相比于前面介绍的i2c子系统（见 http://www.linuxidc.com/Linux/2012-01/52782.htm ），spi子系统相对简单，和i2c的结构也很相似，这里主要介绍一下平台无关部分的代码。先概括的说一下，spi总线或者说是spi控制器用结构体struct spi_master来表述，而一般不会明显的主动实现这个结构而是借助板级的一些信息结构，利用spi的模型填充，存在板级信息的一条链表board_list，上面挂接着板级spi设备的描述信息，其挂接的结构是struct boardinfo，这个结构内部嵌入了具体的板级spi设备所需信息结构struct spi_board_info，对于要操控的spi设备，用struct spidev_data来描述，内嵌了具体设备信息结构struct spi_device，并通过struct spi_device->device_entry成员挂接到全局spi设备链表device_list，结构struct spi_device就是根据前面board_list上所挂的信息填充的，而driver端比较简单，用struct spi_driver来描述，一会儿会看到该结构和标准的platform非常相似，总括的说下一般编写流程：对于soc，spi控制器一般注册成platform，当driver匹配后，会根据platform_device等信息构造出spi_master，一般发生在driver的probe函数，并且注册该master，在master的注册过程中会去遍历board_list找到bus号相同的spi设备信息，并实例化它，好了先就说这么多，下面先看一下具体的数据结构。
一、spi相关的数据结构
先看一下spi设备的板级信息填充的结构：
[cpp]
- struct boardinfo { struct list_head list; //用于挂接到链表头board_list上
- unsigned n_board_info; //设备信息号，spi_board_info成员的编号 struct spi_board_info board_info[0]; //内嵌的spi_board_info结构
- }; //其中内嵌的描述spi设备的具体信息的结构struct spi_board_info为：
- struct spi_board_info { /* the device name and module name are coupled, like platform_bus;
- * "modalias" is normally the driver name. *
- * platform_data goes to spi_device.dev.platform_data, * controller_data goes to spi_device.controller_data,
- * irq is copied too */
- char modalias[SPI_NAME_SIZE]; //名字 const void *platform_data; //如同注释写的指向spi_device.dev.platform_data
- void *controller_data; //指向spi_device.controller_data int irq; //中断号
- /* slower signaling on noisy or low voltage boards */ u32 max_speed_hz; //时钟速率
- /* bus_num is board specific and matches the bus_num of some * spi_master that will probably be registered later.
- * * chip_select reflects how this chip is wired to that master;
- * it's less than num_chipselect. */
- u16 bus_num; //所在的spi总线编号 u16 chip_select;
- /* mode becomes spi_device.mode, and is essential for chips * where the default of SPI_CS_HIGH = 0 is wrong.
- */ u8 mode; //模式
- /* ... may need additional spi_device chip config data here. * avoid stuff protocol drivers can set; but include stuff
- * needed to behave without being bound to a driver: * - quirks like clock rate mattering when not selected
- */ };
利用boardinfo->list成员会将自身挂接到全局的board_list链表上。
再来看一下spi控制器的表述结构：
[cpp]
- struct spi_master { struct device dev; //内嵌的标准dev结构
- /* other than negative (== assign one dynamically), bus_num is fully * board-specific. usually that simplifies to being SOC-specific.
- * example: one SOC has three SPI controllers, numbered 0..2, * and one board's schematics might show it using SPI-2. software
- * would normally use bus_num=2 for that controller. */
- s16 bus_num; //标识的总线号 /* chipselects will be integral to many controllers; some others
- * might use board-specific GPIOs. */
- u16 num_chipselect; /* some SPI controllers pose alignment requirements on DMAable
- * buffers; let protocol drivers know about these requirements. */
- u16 dma_alignment; //dma对其要求 /* spi_device.mode flags understood by this controller driver */
- u16 mode_bits; //代表操作的spi_device.mode /* other constraints relevant to this driver */
- u16 flags; //另外的一些标志 #define SPI_MASTER_HALF_DUPLEX BIT(0) /* can't do full duplex */
- #define SPI_MASTER_NO_RX BIT(1) /* can't do buffer read */ #define SPI_MASTER_NO_TX BIT(2) /* can't do buffer write */
- /* lock and mutex for SPI bus locking */ spinlock_t bus_lock_spinlock;
- struct mutex bus_lock_mutex; /* flag indicating that the SPI bus is locked for exclusive use */
- bool bus_lock_flag; /* Setup mode and clock, etc (spi driver may call many times).
- * * IMPORTANT: this may be called when transfers to another
- * device are active. DO NOT UPDATE SHARED REGISTERS in ways * which could break those transfers.
- */ int (*setup)(struct spi_device *spi); //设置模式
- /* bidirectional bulk transfers *
- * + The transfer() method may not sleep; its main role is * just to add the message to the queue.
- * + For now there's no remove-from-queue operation, or * any other request management
- * + To a given spi_device, message queueing is pure fifo *
- * + The master's main job is to process its message queue, * selecting a chip then transferring data
- * + If there are multiple spi_device children, the i/o queue * arbitration algorithm is unspecified (round robin, fifo,
- * priority, reservations, preemption, etc) *
- * + Chipselect stays active during the entire message * (unless modified by spi_transfer.cs_change != 0).
- * + The message transfers use clock and SPI mode parameters * previously established by setup() for this device
- */ int (*transfer)(struct spi_device *spi, //传输函数
- struct spi_message *mesg); /* called on release() to free memory provided by spi_master */
- void (*cleanup)(struct spi_device *spi); };
而对于要操控的spi总线上的设备，其表述结构为：
[cpp]
- struct spi_device { struct device dev; //内嵌标准device结构体
- struct spi_master *master; //spi主控制器 u32 max_speed_hz; //时钟速率
- u8 chip_select; //设备编号 u8 mode; //模式
- #define SPI_CPHA 0x01 /* clock phase */ #define SPI_CPOL 0x02 /* clock polarity */
- #define SPI_MODE_0 (0|0) /* (original MicroWire) */ #define SPI_MODE_1 (0|SPI_CPHA)
- #define SPI_MODE_2 (SPI_CPOL|0) #define SPI_MODE_3 (SPI_CPOL|SPI_CPHA)
- #define SPI_CS_HIGH 0x04 /* chipselect active high? */ #define SPI_LSB_FIRST 0x08 /* per-word bits-on-wire */
- #define SPI_3WIRE 0x10 /* SI/SO signals shared */ #define SPI_LOOP 0x20 /* loopback mode */
- #define SPI_NO_CS 0x40 /* 1 dev/bus, no chipselect */ #define SPI_READY 0x80 /* slave pulls low to pause */
- u8 bits_per_word; int irq; //中断号
- void *controller_state; //控制状态 void *controller_data; //私有数据
- char modalias[SPI_NAME_SIZE]; //名字 /*
- * likely need more hooks for more protocol options affecting how * the controller talks to each chip, like:
- * - memory packing (12 bit samples into low bits, others zeroed) * - priority
- * - drop chipselect after each word * - chipselect delays
- * - ... */
- };