OMAP3630 Linux I2C总线驱动分析(1)

1 Linux I2C驱动架构
Linux下I2C驱动的架构图如下：

 
图1.1 Linux下I2C驱动架构

    如上图所示，每条I2C总线会对应一个adapter，而每条I2C总线上则可以有多个 client，在linux kernel中，通过I2C core层将I2C client与I2C adapter关联起来，Linux 中I2C驱动代码位于drivers/i2c目录。
    Linux中I2C可以分为三个层次，分别为I2C core层、I2C adapter driver层、I2C device driver层。

 

1.1 I2C core层
    I2C core是用于维护Linux的I2C核心部分，提供了核心的数据结构，I2C适配器驱动和设备驱动的注册、注销管理等API，同时还提供了I2C总线读写访问的一般接口（具体的实现在与I2C控制器相关的I2C adapter中实现）。
    该层为硬件平台无关层，向下屏蔽了物理总线适配器的差异，定义了统一的访问策略和接口；向上则提供了统一的接口，以便I2C设备驱动可以通过总线适配器进行数据收发。
    Linux中，I2C core层的代码位于driver/i2c/ i2c-core.c。由于该层是平台无关层，本文将不再叙述，有兴趣可以查阅相关资料。

 

1.2 I2C adapter driver层
    I2C adapter driver层即I2C适配器驱动层，每种处理器平台都有自己的适配器驱动，属于平台移植相关层。它的职责是为系统中每条I2C总线实现相应的读写方法。但是适配器驱动本身并不会进行任何的通讯，而是等待设备驱动调用其函数。
    在系统开机时，I2C适配器驱动被首先装载。一个适配器驱动用于支持一条特定的I2C总线的读写。一个适配器驱动通常需要两个模块，一个struct i2c_adapter和一个struct i2c_algorithm来描述。
    i2c adapter 构造一个对I2C core层接口的数据结构，并通过相应的接口函数向I2C core注册一个适配器。i2c_algorithm主要实现对I2C总线访问的算法，master_xfer和smbus_xfer即I2C adapter底层对I2C总线读写方法的实现，相关的数据结构如下：

/* * The following structs are for those who like to implement new bus drivers: * i2c_algorithm is the interface to a class of hardware solutions which can * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584 * to name two of the most common. */ struct i2c_algorithm { /* If an adapter algorithm can't do I2C-level access, set master_xfer to NULL. If an adapter algorithm can do SMBus access, set smbus_xfer. If set to NULL, the SMBus protocol is simulated using common I2C messages */ /* master_xfer should return the number of messages successfully processed, or a negative value on error */ int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs, int num); int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr, unsigned short flags, char read_write, u8 command, int size, union i2c_smbus_data *data); /* To determine what the adapter supports */ u32 (*functionality) (struct i2c_adapter *); };

    主要就是master_xfer方法，其和具体的总线控制器相关，不同的CPU在实现上会有差异。

/* * i2c_adapter is the structure used to identify a physical i2c bus along * with the access algorithms necessary to access it. */ struct i2c_adapter { struct module *owner; unsigned int id; unsigned int class; /* classes to allow probing for */ const struct i2c_algorithm *algo; /* the algorithm to access the bus */ void *algo_data; /* data fields that are valid for all devices */ struct rt_mutex bus_lock; int timeout; /* in jiffies */ int retries; struct device dev; /* the adapter device */ int nr; char name[48]; struct completion dev_released; struct list_head userspace_clients; };

    Algo是和底层硬件的接口，标识了具体的物理总线传输的实现。
    Userspace_clients为使用该总线的client链表。
    Nr为该适配器也就是某条I2C总线占据的全局编号。
    bus_lock总线的互斥锁，防止总线冲突。
    Linux中，I2C adapter driver层的代码位于drivers/i2c/busses目录，第3章会详细介绍该层的内容。

1.3 I2C device driver层
    I2C device driver层为用户接口层，其为用户提供了通过I2C总线访问具体设备的接口。
    I2C的device driver层可以用两个模块来描述，struct i2c_driver和struct i2c_client。
    i2c_client和i2c_driver分别构造对I2C core层接口的数据结构，并且通过相关的接口函数向 I2C Core注册I2C设备驱动。相关的数据结构如下：

 /** * struct i2c_driver - represent an I2C device driver * @class: What kind of i2c device we instantiate (for detect) * @attach_adapter: Callback for bus addition (for legacy drivers) * @detach_adapter: Callback for bus removal (for legacy drivers) * @probe: Callback for device binding * @remove: Callback for device unbinding * @shutdown: Callback for device shutdown * @suspend: Callback for device suspend * @resume: Callback for device resume * @command: Callback for bus-wide signaling (optional) * @driver: Device driver model driver * @id_table: List of I2C devices supported by this driver * @detect: Callback for device detection * @address_list: The I2C addresses to probe (for detect) * @clients: List of detected clients we created (for i2c-core use only) * * The driver.owner field should be set to the module owner of this driver. * The driver.name field should be set to the name of this driver. * * For automatic device detection, both @detect and @address_data must * be defined. @class should also be set, otherwise only devices forced * with module parameters will be created. The detect function must * fill at least the name field of the i2c_board_info structure it is * handed upon successful detection, and possibly also the flags field. * * If @detect is missing, the driver will still work fine for enumerated * devices. Detected devices simply won't be supported. This is expected * for the many I2C/SMBus devices which can't be detected reliably, and * the ones which can always be enumerated in practice. * * The i2c_client structure which is handed to the @detect callback is * not a real i2c_client. It is initialized just enough so that you can * call i2c_smbus_read_byte_data and friends on it. Don't do anything * else with it. In particular, calling dev_dbg and friends on it is * not allowed. */ struct i2c_driver { unsigned int class; /* Notifies the driver that a new bus has appeared or is about to be * removed. You should avoid using this if you can, it will probably * be removed in a near future. */ int (*attach_adapter)(struct i2c_adapter *); int (*detach_adapter)(struct i2c_adapter *); /* Standard driver model interfaces */ int (*probe)(struct i2c_client *, const struct i2c_device_id *); int (*remove)(struct i2c_client *); /* driver model interfaces that don't relate to enumeration */ void (*shutdown)(struct i2c_client *); int (*suspend)(struct i2c_client *, pm_message_t mesg); int (*resume)(struct i2c_client *); /* Alert callback, for example for the SMBus alert protocol. * The format and meaning of the data value depends on the protocol. * For the SMBus alert protocol, there is a single bit of data passed * as the alert response's low bit ("event flag"). */ void (*alert)(struct i2c_client *, unsigned int data); /* a ioctl like command that can be used to perform specific functions * with the device. */ int (*command)(struct i2c_client *client, unsigned int cmd, void *arg); struct device_driver driver; const struct i2c_device_id *id_table; /* Device detection callback for automatic device creation */ int (*detect)(struct i2c_client *, struct i2c_board_info *); const unsigned short *address_list; struct list_head clients; };

    Driver是为device服务的，i2c_driver注册时会扫描i2c bus上的设备，进行驱动和设备的绑定。主要有两种接口attach_adapter和probe，二者分别针对旧的和新式的驱动。

 /** * struct i2c_client - represent an I2C slave device * @flags: I2C_CLIENT_TEN indicates the device uses a ten bit chip address; * I2C_CLIENT_PEC indicates it uses SMBus Packet Error Checking * @addr: Address used on the I2C bus connected to the parent adapter. * @name: Indicates the type of the device, usually a chip name that's * generic enough to hide second-sourcing and compatible revisions. * @adapter: manages the bus segment hosting this I2C device * @driver: device's driver, hence pointer to access routines * @dev: Driver model device node for the slave. * @irq: indicates the IRQ generated by this device (if any) * @detected: member of an i2c_driver.clients list or i2c-core's * userspace_devices list * * An i2c_client identifies a single device (i.e. chip) connected to an * i2c bus. The behaviour exposed to Linux is defined by the driver * managing the device. */ struct i2c_client { unsigned short flags; /* div., see below */ unsigned short addr; /* chip address - NOTE: 7bit */ /* addresses are stored in the */ /* _LOWER_ 7 bits */ char name[I2C_NAME_SIZE]; struct i2c_adapter *adapter; /* the adapter we sit on */ struct i2c_driver *driver; /* and our access routines */ struct device dev; /* the device structure */ int irq; /* irq issued by device */ struct list_head detected; };

    通常来说i2c_client对应着I2C总线上某个特定的slave或者是user space的某个用户对应，而此时的slave可以动态变化。
    Linux中，I2C device driver层的代码位于drivers/i2c/chips目录，第4章将详细介绍该层的内容。

 
2 OMAP3630 I2C控制器

    OMAP3630具有4个高速I2C控制器，每个控制器都通过I2C串行总线为本地主机即OAMP3630 MPU和I2C总线兼容设备提供了一个通讯接口，支持多达8-bit的数据传送和接收。
    每个I2C控制器都能配置成一个主机或者从机设备，而且他们都能配置成在一个2线的串行的摄像头控制总线（SCCB总线）上作为主设备，I2C2和I2C3还能配置成在一个3线的SCCB总线上作为主设备。
    I2C4控制器位于PRCM模块，可以进行动态电压控制和电源序列测定。
    OMAP3630的I2C控制器模块图如下：

 

 图2.1 OMAP3630 I2C控制器模块图

 
控制器1，2，3具有以下特征：
 兼容飞利浦I2C 2.1版本
 支持标准I2C标准模式（100Kbps）和快速模式（400Kpbs）
 支持高达3.4Mbps的高速发送模式
 支持I2C2和I2C3 模块的3线/2线的SCCB主从模式，I2C1 模块的2线的SCCB主从模式，高达100kbit/s
 7-bit和10bit的设备地址模式
 多主控发送/从接收模式
 多主控接收/从发送模式
 联合的主机发送/接收和接收/发送模式
 内置FIFO（8，16，32，64字节大小）用于缓存读取和接收
 模块使能/关闭
 可编程的时钟
 8-bit的数据存取
 低功耗的设计
 两个DMA通道
 支持中断机制
 自动空闲机制
 空闲请求和应答握手机制
主从的发送机I2C4控制器有以下特征：
 支持高速和快速模式
 只能支持7-bit地址模式
 只支持主发送模式
关于I2C控制器的详细介绍请参考OMAP36XX_ES1.1_NDA_TRM_V_G.pdf的第17章。

 

3 OMAP3630 I2C adapter驱动
    在Linux内核中，I2C adapter驱动位于drivers/i2c/busses目录下，OMAP3630 的I2C adapter驱动程序为i2c-omap.c。
    I2C adapter驱动，本质上就是实现了具体的总线传输算法并向核心层注册适配器。该驱动的注册采用Platform驱动和设备机制。
3.1 I2C adapter的Platform device
    Andrord 2.1中Platform device的注册的代码位于内核的arch/arm/plat-omap/i2c.c，arch/arm/mach-omap2/board-xxxx.c中。
3.1.1 Platform device的定义
    在文件arch/arm/plat-omap/i2c.c中，Platform device定义如下：

#define OMAP_I2C_SIZE 0x3f #define OMAP1_I2C_BASE 0xfffb3800 #define OMAP2_I2C_BASE1 0x48070000 #define OMAP2_I2C_BASE2 0x48072000 #define OMAP2_I2C_BASE3 0x48060000 static const char name[] = "i2c_omap"; #define I2C_RESOURCE_BUILDER(base, irq) / { / .start = (base), / .end = (base) + OMAP_I2C_SIZE, / .flags = IORESOURCE_MEM, / }, / { / .start = (irq), / .flags = IORESOURCE_IRQ, / }, static struct resource i2c_resources[][2] = { { I2C_RESOURCE_BUILDER(0, 0) }, #if defined(CONFIG_ARCH_OMAP24XX) || defined(CONFIG_ARCH_OMAP34XX) { I2C_RESOURCE_BUILDER(OMAP2_I2C_BASE2, INT_24XX_I2C2_IRQ) }, #endif #if defined(CONFIG_ARCH_OMAP34XX) { I2C_RESOURCE_BUILDER(OMAP2_I2C_BASE3, INT_34XX_I2C3_IRQ) }, #endif }; #define I2C_DEV_BUILDER(bus_id, res, data) / { / .id = (bus_id), / .name = name, / .num_resources = ARRAY_SIZE(res), / .resource = (res), / .dev = { / .platform_data = (data), / }, / } static u32 i2c_rate[ARRAY_SIZE(i2c_resources)]; static struct platform_device omap_i2c_devices[] = { I2C_DEV_BUILDER(1, i2c_resources[0], &i2c_rate[0]), #if defined(CONFIG_ARCH_OMAP24XX) || defined(CONFIG_ARCH_OMAP34XX) I2C_DEV_BUILDER(2, i2c_resources[1], &i2c_rate[1]), #endif #if defined(CONFIG_ARCH_OMAP34XX) I2C_DEV_BUILDER(3, i2c_resources[2], &i2c_rate[2]), #endif };

     可以看到，这边定义了三个I2C适配器的Platform device，id分别为“1，2，3”，name都为“i2c_omap”，变量resource中定义了适配器的寄存器基地址，irq中断号等。
3.1.2 Platform device的注册
    Platform device的注册是由内核启动后，具体产品的板级初始化完成的。xxxx项目的I2C　adapter的Platform device注册过程如下图：

图3.1 Platform device注册过程

 
    函数omap_i2c_add_bus()中，通过函数platform_device_register()注册Platform device到platform bus上，代码如下：

static int __init omap_i2c_add_bus(int bus_id) { struct platform_device *pdev; struct resource *res; resource_size_t base, irq; …… …… return platform_device_register(pdev); }

    注册完成后，中断号及寄存器的基地址等信息会在设备树中描述了，此后只需利用platform_get_resource等标准接口自动获取即可，实现了驱动和资源的分离。
3.2 I2C adapter的Platform driver
    Andrord 2.1中Platform driver的注册的代码位于内核的drivers/i2c/busses/ i2c-omap.c中，该驱动的注册目的是初始化OMAP3630的I2C adapter，提供I2C总线传输的具体实现，并且向I2C core注册I2C adapter。
3.2.1 Platform driver的定义
    在文件drivers/i2c/busses/ i2c-omap.c中，platform driver定义如下：

static struct platform_driver omap_i2c_driver = { .probe = omap_i2c_probe, .remove = omap_i2c_remove, .driver = { .name = "i2c_omap", .owner = THIS_MODULE, }, };

3.2.2 Platform driver的注册
    在文件drivers/i2c/busses/ i2c-omap.c中，platform driver注册如下：

/* I2C may be needed to bring up other drivers */ static int __init omap_i2c_init_driver(void) { return platform_driver_register(&omap_i2c_driver); } subsys_initcall(omap_i2c_init_driver);

    通过platform_driver_register()函数注册Platform driver omap_i2c_driver时，会扫描platform bus上的所有设备，由于匹配因子是name即"i2c_omap"，而之前已经将name为"i2c_omap"的Platform device注册到platform bus上，因此匹配成功，调用函数omap_i2c_probe将设备和驱动绑定起来。
    在drivers/i2c/busses/ i2c-omap.c中会涉及到一个数据结构omap_i2c_dev，这个结构定义了omap3630的I2C控制器，结构如下：

struct omap_i2c_dev { struct device *dev; void __iomem *base; /* virtual */ int irq; struct clk *iclk; /* Interface clock */ struct clk *fclk; /* Functional clock */ struct completion cmd_complete; struct resource *ioarea; u32 speed; /* Speed of bus in Khz */ u16 cmd_err; u8 *buf; size_t buf_len; struct i2c_adapter adapter; u8 fifo_size; /* use as flag and value * fifo_size==0 implies no fifo * if set, should be trsh+1 */ u8 rev; unsigned b_hw:1; /* bad h/w fixes */ unsigned idle:1; u16 iestate; /* Saved interrupt register */ u16 pscstate; u16 scllstate; u16 sclhstate; u16 bufstate; u16 syscstate; u16 westate; };
Base对应I2C控制器寄存器的虚拟地址。
Irq对应I2C控制器的中断号。
Buf对应上层传下来的需要发送数据或者I2C控制接收到数据的缓存空间，buf_len是其大小。
Adapter对应I2C控制器的适配器结构。
U16类型的各个state变量是用于对应I2C控制器的寄存器的值。
    函数omap_i2c_probe的执行流程如下图：

图3.2 omap_i2c_probe的执行流程

    函数omap_i2c_probe的简要代码如下：

static int __init omap_i2c_probe(struct platform_device *pdev) { struct omap_i2c_dev *dev; struct i2c_adapter *adap; struct resource *mem, *irq, *ioarea; irq_handler_t isr; …… /* NOTE: driver uses the static register mapping */ mem = platform_get_resource(pdev, IORESOURCE_MEM, 0); …… irq = platform_get_resource(pdev, IORESOURCE_IRQ, 0); …… dev = kzalloc(sizeof(struct omap_i2c_dev), GFP_KERNEL); …… dev->dev = &pdev->dev; dev->irq = irq->start; dev->base = ioremap(mem->start, mem->end - mem->start + 1); …… /* reset ASAP, clearing any IRQs */ omap_i2c_init(dev); isr = (dev->rev < OMAP_I2C_REV_2) ? omap_i2c_rev1_isr : omap_i2c_isr; r = request_irq(dev->irq, isr, 0, pdev->name, dev); …… adap = &dev->adapter; i2c_set_adapdata(adap, dev); adap->owner = THIS_MODULE; adap->class = I2C_CLASS_HWMON; strncpy(adap->name, "OMAP I2C adapter", sizeof(adap->name)); adap->algo = &omap_i2c_algo; adap->dev.parent = &pdev->dev; /* i2c device drivers may be active on return from add_adapter() */ adap->nr = pdev->id; r = i2c_add_numbered_adapter(adap); …… return 0; …… }

     这里定义了I2C adapter的中断处理函数omap_i2c_isr()，该函数对I2C控制器的中断事件进行响应，主要实现了对I2C数据收发中断事件的处理。
     这边还涉及到了一个i2c_algorithm结构的变量omap_i2c_algo，该变量的定义如下：

static const struct i2c_algorithm omap_i2c_algo = { .master_xfer = omap_i2c_xfer, .functionality = omap_i2c_func, };

 omap_i2c_xfer接口函数实现了底层I2C数据传输的方法。

 omap_i2c_probe函数最后使用了   i2c_add_numbered_adapter()将adapter注册到i2c-core层，adapter的总线号保存在平台设备数组 omap_i2c_devices中，见3.1.1节，由于该数组中有三个成员，即三条I2C总线，所以这里会建立三个I2C adapter，总线号分别为1，2，3。