在分析驅動程序之前我們再來分析一下I2C子系統的模型。I2C的設備驅動中有2中方式，一種是通過通用驅動來編寫用戶驅動。另一種就是直接在I2C子系統中添加一個I2C的設備驅動，比如說針對AT24C02的驅動程序，接下來我們來學習怎麼編寫一個I2C設備驅動。
1.驅動程序分析
- 我們先在Linux內核代碼中打開一個叫做At24.c的文件，只要是屬於AT24開頭的設備都可以使用這個驅動。我們接下來分析這個驅動。
/*-------------------------------------------------------------------------*/
static struct i2c_driver at24_driver = {
.driver = {
.name = "at24",
.owner = THIS_MODULE,
},
.probe = at24_probe,
.remove = __devexit_p(at24_remove),
.id_table = at24_ids,
};
static int __init at24_init(void)
{
io_limit = rounddown_pow_of_two(io_limit);
return i2c_add_driver(&at24_driver);
}
module_init(at24_init);
static void __exit at24_exit(void)
{
i2c_del_driver(&at24_driver);
}
module_exit(at24_exit);
- 初始化函數中主要是註冊一個I2C設備驅動。我們分析(&at24_driver)裏面的成員，比較重要的成員有2個，一個是probe函數，另一個是at24_ids，這裏面存放支持AT24設備的ID列表，比如說AT24C02，AT24C08等等，有興趣的可以看一下這個表。
- 我們接下來分析probe函數。這個函數很長，如果要做到讀懂每一行代碼肯定是非常難的，我們最基本的應該掌握這個函數大概的流程和中心。比如說註冊某個東西，創建某個東西，初始化某個東西。
/*-------------------------------------------------------------------------*/
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;
if (client->dev.platform_data) {
chip = *(struct at24_platform_data *)client->dev.platform_data;
} else {
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;
}
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. */
if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
if (chip.flags & AT24_FLAG_ADDR16) {
err = -EPFNOSUPPORT;
goto err_out;
}
if (!i2c_check_functionality(client->adapter,
I2C_FUNC_SMBUS_READ_I2C_BLOCK)) {
err = -EPFNOSUPPORT;
goto err_out;
}
use_smbus = true;
}
if (chip.flags & AT24_FLAG_TAKE8ADDR)
num_addresses = 8;
else
num_addresses = DIV_ROUND_UP(chip.byte_len,
(chip.flags & AT24_FLAG_ADDR16) ? 65536 : 256);
at24 = kzalloc(sizeof(struct at24_data) +
num_addresses * sizeof(struct i2c_client *), GFP_KERNEL);
if (!at24) {
err = -ENOMEM;
goto err_out;
}
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)
*/
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;
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;
at24->bin.write = at24_bin_write;
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 = 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;
/* 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);
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);
if (err)
goto err_clients;
i2c_set_clientdata(client, at24);
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);
err_out:
dev_dbg(&client->dev, "probe error %d\n", err);
return err;
}
- 這裏好像沒有註冊什麼東西，那麼有沒有創建某個東西呢？可以看到他在sys下面創建了一個文件err = sysfs_create_bin_file(&client->dev.kobj, &at24->bin);這個文件包含什麼東西呢?查看之前對於at24->bin的初始化，可以知道，這個文件叫做eeprom，同時可以猜測，當我們使用這個設備文件來讀寫eeprom時需要使用到文件裏面實現讀寫操作的函數。
- 查看代碼我們也可以知道這2個函數分別是at24_bin_read和at24_bin_write，我們接下來就分析這2個函數。
/*
* 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 */
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;
/* 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;
msg.buf[i++] = offset;
memcpy(&msg.buf[i], buf, count);
msg.len = i + count;
}
/*
* 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) {
status = i2c_smbus_write_i2c_block_data(client,
offset, count, buf);
if (status == 0)
status = count;
} else {
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;
/* REVISIT: at HZ=100, this is sloooow */
msleep(1);
} while (time_before(write_time, timeout));
return -ETIMEDOUT;
}
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 */
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;
/* 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;
msg.buf[i++] = offset;
memcpy(&msg.buf[i], buf, count);
msg.len = i + count;
}
/*
* 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) {
status = i2c_smbus_write_i2c_block_data(client,
offset, count, buf);
if (status == 0)
status = count;
} else {
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;
/* REVISIT: at HZ=100, this is sloooow */
msleep(1);
} while (time_before(write_time, timeout));
return -ETIMEDOUT;
}
- 簡要分析一下可以知道，它主要做了2件事，一個是構造msg，另一件事是使用i2c_transfer函數來傳輸數據，上一節的分析可以知道，i2c設備驅動如果要讀寫數據，都是通過i2c_transfer把它交給i2c總線驅動或者說是i2c控制器驅動。對應msg的構造我們在上一節課已經講的非常清楚了，對於i2c_transfer函數，它是屬於i2c-core裏面的函數，查看他的代碼可以知道，他並沒有做什麼實質性的操作，而是調用i2c適配器裏面的讀寫方法來實現數據的傳輸 ret = adap->algo->master_xfer(adap, msgs, num);
- 接下來分析讀函數，讀函數和寫函數的調用過程也類似，依次調用at24_bin_read->at24_read->at24_eeprom_read來實現eeprom的讀數據，at24_eeprom_read的實現過程也主要分爲msg的構造和數據的傳輸，msg的構造和上一節課講的消息構造差不多，也是需要分爲2個消息，一個是寫消息，然後是讀消息的構造。
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;
msg[1].buf = buf;
msg[1].len = count;
2.驅動程序移植
- 我們現在來對Linux的i2c驅動代碼進行修改和移植。首先是註冊i2c設備，爲什麼要註冊i2c設備呢？我們知道當Linux系統檢測到符合id_table的設備時將會調用probe函數，因此，我們不僅需要註冊i2c驅動，還需要註冊i2c設備。怎麼樣才能註冊一個i2c設備呢？找到Mach-mini2440.c文件，依次添加以下代碼：
// add by david
static struct at24_platform_data at24c08 = {
.byte_len = SZ_8K / 8,
.page_size = 16,
.flags = 0,
};
// add by david
static struct i2c_board_info __initdata mini2440_i2c_devices[] = {
{
I2C_BOARD_INFO("24c08", 0x50),
.platform_data = &at24c08,
},
};
- 由於可能有多個i2c設備，所以把它們保存在一個數組裏面。
- 然後使用i2c_register_board_info函數把我們定義的i2c設備數組註冊到Linux內核中：
i2c_register_board_info(0, mini2440_i2c_devices, ARRAY_SIZE(mini2440_i2c_devices)); // add
- 添加頭文件
// add by david
#include <linux/i2c.h>
#include <linux/i2c/at24.h>
- 這樣就算是移植好了i2c的驅動。
3.內核中啓動對eeprom的支持
- #make menuconfig
- 選擇device driver，選中並進入Misc devices,然後進入 EEPROM Support，選擇裏面全部的配置，這樣就把eeprom的驅動配置好了，然後當註冊i2c設備的時候，就會調用probe函數，從而生成/sys/bus/i2c/devices/0-0050/eeprom文件
- 編譯內核：#make uImage ARCH=arm CROSS_COMPILE=arm-linux-
- 下載到開發板
4.驅動程序測試
- 測試程序我們分爲這幾步：
- 1、打開文件
- 2、寫入數據s
- 3、讀出數據
- 4、打印
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main()
{
int fd, i;
char write_data[256];
char read_data[256];
// 1.打開at24c08對應的sys文件
fd = open("/sys/bus/i2c/devices/0-0050/eeprom", O_RDWR);
if (fd < 0)
{
printf("error\n");
return -1;
}
// 2.寫入數據
for (i = 0; i < 256; i++)
write_data[i] = i;
lseek(fd, 0, SEEK_SET); // 重新定位讀寫指針
write(fd, write_data, 256);
// 3.讀出數據
lseek(fd, 0, SEEK_SET); // 重新定位讀寫指針
lseek(fd, 0, SEEK_SET);
read(fd, read_data, 256);
// 4.打印對比
for (i = 0; i < 256; i++)
{
if (i % 16 == 0)
printf("\n");
printf("%3d ", read_data[i]);
}
printf("\n");
//關閉文件
close(fd);
return 0;
}
- 編譯這個程序：
- #arm-linux-gcc -stati app.c -o app
- 然後把它拷貝到開發板上運行：./i2c-app
- 打印出的信息和寫入eeprom數據的應該是一樣的。
