Linux I2C驅動完全分析（二）

博主按：大熱的天，剛剛負重從五道口走到石板房，大約4公里吧。終於讓我找了一個咖啡屋休息一下，繼續寫這篇驅動分析。單身的生活就是這樣無聊啊。 不發牢騷了，活出個樣兒來給自己看！千難萬險腳下踩，啥也難不倒咱！繼續整！~

 

先說一下，本文中有個疑惑，一直沒有搞懂，寫在這里，望高人指點一二，不勝感激！

#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 */

這里I2C_FUNC_PROTOCOL_MANGLING 是什么意思？為什么定義這些東東？看了注釋也不太理解。求解釋！

 

3. I2C總線驅動代碼分析

   s3c2440的總線驅動代碼在i2c-s3c2410.c中。照例先從init看起。

static int __init i2c_adap_s3c_init(void) { return platform_driver_register(&s3c24xx_i2c_driver); }

在init中只是調用了平台驅動注冊函數注冊了一個i2c的平台驅動s3c24xx_i2c_driver。這個驅動是一個platform_driver的結構體變量。注意這里不是i2c_driver結構體，因為i2c_driver是對設備的驅動，而這里對控制器的驅動要使用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", }, };

同樣的，重要的函數還是那幾個：probe，remove，suspend_late，resume。再加上一個id_table和device_driver結構體變量。

下面逐個分析：

* probe函數

當調用platform_driver_register函數注冊platform_driver結構體時，probe指針指向的s3c24xx_i2c_probe函數將會被調用。這部分詳細解釋參考本博客另一篇文章《S3C2410看門狗驅動分析》。細心的朋友可能會發現，在s3c24xx_i2c_driver中，驅動的名字是"s3c-i2c"，而在板文件中可以看到，設備的名字是"s3c2410-i2c"，這兩個名字不一樣，那驅動和設備是如何match的呢？答案就在於id_table。這個id_table包含了驅動所支持的設備ID表。在match的時候，判斷這個表中的名字是不是和設備一致，一致則match成功。這也是為什么一個驅動可以同時match成功多個設備的原因。如果只是靠platform_driver-->driver中的名字來匹配的話，那么驅動和設備只能是一對一的關系了。

static struct platform_device_id s3c24xx_driver_ids[] = { { .name = "s3c2410-i2c", .driver_data = TYPE_S3C2410, }, { .name = "s3c2440-i2c", .driver_data = TYPE_S3C2440, }, { }, };

扯遠了，還是看看probe的代碼吧~

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; } //給s3c24xx_i2c結構體申請空間 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結構體中各項，包括名稱、所有者、算法、所屬class等等 strlcpy(i2c->adap.name, "s3c2410-i2c", sizeof(i2c->adap.name)); i2c->adap.owner = THIS_MODULE; i2c->adap.algo = &s3c24xx_i2c_algorithm; //這個下面會重點介紹 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始終並且使能它 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結構體變量i2c的必要的信息都填充完了以后，開始進行初始化*/ 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 */ //接下來申請中斷 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？下面調用了i2c-core中的i2c_add_adapter函數來添加一個i2c控制器 //i2c_add_numbered_adapter和i2c_add_adapter的區別在於前者用來添加一個在CPU內 //部集成的適配器，而后者用來添加一個CPU外部的適配器。顯然這里應該用前者。 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函數

這是和probe相反的一個函數，在i2c_adap_s3c_exit時調用。主要功能是注銷適配器，釋放中斷，釋放內存區域，禁止始終等等。看到上邊代碼中的err_的各個部分了吧？ remove是它們的匯總。

 

*suspend函數和resume函數

把這兩個放一起說吧，掛起和恢復函數。掛起時保存狀態並置標志位，恢復時重新初始化i2c適配器並置標志位。

 

Algorithm

 哎呀我去，終於到這了。憋得我難受啊。這里要重點介紹一下，不僅要知其然，還要知其所以然，這樣我們以后自己寫驅動的時候就有把握了。

static const struct i2c_algorithm s3c24xx_i2c_algorithm = { .master_xfer = s3c24xx_i2c_xfer, .functionality = s3c24xx_i2c_func, };

這里實現的就是這個s3c24xx_i2c_xfer。這個是控制器能不能動作的關鍵，缺了這個，控制器就是廢銅爛鐵。

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; }

 

完成任務的函數是s3c24xx_i2c_doxfer()，源碼清單如下，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; }

上面代碼可以分成幾個部分來看：

  * s3c24xx_i2c_set_master() 這個函數每隔1ms查看一次i2c總線狀態，timeout是400ms，如果在這期間總線狀態不忙，則返回零。否則返回-ETIMEDOUT

  * 將要發送的消息和其他信息付給i2c->msg和其他變量，並將狀態設置為STATE_START

  * s3c24xx_i2c_enable_irq() 使能中斷

  * s3c24xx_i2c_message_start() 重中之重啊。在看代碼之前先來看看2440的datasheet上是怎么說的吧。

代碼清單如下：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); }

 

（今天沒寫完啊，明天繼續~）