AT24 I2C EEPROM解析及测试

 关键词：AT24、I2C、nvmem、EEPROM。

1. AT24C介绍

AT24C是一款采用I2C通信的EEPROM，相关驱动涉及到I2C和nvmem。

I2C是读写数据的通道，nvmem将AT24C注册为nvmem设备。

2.源码分析

2.1 DTS

at24是挂在i2c总线下的设备，硬件接到哪个i2c，DTS中也需要对应修改。

其中需要注意的是，status不能是disabled，pinctrl需要配置。

其中at24的campatible需要和代码对应。

        i2c3 {
            compatible = "snps,designware-i2c";
            //status = "disabled";--------------------------注释掉disabled才能使用此i2c。
            reg = <0xfc407000 0x1000>;
            interrupts = <27>;
            clocks = <&high_apb_clk>;
            pinctrl-names = "default";
            pinctrl-0 = <&i2c3_2>;
            #address-cells = <1>;
            #size-cells = <0>;
 　　　　　　 clock-frequency = <400000>;----------------------调整对应i2c的频率，其中400K和1M需要设置时序才能正确使用。

            at24@50 {
                compatible = "at24,24cm02";
                reg = <0x50>;
            };
        };

2.2 AT24初始化

at24的probe主要做数据有效检查、根据i2c的capability决定读写函数、读1字节验证功能、最后注册对应的nvmem设备。

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;
    u8 test_byte;

...
    if (!is_power_of_2(chip.byte_len))-----------------------------------------数据检查
        dev_warn(&client->dev,
            "byte_len looks suspicious (no power of 2)!
");
    if (!chip.page_size) {
        dev_err(&client->dev, "page_size must not be 0!
");
        return -EINVAL;
    }
    if (!is_power_of_2(chip.page_size))
        dev_warn(&client->dev,
            "page_size looks suspicious (no power of 2)!
");

    /* Use I2C operations unless we're stuck with SMBus extensions. */
    if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {-------------是否具备I2C_FUNC_I2C，如果不具备则判断是否具备下面能力。然后决定是否使用SMBus。
...
    }

    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 = devm_kzalloc(&client->dev, sizeof(struct at24_data) +
        num_addresses * sizeof(struct i2c_client *), GFP_KERNEL);
    if (!at24)
        return -ENOMEM;

    mutex_init(&at24->lock);
    at24->use_smbus = use_smbus;
    at24->use_smbus_write = use_smbus_write;
    at24->chip = chip;
    at24->num_addresses = num_addresses;

    if ((chip.flags & AT24_FLAG_SERIAL) && (chip.flags & AT24_FLAG_MAC)) {
        dev_err(&client->dev,
            "invalid device data - cannot have both AT24_FLAG_SERIAL & AT24_FLAG_MAC.");
        return -EINVAL;
    }

    if (chip.flags & AT24_FLAG_SERIAL) {--------------------------------------根据flags，以及是否使用SMBus，选择合适的read_func()/write_func()。
        at24->read_func = at24_eeprom_read_serial;
    } else if (chip.flags & AT24_FLAG_MAC) {
        at24->read_func = at24_eeprom_read_mac;
    } else {
        at24->read_func = at24->use_smbus ? at24_eeprom_read_smbus
                          : at24_eeprom_read_i2c;
    }

    if (at24->use_smbus) {
        if (at24->use_smbus_write == I2C_SMBUS_I2C_BLOCK_DATA)
            at24->write_func = at24_eeprom_write_smbus_block;
        else
            at24->write_func = at24_eeprom_write_smbus_byte;
    } else {
        at24->write_func = at24_eeprom_write_i2c;
    }

    writable = !(chip.flags & AT24_FLAG_READONLY);
    if (writable) {---------------------------------------------------------------------------------write_max决定后面i2c写大小，大于此数字会被拆分。
        if (!use_smbus || use_smbus_write) {

            unsigned write_max = chip.page_size;

            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);
            if (!at24->writebuf)
                return -ENOMEM;
        } 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
",
                    client->addr + i);
            err = -EADDRINUSE;
            goto err_clients;
        }
    }

    i2c_set_clientdata(client, at24);

    /*
     * Perform a one-byte test read to verify that the
     * chip is functional.
     */
    err = at24_read(at24, 0, &test_byte, 1);--------------------------------读一字节进行测试。
    if (err) {
        err = -ENODEV;
        goto err_clients;
    }

    at24->nvmem_config.name = dev_name(&client->dev);-----------------------注册nvmem设备，对应设备/sys/bus/i2c/devices/x-0050/eeprom。
    at24->nvmem_config.dev = &client->dev;
    at24->nvmem_config.read_only = !writable;
    at24->nvmem_config.root_only = true;
    at24->nvmem_config.owner = THIS_MODULE;
    at24->nvmem_config.compat = true;
    at24->nvmem_config.base_dev = &client->dev;
    at24->nvmem_config.reg_read = at24_read;--------------------------------对于非SMBus设备，对应at24_eeprom_read_i2c()。
    at24->nvmem_config.reg_write = at24_write;------------------------------对非SMBus设备，对应at24_eeprom_write_i2c()。
    at24->nvmem_config.priv = at24;
    at24->nvmem_config.stride = 4;
    at24->nvmem_config.word_size = 1;
    at24->nvmem_config.size = chip.byte_len;

    at24->nvmem = nvmem_register(&at24->nvmem_config);

    if (IS_ERR(at24->nvmem)) {
        err = PTR_ERR(at24->nvmem);
        goto err_clients;
    }
...
    /* export data to kernel code */
    if (chip.setup)
        chip.setup(at24->nvmem, 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;
}

static int at24_remove(struct i2c_client *client)
{
    struct at24_data *at24;
    int i;

    at24 = i2c_get_clientdata(client);

    nvmem_unregister(at24->nvmem);

    for (i = 1; i < at24->num_addresses; i++)
        i2c_unregister_device(at24->client[i]);

    return 0;
}

static struct i2c_driver at24_driver = {
    .driver = {
        .name = "at24",
        .acpi_match_table = ACPI_PTR(at24_acpi_ids),
    },
    .probe = at24_probe,
    .remove = at24_remove,
    .id_table = at24_ids,
};

2.3 I2C读写

上层应用的读写，在底层是有限制的。

write就变成了一字节一字节写，read一次只能读取一个128字节大小。

static int at24_read(void *priv, unsigned int off, void *val, size_t count)
{
    struct at24_data *at24 = priv;
    char *buf = val;

    if (unlikely(!count))
        return count;

    mutex_lock(&at24->lock);

    while (count) {---------------------------------------------------上层传入的count大小，交给i2c进行去读。但是每次读取多少字节受限于i2c，一次循环。
        int    status;

        status = at24->read_func(at24, buf, off, count);--------------每次i2c读取操作，buf、off递增，count递减。
        if (status < 0) {
            mutex_unlock(&at24->lock);
            return status;
        }
        buf += status;
        off += status;
        count -= status;
    }

    mutex_unlock(&at24->lock);

    return 0;
}

static int at24_write(void *priv, unsigned int off, void *val, size_t count)
{
    struct at24_data *at24 = priv;
    char *buf = val;

    if (unlikely(!count))
        return -EINVAL;

    /*
     * Write data to chip, protecting against concurrent updates
     * from this host, but not from other I2C masters.
     */
    mutex_lock(&at24->lock);

    while (count) {---------------------------------------------------上层传下来的count，不一定一次写完。此处进行loop。
        int status;

        status = at24->write_func(at24, buf, off, count);--------------每次只写1字节，所以效率很低。
        if (status < 0) {
            mutex_unlock(&at24->lock);
            return status;
        }
        buf += status;
        off += status;
        count -= status;
    }

    mutex_unlock(&at24->lock);

    return 0;
}

static ssize_t at24_eeprom_read_i2c(struct at24_data *at24, char *buf,
                    unsigned int offset, size_t count)
{
    unsigned long timeout, read_time;
    struct i2c_client *client;
    struct i2c_msg msg[2];
    int status, i;
    u8 msgbuf[2];

    memset(msg, 0, sizeof(msg));
    client = at24_translate_offset(at24, &offset);

    if (count > io_limit)
        count = io_limit;------------------------------------------------不管上层传入读取多大数据，都受限于io_limit，这里为一个128字节。

    /*
     * 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;
    msg[1].buf = buf;
    msg[1].len = count;

    loop_until_timeout(timeout, read_time) {
        status = i2c_transfer(client->adapter, msg, 2);
        if (status == 2)
            status = count;

        dev_dbg(&client->dev, "read %zu@%d --> %d (%ld)
",
                count, offset, status, jiffies);

        if (status == count)
            return count;
    }

    return -ETIMEDOUT;
}

static ssize_t at24_eeprom_write_i2c(struct at24_data *at24, const char *buf,
                     unsigned int offset, size_t count)
{
    unsigned long timeout, write_time;
    struct i2c_client *client;
    struct i2c_msg msg;
    ssize_t status = 0;
    int i = 0;

    client = at24_translate_offset(at24, &offset);
    count = at24_adjust_write_count(at24, offset, count);

    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;

    loop_until_timeout(timeout, write_time) {
        status = i2c_transfer(client->adapter, &msg, 1);
        if (status == 1)
            status = count;

        dev_dbg(&client->dev, "write %zu@%d --> %zd (%ld)
",
                count, offset, status, jiffies);

        if (status == count)
            return count;
    }

    return -ETIMEDOUT;
}

3. eeprom测试

不同频率总线，对应的速率通过read较好体现；编写测试程序进行速率验证，中间经过文件系统一些限制。

3.1 测试预期

在at24.c文件中有关于不同速率测试预期，测试读要比写更纯粹一点，因为写只能一个字节一个自己，每次写完地址，再写一个字节。

/*
 * This parameter is to help this driver avoid blocking other drivers out
 * of I2C for potentially troublesome amounts of time. With a 100 kHz I2C
 * clock, one 256 byte read takes about 1/43 second which is excessive;
 * but the 1/170 second it takes at 400 kHz may be quite reasonable; and
 * at 1 MHz (Fm+) a 1/430 second delay could easily be invisible.
 *
 * This value is forced to be a power of two so that writes align on pages.
 */

可以看出一次读256字节，1MHz速率耗时2.3ms；400KHz速率耗时5.9ms；100KHz速率耗时23.3ms。

修改io_limit可以改变一次io字节数。

3.2 测试程序

源代码如下，编译后执行“”teeprom /sys/bus/i2c/devices/1-0050/eeprom value count”即可：

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <linux/i2c-dev.h>
#include <errno.h>
#include <time.h>

//teeprom device value size
#define EEPROM_SIZE 262144
#define PAGE_SIZE   4096
#define EEPROM_PAGES (EEPROM_SIZE/PAGE_SIZE)
int main(int argc, char *argv[])
{
    int num, value;
    int fd, pFile, page_index = 0;
    char *device_name, *buff, *out_buf;
    struct timespec time_0, time_1, time_2;

    printf("Please input as:");
    printf("teeprom device_name value size
");
    fflush(stdout);
    
    if(argc < 3){
        printf("arg error
");
        return -1;    
    }

    device_name = argv[1];
    value = atoi(argv[2]);
    num = atoi(argv[3]);
 
    buff = calloc(sizeof(char), num);
    if(buff < 0){
        printf("alloc failed
");
        return -1;
    }
    memset(buff, value, num);

    out_buf = calloc(sizeof(char), EEPROM_SIZE);

    fd = open(device_name,O_RDWR);
    if(fd < 0){
        printf("device open failed
");
        return -1;    
    }
    clock_gettime(CLOCK_MONOTONIC, &time_0);
    printf("%8d.%8d Write %d to 0x00 for %d bytes.
", time_0.tv_sec, time_0.tv_nsec, value, num);
    write(fd, buff, num);----------------------------------虽然此处希望写入num个字节，但是在系统调用到i2c写入中间有些限制了大小。

    clock_gettime(CLOCK_MONOTONIC, &time_1);
    printf("%8d.%8d Read from 0x00 for %d bytes.
", time_1.tv_sec, time_1.tv_nsec, EEPROM_SIZE);
    lseek(fd, 0, SEEK_SET);
    for(page_index = 0; page_index < EEPROM_PAGES; page_index++)
    {
        read(fd, out_buf, PAGE_SIZE);----------------------对于读也同样的有限制，只能循环读page大小。
    }
    close(fd);

    clock_gettime(CLOCK_MONOTONIC, &time_2);
    printf("%8d.%8d Write to eeprom.bin.
", time_2.tv_sec, time_2.tv_nsec);
    pFile = fopen("eeprom.bin","wb");
    if(pFile < 0){
        printf("device open failed
");
        return -1;    
    }
    fwrite(out_buf,EEPROM_SIZE,1,pFile);
    fclose(pFile);

    return 0;
}

3.3 结果分析

eeprom对应的nvmem节点如下，可以直接对其open/read/write/close操作。

/sys/bus/i2c/devices/1-0050/eeprom

/sys/bus/i2c/devices/3-0050/eeprom

对eeprom进行读写的backtrace如下：

#0  at24_eeprom_read_i2c (at24=0xbda6b18c, buf=0xbdbc4380 '177' <repeats 200 times>..., offset=896, count=3200) at drivers/misc/eeprom/at24.c:254
#1  0x802673fe in at24_read (priv=0xbda6b18c, off=3183231872, val=0x380, count=3200) at drivers/misc/eeprom/at24.c:519
#2  0x803847f0 in nvmem_reg_read (nvmem=<optimized out>, nvmem=<optimized out>, bytes=<optimized out>, val=<optimized out>, offset=<optimized out>) at drivers/nvmem/core.c:74
#3  bin_attr_nvmem_read (filp=<optimized out>, kobj=<optimized out>, attr=<optimized out>, buf=<optimized out>, pos=0, count=4096) at drivers/nvmem/core.c:114
#4  0x800fc0b2 in sysfs_kf_bin_read (of=<optimized out>, buf=<optimized out>, count=<optimized out>, pos=-9218947567704862720) at fs/sysfs/file.c:102
#5  0x800fb808 in kernfs_file_direct_read (ppos=<optimized out>, count=<optimized out>, user_buf=<optimized out>, of=<optimized out>) at fs/kernfs/file.c:214
#6  kernfs_fop_read (file=<optimized out>, user_buf=0xbdbc4380 '177' <repeats 200 times>..., count=<optimized out>, ppos=0xc80) at fs/kernfs/file.c:254
#7  0x800a9436 in __vfs_read (file=0xbda6b18c, buf=<optimized out>, count=<optimized out>, pos=0xc80) at fs/read_write.c:452
#8  0x800a9fe8 in vfs_read (file=0xbdb2ed20, buf=0xbdbc4380 '177' <repeats 200 times>..., count=262144, pos=0xbdb71f44) at fs/read_write.c:475
#9  0x800aad02 in SYSC_read (count=<optimized out>, buf=<optimized out>, fd=<optimized out>) at fs/read_write.c:591
#10 SyS_read (fd=<optimized out>, buf=717357064, count=262144) at fs/read_write.c:584
#11 0x80020b40 in csky_systemcall () at arch/csky/kernel/entry.S:154
#12 0x2ac20008 in ?? ()


#0  at24_eeprom_write_i2c (at24=0xbda6b18c, buf=0xbdb9a2c0 "177", offset=0, count=1) at drivers/misc/eeprom/at24.c:465
#1  0x8026739e in at24_write (priv=0xbda6b18c, off=3183059648, val=0x0, count=1) at drivers/misc/eeprom/at24.c:551
#2  0x80384898 in nvmem_reg_write (nvmem=<optimized out>, nvmem=<optimized out>, bytes=<optimized out>, val=<optimized out>, offset=<optimized out>) at drivers/nvmem/core.c:83
#3  bin_attr_nvmem_write (filp=<optimized out>, kobj=<optimized out>, attr=<optimized out>, buf=<optimized out>, pos=0, count=1) at drivers/nvmem/core.c:148
#4  0x800fc1a6 in sysfs_kf_bin_write (of=<optimized out>, buf=<optimized out>, count=<optimized out>, pos=-9218948675806429183) at fs/sysfs/file.c:163
#5  0x800fb706 in kernfs_fop_write (file=<optimized out>, user_buf=<optimized out>, count=1, ppos=0xbdb71f44) at fs/kernfs/file.c:316
#6  0x800a9506 in __vfs_write (file=0xbda6b18c, p=<optimized out>, count=<optimized out>, pos=0x1) at fs/read_write.c:510
#7  0x800aa106 in vfs_write (file=0xbdb2ed20, buf=0xbdb9a2c0 "177", count=<optimized out>, pos=0xbdb71f44) at fs/read_write.c:560
#8  0x800aad9e in SYSC_write (count=<optimized out>, buf=<optimized out>, fd=<optimized out>) at fs/read_write.c:607
#9  SyS_write (fd=<optimized out>, buf=49496, count=1) at fs/read_write.c:599
#10 0x80020b40 in csky_systemcall () at arch/csky/kernel/entry.S:154
#11 0x0000c158 in ?? ()

csky_systemcall()->SyS_read()->SYSC_read()->vfs_read()->__vfs_read()->kernfs_fop_read()->kernfs_file_direct_read()->sysfs_kf_bin_read()->bin_attr_nvmem_read()->nvmem_reg_read()->at24_read()->at24_eeprom_read_i2c()

其中kernfs_file_direct_read()对读的大小进行了限制：

static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
                       char __user *user_buf, size_t count,
                       loff_t *ppos)
{
    ssize_t len = min_t(size_t, count, PAGE_SIZE);-------------------------可以看出实际读大小取count和PAGE_SIZE小者。
...
    of->event = atomic_read(&of->kn->attr.open->event);
    ops = kernfs_ops(of->kn);
　　 if (ops->read)
　　　 　len = ops->read(of, buf, len, *ppos);
 　　else
　 　　　len = -EINVAL;
...

}

到at24这一层，at24_eeprom_read_i2c()又对read进行了限制，每次只能读取最大io_limit个字节。

在at24_read()中循环读取。

cky_systemcall()->SyS_write()->SYSC_write()->vfs_write()->__vfs_write()->kernfs_fop_write()->sysfs_kf_bin_write()->bin_attr_nvmem_write()->nvmem_reg_write()->at24_write()->at24_eeprom_write_i2c()

写实际上也限制了大小：

static ssize_t kernfs_fop_write(struct file *file, const char __user *user_buf,
                size_t count, loff_t *ppos)
{
...
    if (of->atomic_write_len) {
        len = count;
        if (len > of->atomic_write_len)
            return -E2BIG;
    } else {
        len = min_t(size_t, count, PAGE_SIZE);-----------------------------此处可以看出实际写大小取count和PAGE_SIZE小者。
    }
...
    ops = kernfs_ops(of->kn);
    if (ops->write)
        len = ops->write(of, buf, len, *ppos);
    else
        len = -EINVAL;
...
}

在at24_eeprom_write_i2c()中每次只能写入一个字节，at24_write()循环写入。

100KHz读256字节大概23.3ms，下面实际速度在22ms左右，符合预期。

[ 1105.099343] at24 1-0050: read 256@0 --> 256 (201274)
[ 1105.121371] at24 1-0050: read 256@256 --> 256 (201279)
[ 1105.143395] at24 1-0050: read 256@512 --> 256 (201285)
[ 1105.165428] at24 1-0050: read 256@768 --> 256 (201290)

400KHz读256字节大概在5.9ms，下面实际速度在6.25ms，符合预期。

[ 3616.209011] at24 3-0050: read 256@0 --> 256 (829051)
[ 3616.215264] at24 3-0050: read 256@256 --> 256 (829053)
[ 3616.221512] at24 3-0050: read 256@512 --> 256 (829054)
[ 3616.227760] at24 3-0050: read 256@768 --> 256 (829056)
[ 3616.234007] at24 3-0050: read 256@1024 --> 256 (829057)

在实际测试中，虽然上层希望读写一大块内容，但是底层对其进行了几次拆分。

实际结果可能会合期望有较大差别，这时候就需要跟踪读写流程。

参考文档：《I2C子系统之at24c02读写测试》