Linux I2C驱动完全分析（二）
版权声明：本文为博主原创文章，未经博主允许不得转载。
博主按：大热的天，刚刚负重从五道口走到石板房，大约4公里吧。终于让我找了一个咖啡屋休息一下，继续写这篇驱动分析。单身的生活就是这样无聊啊。 不发牢骚了，活出个样儿来给自己看！千难万险脚下踩，啥也难不倒咱！继续整！~
先说一下，本文中有个疑惑，一直没有搞懂，写在这里，望高人指点一二，不胜感激！
#define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */
这里I2C_FUNC_PROTOCOL_MANGLING 是什么意思？为什么定义这些东东？看了注释也不太理解。求解释！
3. I2C总线驱动代码分析
s3c2440的总线驱动代码在i2c-s3c2410.c中。照例先从init看起。
- static int __init i2c_adap_s3c_init(void)
- {
- return platform_driver_register(&s3c24xx_i2c_driver);
- }
在init中只是调用了平台驱动注册函数注册了一个i2c的平台驱动s3c24xx_i2c_driver。这个驱动是一个platform_driver的结构体变量。注意这里不是i2c_driver结构体，因为i2c_driver是对设备的驱动，而这里对控制器的驱动要使用platform_driver
- static struct platform_driver s3c24xx_i2c_driver = {
- .probe = s3c24xx_i2c_probe,
- .remove = s3c24xx_i2c_remove,
- .suspend_late = s3c24xx_i2c_suspend_late,
- .resume = s3c24xx_i2c_resume,
- .id_table = s3c24xx_driver_ids,
- .driver = {
- .owner = THIS_MODULE,
- .name = "s3c-i2c",
- },
- };
同样的，重要的函数还是那几个：probe，remove，suspend_late，resume。再加上一个id_table和device_driver结构体变量。
下面逐个分析：
* probe函数
当调用platform_driver_register函数注册platform_driver结构体时，probe指针指向的s3c24xx_i2c_probe函数将会被调用。这部分详细解释参考本博客另一篇文章《S3C2410看门狗驱动分析》。细心的朋友可能会发现，在s3c24xx_i2c_driver中，驱动的名字是"s3c-i2c"，而在板文件中可以看到，设备的名字是"s3c2410-i2c"，这两个名字不一样，那驱动和设备是如何match的呢？答案就在于id_table。这个id_table包含了驱动所支持的设备ID表。在match的时候，判断这个表中的名字是不是和设备一致，一致则match成功。这也是为什么一个驱动可以同时match成功多个设备的原因。如果只是靠platform_driver-->driver中的名字来匹配的话，那么驱动和设备只能是一对一的关系了。
- static struct platform_device_id s3c24xx_driver_ids[] = {
- {
- .name = "s3c2410-i2c",
- .driver_data = TYPE_S3C2410,
- }, {
- .name = "s3c2440-i2c",
- .driver_data = TYPE_S3C2440,
- }, { },
- };
扯远了，还是看看probe的代码吧~
- static int s3c24xx_i2c_probe(struct platform_device *pdev)
- {
- struct s3c24xx_i2c *i2c;
- struct s3c2410_platform_i2c *pdata;
- struct resource *res;
- int ret;
- pdata = pdev->dev.platform_data;
- if (!pdata) {
- dev_err(&pdev->dev, "no platform data/n");
- return -EINVAL;
- }
- //给s3c24xx_i2c结构体申请空间
- i2c = kzalloc(sizeof(struct s3c24xx_i2c), GFP_KERNEL);
- if (!i2c) {
- dev_err(&pdev->dev, "no memory for state/n");
- return -ENOMEM;
- }
- //填充s3c24xx_i2c结构体中各项，包括名称、所有者、算法、所属class等等
- strlcpy(i2c->adap.name, "s3c2410-i2c", sizeof(i2c->adap.name));
- i2c->adap.owner = THIS_MODULE;
- i2c->adap.algo = &s3c24xx_i2c_algorithm; //这个下面会重点介绍
- i2c->adap.retries = 2;
- i2c->adap.class = I2C_CLASS_HWMON | I2C_CLASS_SPD;
- i2c->tx_setup = 50;
- spin_lock_init(&i2c->lock);
- init_waitqueue_head(&i2c->wait);
- /* find the clock and enable it */
- // 找到i2c始终并且使能它
- i2c->dev = &pdev->dev;
- i2c->clk = clk_get(&pdev->dev, "i2c");
- if (IS_ERR(i2c->clk)) {
- dev_err(&pdev->dev, "cannot get clock/n");
- ret = -ENOENT;
- goto err_noclk;
- }
- dev_dbg(&pdev->dev, "clock source %p/n", i2c->clk);
- clk_enable(i2c->clk);
- /* map the registers */
- /*映射寄存器*/
- res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
- if (res == NULL) {
- dev_err(&pdev->dev, "cannot find IO resource/n");
- ret = -ENOENT;
- goto err_clk;
- }
- i2c->ioarea = request_mem_region(res->start, resource_size(res),
- pdev->name);
- if (i2c->ioarea == NULL) {
- dev_err(&pdev->dev, "cannot request IO/n");
- ret = -ENXIO;
- goto err_clk;
- }
- i2c->regs = ioremap(res->start, resource_size(res));
- if (i2c->regs == NULL) {
- dev_err(&pdev->dev, "cannot map IO/n");
- ret = -ENXIO;
- goto err_ioarea;
- }
- dev_dbg(&pdev->dev, "registers %p (%p, %p)/n",
- i2c->regs, i2c->ioarea, res);
- /* setup info block for the i2c core */
- i2c->adap.algo_data = i2c;
- i2c->adap.dev.parent = &pdev->dev;
- /* initialise the i2c controller */
- /*s3c24xx_i2c结构体变量i2c的必要的信息都填充完了以后，开始进行初始化*/
- ret = s3c24xx_i2c_init(i2c);
- if (ret != 0)
- goto err_iomap;
- /* find the IRQ for this unit (note, this relies on the init call to
- * ensure no current IRQs pending
- */
- //接下来申请中断
- i2c->irq = ret = platform_get_irq(pdev, 0);
- if (ret <= 0) {
- dev_err(&pdev->dev, "cannot find IRQ/n");
- goto err_iomap;
- }
- ret = request_irq(i2c->irq, s3c24xx_i2c_irq, IRQF_DISABLED,
- dev_name(&pdev->dev), i2c);
- if (ret != 0) {
- dev_err(&pdev->dev, "cannot claim IRQ %d/n", i2c->irq);
- goto err_iomap;
- }
- ret = s3c24xx_i2c_register_cpufreq(i2c);
- if (ret < 0) {
- dev_err(&pdev->dev, "failed to register cpufreq notifier/n");
- goto err_irq;
- }
- /* Note, previous versions of the driver used i2c_add_adapter()
- * to add the bus at any number. We now pass the bus number via
- * the platform data, so if unset it will now default to always
- * being bus 0.
- */
- i2c->adap.nr = pdata->bus_num;
- //看到了吧？下面调用了i2c-core中的i2c_add_adapter函数来添加一个i2c控制器
- //i2c_add_numbered_adapter和i2c_add_adapter的区别在于前者用来添加一个在CPU内
- //部集成的适配器，而后者用来添加一个CPU外部的适配器。显然这里应该用前者。
- ret = i2c_add_numbered_adapter(&i2c->adap);
- if (ret < 0) {
- dev_err(&pdev->dev, "failed to add bus to i2c core/n");
- goto err_cpufreq;
- }
- platform_set_drvdata(pdev, i2c);
- dev_info(&pdev->dev, "%s: S3C I2C adapter/n", dev_name(&i2c->adap.dev));
- return 0;
- err_cpufreq:
- s3c24xx_i2c_deregister_cpufreq(i2c);
- err_irq:
- free_irq(i2c->irq, i2c);
- err_iomap:
- iounmap(i2c->regs);
- err_ioarea:
- release_resource(i2c->ioarea);
- kfree(i2c->ioarea);
- err_clk:
- clk_disable(i2c->clk);
- clk_put(i2c->clk);
- err_noclk:
- kfree(i2c);
- return ret;
- }
*remove函数
这是和probe相反的一个函数，在i2c_adap_s3c_exit时调用。主要功能是注销适配器，释放中断，释放内存区域，禁止始终等等。看到上边代码中的err_的各个部分了吧？ remove是它们的汇总。
*suspend函数和resume函数
把这两个放一起说吧，挂起和恢复函数。挂起时保存状态并置标志位，恢复时重新初始化i2c适配器并置标志位。
Algorithm
哎呀我去，终于到这了。憋得我难受啊。这里要重点介绍一下，不仅要知其然，还要知其所以然，这样我们以后自己写驱动的时候就有把握了。
- static const struct i2c_algorithm s3c24xx_i2c_algorithm = {
- .master_xfer = s3c24xx_i2c_xfer,
- .functionality = s3c24xx_i2c_func,
- };
这里实现的就是这个s3c24xx_i2c_xfer。这个是控制器能不能动作的关键，缺了这个，控制器就是废铜烂铁。
- static int s3c24xx_i2c_xfer(struct i2c_adapter *adap,
- struct i2c_msg *msgs, int num)
- {
- struct s3c24xx_i2c *i2c = (struct s3c24xx_i2c *)adap->algo_data;
- int retry;
- int ret;
- for (retry = 0; retry < adap->retries; retry++) {
- ret = s3c24xx_i2c_doxfer(i2c, msgs, num);
- if (ret != -EAGAIN)
- return ret;
- dev_dbg(i2c->dev, "Retrying transmission (%d)/n", retry);
- udelay(100);
- }
- return -EREMOTEIO;
- }
完成任务的函数是s3c24xx_i2c_doxfer()，源码清单如下，
- static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
- struct i2c_msg *msgs, int num)
- {
- unsigned long timeout;
- int ret;
- if (i2c->suspended)
- return -EIO;
- ret = s3c24xx_i2c_set_master(i2c);
- if (ret != 0) {
- dev_err(i2c->dev, "cannot get bus (error %d)/n", ret);
- ret = -EAGAIN;
- goto out;
- }
- spin_lock_irq(&i2c->lock);
- i2c->msg = msgs;
- i2c->msg_num = num;
- i2c->msg_ptr = 0;
- i2c->msg_idx = 0;
- i2c->state = STATE_START;
- s3c24xx_i2c_enable_irq(i2c);
- s3c24xx_i2c_message_start(i2c, msgs);
- spin_unlock_irq(&i2c->lock);
- timeout = wait_event_timeout(i2c->wait, i2c->msg_num == 0, HZ * 5);
- ret = i2c->msg_idx;
- /* having these next two as dev_err() makes life very
- * noisy when doing an i2cdetect */
- if (timeout == 0)
- dev_dbg(i2c->dev, "timeout/n");
- else if (ret != num)
- dev_dbg(i2c->dev, "incomplete xfer (%d)/n", ret);
- /* ensure the stop has been through the bus */
- msleep(1);
- out:
- return ret;
- }
上面代码可以分成几个部分来看：
* s3c24xx_i2c_set_master() 这个函数每隔1ms查看一次i2c总线状态，timeout是400ms，如果在这期间总线状态不忙，则返回零。否则返回-ETIMEDOUT
* 将要发送的消息和其他信息付给i2c->msg和其他变量，并将状态设置为STATE_START
* s3c24xx_i2c_enable_irq() 使能中断
* s3c24xx_i2c_message_start() 重中之重啊。在看代码之前先来看看2440的datasheet上是怎么说的吧。
代码清单如下：
- static void s3c24xx_i2c_message_start(struct s3c24xx_i2c *i2c,
- struct i2c_msg *msg)
- {
- unsigned int addr = (msg->addr & 0x7f) << 1;
- unsigned long stat;
- unsigned long iiccon;
- stat = 0;
- stat |= S3C2410_IICSTAT_TXRXEN;
- if (msg->flags & I2C_M_RD) {//如果是read data, from slave to master stat |= S3C2410_IICSTAT_MASTER_RX;
- addr |= 1;
- } else
- stat |= S3C2410_IICSTAT_MASTER_TX;
- if (msg->flags & I2C_M_REV_DIR_ADDR)
- addr ^= 1;
- /* todo - check for wether ack wanted or not */
- s3c24xx_i2c_enable_ack(i2c);
- iiccon = readl(i2c->regs + S3C2410_IICCON);
- writel(stat, i2c->regs + S3C2410_IICSTAT);
- dev_dbg(i2c->dev, "START: %08lx to IICSTAT, %02x to DS/n", stat, addr);
- writeb(addr, i2c->regs + S3C2410_IICDS);
- /* delay here to ensure the data byte has gotten onto the bus
- * before the transaction is started */
- ndelay(i2c->tx_setup);
- dev_dbg(i2c->dev, "iiccon, %08lx/n", iiccon);
- writel(iiccon, i2c->regs + S3C2410_IICCON);
- stat |= S3C2410_IICSTAT_START;
- writel(stat, i2c->regs + S3C2410_IICSTAT);
- }
（今天没写完啊，明天继续~）
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猜你在找
这几个宏定义是GPIO模拟i2c总线算法实现用的
Document/i2c/i2c-protocol
文件找到说明。