一、概述
基于子系统去开发驱动程序已经是linux内核中普遍的做法了。前面写过基于I2C子系统的驱动开发。本文介绍另外一种常用总线SPI的开发方法。SPI子系统的开发和I2C有很多的相似性，大家可以对比学习。本主题分为两个部分叙述，第一部分介绍基于SPI子系统开发的理论框架；第二部分以华清远见教学平台FS_S5PC100上的M25P10芯片为例（内核版本2.6.29），编写一个SPI驱动程序实例。
二、SPI总线协议简介
介绍驱动开发前，需要先熟悉下SPI通讯协议中的几个关键的地方，后面在编写驱动时，需要考虑相关因素。
SPI总线由MISO（串行数据输入）、MOSI（串行数据输出）、SCK（串行移位时钟）、CS（使能信号）4个信号线组成。如FS_S5PC100上的M25P10芯片接线为：
上图中M25P10的D脚为它的数据输入脚，Q为数据输出脚，C为时钟脚。
SPI常用四种数据传输模式，主要差别在于：输出串行同步时钟极性（CPOL）和相位（CPHA）可以进行配置。如果CPOL= 0，串行同步时钟的空闲状态为低电平；如果CPOL= 1，串行同步时钟的空闲状态为高电平。如果CPHA= 0，在串行同步时钟的前沿（上升或下降）数据被采样；如果CPHA = 1，在串行同步时钟的后沿（上升或下降）数据被采样。
这四种模式中究竟选择哪种模式取决于设备。如M25P10的手册中明确它可以支持的两种模式为：CPOL=0 CPHA=0 和 CPOL=1 CPHA=1
三、linux下SPI驱动开发
首先明确SPI驱动层次，如下图：
我们以上面的这个图为思路
1、 Platform bus
Platform bus对应的结构是platform_bus_type，这个内核开始就定义好的。我们不需要定义。
2、Platform_device
SPI控制器对应platform_device的定义方式，同样以S5PC100中的SPI控制器为例，参看arch/arm/plat-s5pc1xx/dev-spi.c文件
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struct platform_device s3c_device_spi0 = { .name = "s3c64xx-spi", //名称，要和Platform_driver匹配 .id = 0, //第0个控制器，S5PC100中有3个控制器 .num_resources = ARRAY_SIZE(s5pc1xx_spi0_resource), //占用资源的种类 .resource = s5pc1xx_spi0_resource, //指向资源结构数组的指针 .dev = { .dma_mask = &spi_dmamask, //dma寻址范围 .coherent_dma_mask = DMA_BIT_MASK(32), //可以通过关闭cache等措施保证一致性的dma寻址范围 .platform_data = &s5pc1xx_spi0_pdata, //特殊的平台数据，参看后文 }, }; static struct s3c64xx_spi_cntrlr_info s5pc1xx_spi0_pdata = { .cfg_gpio = s5pc1xx_spi_cfg_gpio, //用于控制器管脚的IO配置 .fifo_lvl_mask = 0x7f, .rx_lvl_offset = 13, }; static int s5pc1xx_spi_cfg_gpio(struct platform_device *pdev) { switch (pdev->id) { case 0: s3c_gpio_cfgpin(S5PC1XX_GPB(0), S5PC1XX_GPB0_SPI_MISO0); s3c_gpio_cfgpin(S5PC1XX_GPB(1), S5PC1XX_GPB1_SPI_CLK0); s3c_gpio_cfgpin(S5PC1XX_GPB(2), S5PC1XX_GPB2_SPI_MOSI0); s3c_gpio_setpull(S5PC1XX_GPB(0), S3C_GPIO_PULL_UP); s3c_gpio_setpull(S5PC1XX_GPB(1), S3C_GPIO_PULL_UP); s3c_gpio_setpull(S5PC1XX_GPB(2), S3C_GPIO_PULL_UP); break; case 1: s3c_gpio_cfgpin(S5PC1XX_GPB(4), S5PC1XX_GPB4_SPI_MISO1); s3c_gpio_cfgpin(S5PC1XX_GPB(5), S5PC1XX_GPB5_SPI_CLK1); s3c_gpio_cfgpin(S5PC1XX_GPB(6), S5PC1XX_GPB6_SPI_MOSI1); s3c_gpio_setpull(S5PC1XX_GPB(4), S3C_GPIO_PULL_UP); s3c_gpio_setpull(S5PC1XX_GPB(5), S3C_GPIO_PULL_UP); s3c_gpio_setpull(S5PC1XX_GPB(6), S3C_GPIO_PULL_UP); break; case 2: s3c_gpio_cfgpin(S5PC1XX_GPG3(0), S5PC1XX_GPG3_0_SPI_CLK2); s3c_gpio_cfgpin(S5PC1XX_GPG3(2), S5PC1XX_GPG3_2_SPI_MISO2); s3c_gpio_cfgpin(S5PC1XX_GPG3(3), S5PC1XX_GPG3_3_SPI_MOSI2); s3c_gpio_setpull(S5PC1XX_GPG3(0), S3C_GPIO_PULL_UP); s3c_gpio_setpull(S5PC1XX_GPG3(2), S3C_GPIO_PULL_UP); s3c_gpio_setpull(S5PC1XX_GPG3(3), S3C_GPIO_PULL_UP); break; default: dev_err(&pdev->dev, "Invalid SPI Controller number!"); return -EINVAL; }3、Platform_driver
再看platform_driver，参看drivers/spi/spi_s3c64xx.c文件
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static struct platform_driver s3c64xx_spi_driver = { .driver = { .name = "s3c64xx-spi", //名称，和platform_device对应 .owner = THIS_MODULE, }, .remove = s3c64xx_spi_remove, .suspend = s3c64xx_spi_suspend, .resume = s3c64xx_spi_resume, }; platform_driver_probe(&s3c64xx_spi_driver, s3c64xx_spi_probe)；//注册s3c64xx_spi_driver和平台中注册的platform_device匹配后，调用s3c64xx_spi_probe。然后根据传入的platform_device参数，构建一个用于描述SPI控制器的结构体spi_master，并注册。spi_register_master(master)。后续注册的spi_device需要选定自己的spi_master，并利用spi_master提供的传输功能传输spi数据。
和I2C类似，SPI也有一个描述控制器的对象叫spi_master。其主要成员是主机控制器的序号（系统中可能存在多个SPI主机控制器）、片选数量、SPI模式和时钟设置用到的函数、数据传输用到的函数等。
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struct spi_master { struct device dev; s16 bus_num; //表示是SPI主机控制器的编号。由平台代码决定 u16 num_chipselect; //控制器支持的片选数量，即能支持多少个spi设备 int (*setup)(struct spi_device *spi); //针对设备设置SPI的工作时钟及数据传输模式等。在spi_add_device函数中调用。 int (*transfer)(struct spi_device *spi, struct spi_message *mesg); //实现数据的双向传输，可能会睡眠 void (*cleanup)(struct spi_device *spi); //注销时调用 };4、Spi bus
Spi总线对应的总线类型为spi_bus_type，在内核的drivers/spi/spi.c中定义
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struct bus_type spi_bus_type = { .name = "spi", .dev_attrs = spi_dev_attrs, .match = spi_match_device, .uevent = spi_uevent, .suspend = spi_suspend, .resume = spi_resume, };对应的匹配规则是（高版本中的匹配规则会稍有变化，引入了id_table，可以匹配多个spi设备名称）：
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static int spi_match_device(struct device *dev, struct device_driver *drv) { const struct spi_device *spi = to_spi_device(dev); return strcmp(spi->modalias, drv->name) == 0; }5、spi_device
下面该讲到spi_device的构建与注册了。spi_device对应的含义是挂接在spi总线上的一个设备，所以描述它的时候应该明确它自身的设备特性、传输要求、及挂接在哪个总线上。
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static struct spi_board_info s3c_spi_devs[] __initdata = { { .modalias = "m25p10", .mode = SPI_MODE_0, //CPOL=0, CPHA=0 此处选择具体数据传输模式 .max_speed_hz = 10000000, //最大的spi时钟频率 /* Connected to SPI-0 as 1st Slave */ .bus_num = 0, //设备连接在spi控制器0上 .chip_select = 0, //片选线号，在S5PC100的控制器驱动中没有使用它作为片选的依据，而是选择了下文controller_data里的方法。 .controller_data = &smdk_spi0_csi[0], }, }; static struct s3c64xx_spi_csinfo smdk_spi0_csi[] = { [0] = { .set_level = smdk_m25p10_cs_set_level, .fb_delay = 0x3, }, }; static void smdk_m25p10_cs_set_level(int high) //spi控制器会用这个方法设置cs { u32 val; val = readl(S5PC1XX_GPBDAT); if (high) val |= (1<<3); else val &= ~(1<<3); writel(val, S5PC1XX_GPBDAT); } spi_register_board_info(s3c_spi_devs, ARRAY_SIZE(s3c_spi_devs));//注册spi_board_info。这个代码会把spi_board_info注册要链表board_list上。事实上上文提到的spi_master的注册会在spi_register_board_info之后，spi_master注册的过程中会调用scan_boardinfo扫描board_list，找到挂接在它上面的spi设备，然后创建并注册spi_device。
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static void scan_boardinfo(struct spi_master *master) { struct boardinfo *bi; mutex_lock(&board_lock); list_for_each_entry(bi, &board_list, list) { struct spi_board_info *chip = bi->board_info; unsigned n; for (n = bi->n_board_info; n > 0; n--, chip++) { if (chip->bus_num != master->bus_num) continue; /* NOTE: this relies on spi_new_device to * issue diagnostics when given bogus inputs */ (void) spi_new_device(master, chip); //创建并注册了spi_device } } mutex_unlock(&board_lock); }6、spi_driver
本文先以linux内核中的/driver/mtd/devices/m25p80.c驱动为参考。
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static struct spi_driver m25p80_driver = { //spi_driver的构建 .driver = { .name = "m25p80", .bus = &spi_bus_type, .owner = THIS_MODULE, }, .probe = m25p_probe, .remove = __devexit_p(m25p_remove), */ }; spi_register_driver(&m25p80_driver);//spi driver的注册 在有匹配的spi device时，会调用m25p_probe static int __devinit m25p_probe(struct spi_device *spi) { …… }根据传入的spi_device参数，可以找到对应的spi_master。接下来就可以利用spi子系统为我们完成数据交互了。可以参看m25p80_read函数。要完成传输，先理解下面几个结构的含义：（这两个结构的定义及详细注释参见include/linux/spi/spi.h）
spi_message：描述一次完整的传输，即cs信号从高->底->高的传输 spi_transfer：多个spi_transfer够成一个spi_message 举例说明：m25p80的读过程如下图
可以分解为两个spi_ transfer一个是写命令，另一个是读数据。具体实现参见m25p80.c中的m25p80_read函数。下面内容摘取之此函数。
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struct spi_transfer t[2]; //定义了两个spi_transfer struct spi_message m; //定义了两个spi_message spi_message_init(&m); //初始化其transfers链表 t[0].tx_buf = flash->command; t[0].len = CMD_SIZE + FAST_READ_DUMMY_BYTE; //定义第一个transfer的写指针和长度 spi_message_add_tail(&t[0], &m); //添加到spi_message t[1].rx_buf = buf; t[1].len = len; //定义第二个transfer的读指针和长度 spi_message_add_tail(&t[1], &m); //添加到spi_message flash->command[0] = OPCODE_READ; flash->command[1] = from >> 16; flash->command[2] = from >> 8; flash->command[3] = from; //初始化前面写buf的内容 spi_sync(flash->spi, &m); //调用spi_master发送spi_message // spi_sync为同步方式发送，还可以用spi_async异步方式，那样的话，需要设置回调完成函数。 另外你也可以选择一些封装好的更容易使用的函数，这些函数可以在include/linux/spi/spi.h文件中找到，如： extern int spi_write_then_read(struct spi_device *spi, const u8 *txbuf, unsigned n_tx, u8 *rxbuf, unsigned n_rx);这篇博文就到这了，下篇给出一个针对m25p10完整的驱动程序。 Linux下spi驱动开发之m25p10驱动测试
目标：在华清远见的FS_S5PC100平台上编写一个简单的spi驱动模块，在probe阶段实现对m25p10的ID号探测、flash擦除、flash状态读取、flash写入、flash读取等操作。代码已经经过测试，运行于2.6.35内核。理解下面代码需要参照m25p10的芯片手册。其实下面的代码和处理器没有太大关系，这也是spi子系统的分层特点。
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#include <linux/platform_device.h> #include <linux/spi/spi.h> #include <linux/init.h> #include <linux/module.h> #include <linux/device.h> #include <linux/interrupt.h> #include <linux/mutex.h> #include <linux/slab.h> // kzalloc #include <linux/delay.h> #define FLASH_PAGE_SIZE 256 /* Flash Operating Commands */ #define CMD_READ_ID 0x9f #define CMD_WRITE_ENABLE 0x06 #define CMD_BULK_ERASE 0xc7 #define CMD_READ_BYTES 0x03 #define CMD_PAGE_PROGRAM 0x02 #define CMD_RDSR 0x05 /* Status Register bits. */ #define SR_WIP 1 /* Write in progress */ #define SR_WEL 2 /* Write enable latch */ /* ID Numbers */ #define MANUFACTURER_ID 0x20 #define DEVICE_ID 0x1120 /* Define max times to check status register before we give up. */ #define MAX_READY_WAIT_COUNT 100000 #define CMD_SZ 4 struct m25p10a { struct spi_device *spi; struct mutex lock; char erase_opcode; char cmd[ CMD_SZ ]; }; /* * Internal Helper functions */ /* * Read the status register, returning its value in the location * Return the status register value. * Returns negative if error occurred. */ static int read_sr(struct m25p10a *flash) { ssize_t retval; u8 code = CMD_RDSR; u8 val; retval = spi_write_then_read(flash->spi, &code, 1, &val, 1); if (retval < 0) { dev_err(&flash->spi->dev, "error %d reading SR\n", (int) retval); return retval; } return val; } /* * Service routine to read status register until ready, or timeout occurs. * Returns non-zero if error. */ static int wait_till_ready(struct m25p10a *flash) { int count; int sr; /* one chip guarantees max 5 msec wait here after page writes, * but potentially three seconds (!) after page erase. */ for (count = 0; count < MAX_READY_WAIT_COUNT; count++) { if ((sr = read_sr(flash)) < 0) break; else if (!(sr & SR_WIP)) return 0; /* REVISIT sometimes sleeping would be best */ } printk( "in (%s): count = %d\n", count ); return 1; } /* * Set write enable latch with Write Enable command. * Returns negative if error occurred. */ static inline int write_enable( struct m25p10a *flash ) { flash->cmd[0] = CMD_WRITE_ENABLE; return spi_write( flash->spi, flash->cmd, 1 ); } /* * Erase the whole flash memory * * Returns 0 if successful, non-zero otherwise. */ static int erase_chip( struct m25p10a *flash ) { /* Wait until finished previous write command. */ if (wait_till_ready(flash)) return -1; /* Send write enable, then erase commands. */ write_enable( flash ); flash->cmd[0] = CMD_BULK_ERASE; return spi_write( flash->spi, flash->cmd, 1 ); } /* * Read an address range from the flash chip. The address range * may be any size provided it is within the physical boundaries. */ static int m25p10a_read( struct m25p10a *flash, loff_t from, size_t len, char *buf ) { int r_count = 0, i; flash->cmd[0] = CMD_READ_BYTES; flash->cmd[1] = from >> 16; flash->cmd[2] = from >> 8; flash->cmd[3] = from; #if 1 struct spi_transfer st[2]; struct spi_message msg; spi_message_init( &msg ); memset( st, 0, sizeof(st) ); flash->cmd[0] = CMD_READ_BYTES; flash->cmd[1] = from >> 16; flash->cmd[2] = from >> 8; flash->cmd[3] = from; st[ 0 ].tx_buf = flash->cmd; st[ 0 ].len = CMD_SZ; spi_message_add_tail( &st[0], &msg ); st[ 1 ].rx_buf = buf; st[ 1 ].len = len; spi_message_add_tail( &st[1], &msg ); mutex_lock( &flash->lock ); /* Wait until finished previous write command. */ if (wait_till_ready(flash)) { mutex_unlock( &flash->lock ); return -1; } spi_sync( flash->spi, &msg ); r_count = msg.actual_length - CMD_SZ; printk( "in (%s): read %d bytes\n", __func__, r_count ); for( i = 0; i < r_count; i++ ) { printk( "0xx\n", buf[ i ] ); } mutex_unlock( &flash->lock ); #endif return 0; } /* * Write an address range to the flash chip. Data must be written in * FLASH_PAGE_SIZE chunks. The address range may be any size provided * it is within the physical boundaries. */ static int m25p10a_write( struct m25p10a *flash, loff_t to, size_t len, const char *buf ) { int w_count = 0, i, page_offset; struct spi_transfer st[2]; struct spi_message msg; #if 1 if (wait_till_ready(flash)) { //读状态，等待ready mutex_unlock( &flash->lock ); return -1; } #endif write_enable( flash ); //写使能 spi_message_init( &msg ); memset( st, 0, sizeof(st) ); flash->cmd[0] = CMD_PAGE_PROGRAM; flash->cmd[1] = to >> 16; flash->cmd[2] = to >> 8; flash->cmd[3] = to; st[ 0 ].tx_buf = flash->cmd; st[ 0 ].len = CMD_SZ; spi_message_add_tail( &st[0], &msg ); st[ 1 ].tx_buf = buf; st[ 1 ].len = len; spi_message_add_tail( &st[1], &msg ); mutex_lock( &flash->lock ); /* get offset address inside a page */ page_offset = to % FLASH_PAGE_SIZE; /* do all the bytes fit onto one page? */ if( page_offset + len <= FLASH_PAGE_SIZE ) { // yes st[ 1 ].len = len; printk("%d, cmd = %d\n", st[ 1 ].len, *(char *)st[0].tx_buf); //while(1) { spi_sync( flash->spi, &msg ); } w_count = msg.actual_length - CMD_SZ; } else { // no } printk( "in (%s): write %d bytes to flash in total\n", __func__, w_count ); mutex_unlock( &flash->lock ); return 0; } static int check_id( struct m25p10a *flash ) { char buf[10] = {0}; flash->cmd[0] = CMD_READ_ID; spi_write_then_read( flash->spi, flash->cmd, 1, buf, 3 ); printk( "Manufacture ID: 0x%x\n", buf[0] ); printk( "Device ID: 0x%x\n", buf[1] | buf[2] << 8 ); return buf[2] << 16 | buf[1] << 8 | buf[0]; } static int m25p10a_probe(struct spi_device *spi) { int ret = 0; struct m25p10a *flash; char buf[ 256 ]; printk( "%s was called\n", __func__ ); flash = kzalloc( sizeof(struct m25p10a), GFP_KERNEL ); if( !flash ) { return -ENOMEM; } flash->spi = spi; mutex_init( &flash->lock ); /* save flash as driver's private data */ spi_set_drvdata( spi, flash ); check_id( flash ); //读取ID #if 1 ret = erase_chip( flash ); //擦除 if( ret < 0 ) { printk( "erase the entirely chip failed\n" ); } printk( "erase the whole chip done\n" ); memset( buf, 0x7, 256 ); m25p10a_write( flash, 0, 20, buf); //0地址写入20个7 memset( buf, 0, 256 ); m25p10a_read( flash, 0, 25, buf ); //0地址读出25个数 #endif return 0; } static int m25p10a_remove(struct spi_device *spi) { return 0; } static struct spi_driver m25p10a_driver = { .probe = m25p10a_probe, .remove = m25p10a_remove, .driver = { .name = "m25p10a", }, }; static int __init m25p10a_init(void) { return spi_register_driver(&m25p10a_driver); } static void __exit m25p10a_exit(void) { spi_unregister_driver(&m25p10a_driver); } module_init(m25p10a_init); module_exit(m25p10a_exit); MODULE_DESCRIPTION("m25p10a driver for FS_S5PC100"); MODULE_LICENSE("GPL");