG_ADDR16)
msg.buf[i++] = offset >> 8; // 16位地址高8位
msg.buf[i++] = offset; // 16位地址低8位或者8位地址
memcpy(&msg.buf[i], buf, count); // 拷贝实际要写入的数据
msg.len = i + count; // 实际消息的长度: eeprom目标地址大小+数据大小
}
/*
* 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_write) {
switch (at24->use_smbus_write) {
case I2C_SMBUS_I2C_BLOCK_DATA:
status = i2c_smbus_write_i2c_block_data(client,
offset, count, buf);
break;
case I2C_SMBUS_BYTE_DATA:
status = i2c_smbus_write_byte_data(client,
offset, buf[0]);
break;
}
if (status == 0)
status = count;
} else {
status = i2c_transfer(client->adapter, &msg, 1); // 调用i2c框架传输接口，执行实际数据传输操作
if (status == 1) // i2c_transfer返回执行成功的msg个数
status = count;
}
dev_dbg(&client->dev, "write %zu@%d --> %zd (%ld)\n",
count, offset, status, jiffies);
if (status == count)
return count; // 返回实际写入的数据长度
/* REVISIT: at HZ=100, this is sloooow */
msleep(1);
} while (time_before(write_time, timeout));
return -ETIMEDOUT;
}
I2C eeprom的设备驱动的读取和写入的协议都算是比较简单的。在实际中，也可能遇到更加复杂的i2c设备驱动，这就需要我们熟悉具体设备的业务协议，通过sysfs或者字符设备等，提供给用户操作的api接口。
从这个例子中，我们看到了kernel中随处可见的抽象思想：I2C核心框架抽象出一套平台无关的接口，I2C adapter实现具体总线的操作algorithm，而各个具体的i2c设备又在此I2C总线的操作基础之上，实现各自的功能。