1 Linux I2C驅動架構
Linux下I2C驅動的架構圖如下:
圖1.1 Linux下I2C驅動架構
如上圖所示,每條I2C總線會對應一個adapter,而每條I2C總線上則可以有多個 client,在linux kernel中,通過I2C core層將I2C client與I2C adapter關聯起來,Linux 中I2C驅動代碼位於drivers/i2c目錄。
Linux中I2C可以分爲三個層次,分別爲I2C core層、I2C adapter driver層、I2C device driver層。
1.1 I2C core層
I2C core是用於維護Linux的I2C核心部分,提供了核心的數據結構,I2C適配器驅動和設備驅動的註冊、註銷管理等API,同時還提供了I2C總線讀寫訪問的一般接口(具體的實現在與I2C控制器相關的I2C adapter中實現)。
該層爲硬件平臺無關層,向下屏蔽了物理總線適配器的差異,定義了統一的訪問策略和接口;向上則提供了統一的接口,以便I2C設備驅動可以通過總線適配器進行數據收發。
Linux中,I2C core層的代碼位於driver/i2c/ i2c-core.c。由於該層是平臺無關層,本文將不再敘述,有興趣可以查閱相關資料。
1.2 I2C adapter driver層
I2C adapter driver層即I2C適配器驅動層,每種處理器平臺都有自己的適配器驅動,屬於平臺移植相關層。它的職責是爲系統中每條I2C總線實現相應的讀寫方法。但是適配器驅動本身並不會進行任何的通訊,而是等待設備驅動調用其函數。
在系統開機時,I2C適配器驅動被首先裝載。一個適配器驅動用於支持一條特定的I2C總線的讀寫。一個適配器驅動通常需要兩個模塊,一個struct i2c_adapter和一個struct i2c_algorithm來描述。
i2c adapter 構造一個對I2C core層接口的數據結構,並通過相應的接口函數向I2C core註冊一個適配器。i2c_algorithm主要實現對I2C總線訪問的算法,master_xfer和smbus_xfer即I2C adapter底層對I2C總線讀寫方法的實現,相關的數據結構如下:
- /*
- * 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,其和具體的總線控制器相關,不同的CPU在實現上會有差異。
- /*
- * 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是和底層硬件的接口,標識了具體的物理總線傳輸的實現。
Userspace_clients爲使用該總線的client鏈表。
Nr爲該適配器也就是某條I2C總線佔據的全局編號。
bus_lock總線的互斥鎖,防止總線衝突。
Linux中,I2C adapter driver層的代碼位於drivers/i2c/busses目錄,第3章會詳細介紹該層的內容。
1.3 I2C device driver層
I2C device driver層爲用戶接口層,其爲用戶提供了通過I2C總線訪問具體設備的接口。
I2C的device driver層可以用兩個模塊來描述,struct i2c_driver和struct i2c_client。
i2c_client和i2c_driver分別構造對I2C core層接口的數據結構,並且通過相關的接口函數向 I2C Core註冊I2C設備驅動。相關的數據結構如下:
- /**
- * 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是爲device服務的,i2c_driver註冊時會掃描i2c bus上的設備,進行驅動和設備的綁定。主要有兩種接口attach_adapter和probe,二者分別針對舊的和新式的驅動。
- /**
- * 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;
- };
通常來說i2c_client對應着I2C總線上某個特定的slave或者是user space的某個用戶對應,而此時的slave可以動態變化。
Linux中,I2C device driver層的代碼位於drivers/i2c/chips目錄,第4章將詳細介紹該層的內容。
2 OMAP3630 I2C控制器
OMAP3630具有4個高速I2C控制器,每個控制器都通過I2C串行總線爲本地主機即OAMP3630 MPU和I2C總線兼容設備提供了一個通訊接口,支持多達8-bit的數據傳送和接收。
每個I2C控制器都能配置成一個主機或者從機設備,而且他們都能配置成在一個2線的串行的攝像頭控制總線(SCCB總線)上作爲主設備,I2C2和I2C3還能配置成在一個3線的SCCB總線上作爲主設備。
I2C4控制器位於PRCM模塊,可以進行動態電壓控制和電源序列測定。
OMAP3630的I2C控制器模塊圖如下:
圖2.1 OMAP3630 I2C控制器模塊圖
控制器1,2,3具有以下特徵:
兼容飛利浦I2C 2.1版本
支持標準I2C標準模式(100Kbps)和快速模式(400Kpbs)
支持高達3.4Mbps的高速發送模式
支持I2C2和I2C3 模塊的3線/2線的SCCB主從模式,I2C1 模塊的2線的SCCB主從模式,高達100kbit/s
7-bit和10bit的設備地址模式
多主控發送/從接收模式
多主控接收/從發送模式
聯合的主機發送/接收和接收/發送模式
內置FIFO(8,16,32,64字節大小)用於緩存讀取和接收
模塊使能/關閉
可編程的時鐘
8-bit的數據存取
低功耗的設計
兩個DMA通道
支持中斷機制
自動空閒機制
空閒請求和應答握手機制
主從的發送機I2C4控制器有以下特徵:
支持高速和快速模式
只能支持7-bit地址模式
只支持主發送模式
關於I2C控制器的詳細介紹請參考OMAP36XX_ES1.1_NDA_TRM_V_G.pdf的第17章。
3 OMAP3630 I2C adapter驅動
在Linux內核中,I2C adapter驅動位於drivers/i2c/busses目錄下,OMAP3630 的I2C adapter驅動程序爲i2c-omap.c。
I2C adapter驅動,本質上就是實現了具體的總線傳輸算法並向核心層註冊適配器。該驅動的註冊採用Platform驅動和設備機制。
3.1 I2C adapter的Platform device
Andrord 2.1中Platform device的註冊的代碼位於內核的arch/arm/plat-omap/i2c.c,arch/arm/mach-omap2/board-xxxx.c中。
3.1.1 Platform device的定義
在文件arch/arm/plat-omap/i2c.c中,Platform device定義如下:
- #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,這邊定義了三個I2C適配器的Platform device,id分別爲“1,2,3”,name都爲“i2c_omap”,變量resource中定義了適配器的寄存器基地址,irq中斷號等。
3.1.2 Platform device的註冊
Platform device的註冊是由內核啓動後,具體產品的板級初始化完成的。xxxx項目的I2C adapter的Platform device註冊過程如下圖:
圖3.1 Platform device註冊過程
函數omap_i2c_add_bus()中,通過函數platform_device_register()註冊Platform device到platform bus上,代碼如下:
- 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);
- }
註冊完成後,中斷號及寄存器的基地址等信息會在設備樹中描述了,此後只需利用platform_get_resource等標準接口自動獲取即可,實現了驅動和資源的分離。
3.2 I2C adapter的Platform driver
Andrord 2.1中Platform driver的註冊的代碼位於內核的drivers/i2c/busses/ i2c-omap.c中,該驅動的註冊目的是初始化OMAP3630的I2C adapter,提供I2C總線傳輸的具體實現,並且向I2C core註冊I2C adapter。
3.2.1 Platform driver的定義
在文件drivers/i2c/busses/ i2c-omap.c中,platform driver定義如下:
- 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的註冊
在文件drivers/i2c/busses/ i2c-omap.c中,platform driver註冊如下:
- /* 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);
通過platform_driver_register()函數註冊Platform driver omap_i2c_driver時,會掃描platform bus上的所有設備,由於匹配因子是name即"i2c_omap",而之前已經將name爲"i2c_omap"的Platform device註冊到platform bus上,因此匹配成功,調用函數omap_i2c_probe將設備和驅動綁定起來。
在drivers/i2c/busses/ i2c-omap.c中會涉及到一個數據結構omap_i2c_dev,這個結構定義了omap3630的I2C控制器,結構如下:
- 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對應I2C控制器寄存器的虛擬地址。
Irq對應I2C控制器的中斷號。
Buf對應上層傳下來的需要發送數據或者I2C控制接收到數據的緩存空間,buf_len是其大小。
Adapter對應I2C控制器的適配器結構。
U16類型的各個state變量是用於對應I2C控制器的寄存器的值。
函數omap_i2c_probe的執行流程如下圖:
圖3.2 omap_i2c_probe的執行流程
函數omap_i2c_probe的簡要代碼如下:
- 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;
- ……
- }
這裏定義了I2C adapter的中斷處理函數omap_i2c_isr(),該函數對I2C控制器的中斷事件進行響應,主要實現了對I2C數據收發中斷事件的處理。
這邊還涉及到了一個i2c_algorithm結構的變量omap_i2c_algo,該變量的定義如下:
- static const struct i2c_algorithm omap_i2c_algo = {
- .master_xfer = omap_i2c_xfer,
- .functionality = omap_i2c_func,
- };
omap_i2c_xfer接口函數實現了底層I2C數據傳輸的方法。
omap_i2c_probe函數最後使用了 i2c_add_numbered_adapter()將adapter註冊到i2c-core層,adapter的總線號保存在平臺設備數組 omap_i2c_devices中,見3.1.1節,由於該數組中有三個成員,即三條I2C總線,所以這裏會建立三個I2C adapter,總線號分別爲1,2,3。