基于S3C2440的嵌入式Linux驱动——AT24C02(EEPROM I2C接口)驱动解读

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:

S3C2440驱动移植——AT24C02(EEPROM)移植


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