I2C总线架构之 I2C核心

作者: 拂去尘世尘 | 来源:发表于2020-08-21 08:47 被阅读0次

引言

在之前的 《I2C总线架构之设备驱动》 和 《I2C总线架构之总线驱动》 中一再提到i2c核心，本篇文章就总结一下i2c核心的主要功能。

分层思想

在之前的篇章中，大致能够了解i2c核心功能主要是为i2c设备驱动和i2c总线驱动的注册操作提供API。
这种设计的好处在于实现：高内聚，低耦合。i2c总线与i2c设备相互独立，互无联系；两者都是通过调用i2c核心提供的API实现匹配。即两者只与i2c核心有联系，这样也就可以实现一个固定i2c总线驱动可以和任一符合条件的设备驱动匹配。

具体API

这里对i2c总线驱动和i2c设备驱动进行分析，关键在于分析i2c_client与i2c_driver如何实现匹配的。

i2c总线驱动：

i2c适配器注册
在总线篇是利用 i2c_add_numbered_adapter注册到内核中：

 i2c_add_numbered_adapter --> __i2c_add_numbered_adapter --> i2c_register_adapter

static int i2c_register_adapter(struct i2c_adapter *adap) 
{
    /* 初始化链表节点 */
    INIT_LIST_HEAD(&adap->userspace_clients);
……
    adap->dev.bus = &i2c_bus_type;
    adap->dev.type = &i2c_adapter_type;
    res = device_register(&adap->dev);
……
    i2c_scan_static_board_info(adap);

    bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
……
}

(1) device_register注册设备

int device_register(struct device *dev)
{
    device_initialize(dev);
    return device_add(dev);
}

int device_add(struct device *dev) 
{
    bus_add_device(dev);
    bus_probe_device(dev);
}

int bus_add_device(struct device *dev)
{
    /* 将设备添加到总线链表中 */
    klist_add_tail(&dev->p->knode_bus, &bus->p->klist_devices);
}

void bus_probe_device(struct device *dev)
{ 
    if (bus->p->drivers_autoprobe)
        device_initial_probe(dev);

    list_for_each_entry(sif, &bus->p->interfaces, node)
    if (sif->add_dev) sif->add_dev(dev, sif);
}

将设备信息的链表成员添加到对应内核链表中，即将device资源注册到内核中。

void device_initial_probe(struct device *dev)
{
    __device_attach(dev, true);
}

static int __device_attach(struct device *dev, bool allow_async)
{
    bus_for_each_drv(dev->bus, NULL, &data, __device_attach_driver);
}

i2c_scan_static_board_info //扫描硬件信息，生成与adapter总线号相同的i2c_client设备

static void i2c_scan_static_board_info(struct i2c_adapter *adapter)
{
    list_for_each_entry(devinfo, &__i2c_board_list, list) { //遍历i2c链表
        if (devinfo->busnum == adapter->nr
                && !i2c_new_device(adapter,
                        &devinfo->board_info))
    }
}

struct i2c_client *
i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)
{
    client->adapter = adap; //i2c_client绑定adapter

    client->dev.parent = &client->adapter->dev;
    client->dev.bus = &i2c_bus_type;
    client->dev.type = &i2c_client_type;
    client->dev.of_node = info->of_node;
    client->dev.fwnode = info->fwnode;

    i2c_dev_set_name(adap, client);
    status = device_register(&client->dev);

    return client;

在生成一个新的i2c_client的同时，会将adapter与i2c_client绑定。然后将i2c_client注册到Linux。

bus_for_each_drv：匹配机制。遍历整个driver链表，进行i2c_client与i2c_driver的匹配工作

int bus_for_each_drv(struct bus_type *bus, struct device_driver *start,
             void *data, int (*fn)(struct device_driver *, void *))
{
    while ((drv = next_driver(&i)) && !error)
        error = fn(drv, data);
}

fn指向的参数为__process_new_adapter

static int __process_new_adapter(struct device_driver *d, void *data)
{
    return i2c_do_add_adapter(to_i2c_driver(d), data);
}

static int i2c_do_add_adapter(struct i2c_driver *driver,
                  struct i2c_adapter *adap)
{
    i2c_detect(adap, driver);
}

i2c_detect在设备驱动注册的时候也会使用到，这里也不过分追究，只需要知道其功能是为i2c_driver匹配合适的i2c_client。在文章后续i2c设备驱动会详细追一下这个函数。

总结：

总线驱动最终会调用i2c-core接口i2c_register_adapter，生成i2c_client设备，并且绑定总线号相同的adapter。
归根到底并不是对adapter的注册，而是对绑定过adapter的i2c_client的注册。由于adapter已经依附给i2c_client，所以可以通过注册过的i2c_client找到对应的adapter。
i2c_client的数据信息就是i2c硬件设备的抽象，其数据主要记录在设备树中:

&i2c2 {
    clock_frequency = <100000>;
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_i2c2>;
    status = "okay";

    sii902x: sii902x@39 {
          compatible = "SiI,sii902x";
          pinctrl-names = "default";
          reset-names="sii902x";
          pinctrl-0 = <&pinctrl_sii902x>;
          reg = <0x39>;
          status = "okay";    
         };

    gt9xx@5d {
         compatible = "goodix,gt9xx";
         reg = <0x5d>;
         status = "okay";
         interrupt-parent = <&gpio1>;
         interrupts = <5 IRQ_TYPE_EDGE_FALLING>;
         pinctrl-names = "default";
         pinctrl-0 = <&pinctrl_tsc_reset &pinctrl_touchscreen_int>;
    };
};

以上设备树代码记录i2c2总线下挂接的2个硬件设备信息；
linux在启动uboot的时候，会解析设备树，自动调用i2c_register_board_info函数将硬件设备注册进设备链表中。故在总线驱动注册时，会扫描设备链表域，并获取链表中的数据生成对应的与adapter总线相同的i2c_client。

在i2c_client生成完毕之后，就会开始遍历i2c_driver链表，查找是否与i2c_client相匹配的i2c_driver。如果有匹配成功，则最终会进入driver->probe。

i2c设备驱动：

设备驱动注册：
通过i2c_add_driver注册：

#define i2c_add_driver(driver) \
    i2c_register_driver(THIS_MODULE, driver)

int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
    driver->driver.owner = owner;
    driver->driver.bus = &i2c_bus_type;
      /* 初始化链表节点 */
    INIT_LIST_HEAD(&driver->clients);

    res = driver_register(&driver->driver);
    i2c_for_each_dev(driver, __process_new_driver);
}

看一下driver_registe如何注册驱动:

int driver_register(struct device_driver *drv)
{
    /* 检查总线上是否已经存在需要注册的驱动名，如果已经被注册，就会报错 */
    other = driver_find(drv->name, drv->bus);
    if (other) {
        printk(KERN_ERR "Error: Driver '%s' is already registered, "
            "aborting...\n", drv->name);
        return -EBUSY;
    }

    ret = bus_add_driver(drv);
    //ret = driver_add_groups(drv, drv->groups);
}

int bus_add_driver(struct device_driver *drv)
{
    klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);

    error = driver_attach(drv);
   /* 在 /sys 添加节点 */
    error = driver_create_file(drv, &driver_attr_uevent);
    error = driver_add_groups(drv, bus->drv_groups);
}

int driver_attach(struct device_driver *drv)
{
    return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);
}

int bus_for_each_dev(struct bus_type *bus, struct device *start,
             void *data, int (*fn)(struct device *, void *))
{
    while ((dev = next_device(&i)) && !error)
        error = fn(dev, data);
}

fn指向__driver_attach

static int __driver_attach(struct device *dev, void *data)
{
    /* 匹配driver与device */
    ret = driver_match_device(drv, dev);
    /* 匹配成功后，进入driver_probe_device */
    if (!dev->driver)
        driver_probe_device(drv, dev);
}

static inline int driver_match_device(struct device_driver *drv,
                      struct device *dev)
{
    /* 调用macth函数 */
    return drv->bus->match ? drv->bus->match(dev, drv) : 1;
}

这里调用i2c-core封装的接口

struct bus_type i2c_bus_type = {
    .name       = "i2c",
    .match      = i2c_device_match,
    .probe      = i2c_device_probe,
    .remove     = i2c_device_remove,
    .shutdown   = i2c_device_shutdown,
};

看下匹配函数：

static int i2c_device_match(struct device *dev, struct device_driver *drv)
{
    /* 通过id_table名进行匹配 */
    return i2c_match_id(driver->id_table, client) != NULL;
}

static const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,
                        const struct i2c_client *client)
{
    if (strcmp(client->name, id->name) == 0)
}

发现是通过client->name与id->name匹配。匹配成功后，再追driver_probe_device(drv, dev)

int driver_probe_device(struct device_driver *drv, struct device *dev)
{
    ret = really_probe(dev, drv);
}

static int really_probe(struct device *dev, struct device_driver *drv)
{
    if (dev->bus->probe) {
        ret = dev->bus->probe(dev);
    } else if (drv->probe) {
        ret = drv->probe(dev);
    }
}

struct bus_type i2c_bus_type = {
    .name       = "i2c",
    .match      = i2c_device_match,
    .probe      = i2c_device_probe,
    .remove     = i2c_device_remove,
    .shutdown   = i2c_device_shutdown,
};

static int i2c_device_probe(struct device *dev)
{
    status = driver->probe(client, i2c_match_id(driver->id_table, client));
}

小结： 通过以上分析，基本上可以了解到i2c_driver与i2c_client匹配机制的流程。在i2c_driver驱动注册时，会先将过of_device_id与client->name进行匹配，当发现匹配成功后，会进入driver->probe。

回到第一步，再看i2c_for_each_dev

int i2c_for_each_dev(void *data, int (*fn)(struct device *, void *))
{
    res = bus_for_each_dev(&i2c_bus_type, NULL, data, fn);
}

int bus_for_each_dev(struct bus_type *bus, struct device *start,
             void *data, int (*fn)(struct device *, void *))
{
    while ((dev = next_device(&i)) && !error)
        error = fn(dev, data);
}

fn指向__process_new_driver

static int __process_new_driver(struct device *dev, void *data)
{
    return i2c_do_add_adapter(data, to_i2c_adapter(dev));
}

static int i2c_do_add_adapter(struct i2c_driver *driver,
                  struct i2c_adapter *adap)
{
    i2c_detect(adap, driver);
}

static int i2c_detect(struct i2c_adapter *adapter, struct i2c_driver *driver)
{
    address_list = driver->address_list;
    temp_client->adapter = adapter;

    for (i = 0; address_list[i] != I2C_CLIENT_END; i += 1) {
        err = i2c_detect_address(temp_client, driver);
    }
}

static int i2c_detect_address(struct i2c_client *temp_client,
                  struct i2c_driver *driver)
{
    /* 检测是否忙状态 */
    i2c_check_addr_busy(adapter, addr);

    /* 说明i2c总线存在， 调用detect再次确认 */
    err = driver->detect(temp_client, &info);

    /* 确认完毕，生成i2c设备i2c_client ，并添加到链表中*/
    client = i2c_new_device(adapter, &info);
    list_add_tail(&client->detected, &driver->clients);
}

看似driver_register已经把注册的操作实现完了，i2c_for_each_dev似乎有点多余。其实i2c_for_each_dev是另一种注册方式，也可称为动态注册，在driver_register注册失败，就会启动此函数进行注册。其实现功能与driver_register相同，不同的是i2c_client的数据是存在驱动文件中，因此子函数的注册也需要i2c_driver的配合。

传输数据： i2c_transfer
此函数是设备驱动文件用来与i2c器件传输i2c协议数据的，封装好的i2c数据传输API。

int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
    ret = __i2c_transfer(adap, msgs, num);
}

int __i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
    unsigned long orig_jiffies;
    int ret, try;

    if (adap->quirks && i2c_check_for_quirks(adap, msgs, num))
        return -EOPNOTSUPP;

    /* i2c_trace_msg gets enabled when tracepoint i2c_transfer gets
     * enabled.  This is an efficient way of keeping the for-loop from
     * being executed when not needed.
     */
    if (static_key_false(&i2c_trace_msg)) {
        int i;
        for (i = 0; i < num; i++)
            if (msgs[i].flags & I2C_M_RD)
                trace_i2c_read(adap, &msgs[i], i);
            else
                trace_i2c_write(adap, &msgs[i], i);
    }

    /* Retry automatically on arbitration loss */
    orig_jiffies = jiffies;
    for (ret = 0, try = 0; try <= adap->retries; try++) {
        /*调用adapter下的i2c数据传输函数*/
        ret = adap->algo->master_xfer(adap, msgs, num); 
        if (ret != -EAGAIN)
            break;
        if (time_after(jiffies, orig_jiffies + adap->timeout))
            break;
    }

    if (static_key_false(&i2c_trace_msg)) {
        int i;
        for (i = 0; i < ret; i++)
            if (msgs[i].flags & I2C_M_RD)   
                trace_i2c_reply(adap, &msgs[i], i);
        trace_i2c_result(adap, i, ret);
    }

    return ret;
}

在之前i2c总线驱动中，能发现master_xfer指向的传输函数

static int i2c_imx_probe(struct platform_device *pdev)
{
    i2c_imx->adapter.algo       = &i2c_imx_algo;
}

static struct i2c_algorithm i2c_imx_algo = {
    .master_xfer    = i2c_imx_xfer,
    .functionality  = i2c_imx_func,
};

i2c_imx_xfer具体实现i2c数据传输的细节。这个就属于i2c字节传输的逻辑实现，不过多分析，代码很容易理解。

static int i2c_imx_xfer(struct i2c_adapter *adapter,
                        struct i2c_msg *msgs, int num)
{
    unsigned int i, temp;
    int result;
    bool is_lastmsg = false;
    bool enable_runtime_pm = false;
    struct imx_i2c_struct *i2c_imx = i2c_get_adapdata(adapter);

    dev_dbg(&i2c_imx->adapter.dev, "<%s>\n", __func__);


    if (!pm_runtime_enabled(i2c_imx->adapter.dev.parent)) {
        pm_runtime_enable(i2c_imx->adapter.dev.parent);
        enable_runtime_pm = true;
    }

    result = pm_runtime_get_sync(i2c_imx->adapter.dev.parent);
    if (result < 0)
        goto out;

    /* Start I2C transfer */
    result = i2c_imx_start(i2c_imx);
    if (result) {
        if (i2c_imx->adapter.bus_recovery_info) {
            i2c_recover_bus(&i2c_imx->adapter);
            result = i2c_imx_start(i2c_imx);
        }
    }

    if (result)
        goto fail0;

    /* read/write data */
    for (i = 0; i < num; i++) {
        if (i == num - 1)
            is_lastmsg = true;

        if (i) {
            dev_dbg(&i2c_imx->adapter.dev,
                "<%s> repeated start\n", __func__);
            temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
            temp |= I2CR_RSTA;
            imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
            result = i2c_imx_bus_busy(i2c_imx, 1);
            if (result)
                goto fail0;
        }
        dev_dbg(&i2c_imx->adapter.dev,
            "<%s> transfer message: %d\n", __func__, i);
        /* write/read data */
#ifdef CONFIG_I2C_DEBUG_BUS
        temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
        dev_dbg(&i2c_imx->adapter.dev,
            "<%s> CONTROL: IEN=%d, IIEN=%d, MSTA=%d, MTX=%d, TXAK=%d, RSTA=%d\n",
            __func__,
            (temp & I2CR_IEN ? 1 : 0), (temp & I2CR_IIEN ? 1 : 0),
            (temp & I2CR_MSTA ? 1 : 0), (temp & I2CR_MTX ? 1 : 0),
            (temp & I2CR_TXAK ? 1 : 0), (temp & I2CR_RSTA ? 1 : 0));
        temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR);
        dev_dbg(&i2c_imx->adapter.dev,
            "<%s> STATUS: ICF=%d, IAAS=%d, IBB=%d, IAL=%d, SRW=%d, IIF=%d, RXAK=%d\n",
            __func__,
            (temp & I2SR_ICF ? 1 : 0), (temp & I2SR_IAAS ? 1 : 0),
            (temp & I2SR_IBB ? 1 : 0), (temp & I2SR_IAL ? 1 : 0),
            (temp & I2SR_SRW ? 1 : 0), (temp & I2SR_IIF ? 1 : 0),
            (temp & I2SR_RXAK ? 1 : 0));
#endif
        if (msgs[i].flags & I2C_M_RD)
            result = i2c_imx_read(i2c_imx, &msgs[i], is_lastmsg);
        else {
            if (i2c_imx->dma && msgs[i].len >= DMA_THRESHOLD)
                result = i2c_imx_dma_write(i2c_imx, &msgs[i]);
            else
                result = i2c_imx_write(i2c_imx, &msgs[i]);
        }
        if (result)
            goto fail0;
    }

fail0:
    /* Stop I2C transfer */
    i2c_imx_stop(i2c_imx);

    pm_runtime_mark_last_busy(i2c_imx->adapter.dev.parent);
    pm_runtime_put_autosuspend(i2c_imx->adapter.dev.parent);

out:
    if (enable_runtime_pm)
        pm_runtime_disable(i2c_imx->adapter.dev.parent);

    dev_dbg(&i2c_imx->adapter.dev, "<%s> exit with: %s: %d\n", __func__,
        (result < 0) ? "error" : "success msg",
            (result < 0) ? result : num);
    return (result < 0) ? result : num;
}

由以上逻辑可以发现，不同平台提供的i2c总线驱动不一样，所以其具体的i2c数据传输的实现也会不一样，但是对于具体的i2c设备驱动是没有任何影响的。

总结：

对于i2c驱动核心的整个分析，就主要是这些。期间查阅了很多优质的博客以及内核代码，才有了本篇文章。此篇文章整理的还有些瑕疵，但是整体的逻辑流程基本上是完整的，如有错误，欢迎指出！

参考:
https://blog.csdn.net/ordinaryjoe/article/details/6678871
https://blog.csdn.net/chihunqi5879/article/details/79971034
https://blog.csdn.net/weixin_42462202/article/details/91128914
代码路径：
linux/include/linux/i2c.h
linux/driver/i2c
等

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I2C总线架构之 I2C核心

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