一：
SPI核心，就是指/drivers/spi/目录下spi.c文件中提供给其他文件的函数，首先看下spi核心的初始化函数spi_init(void)。
1: static int __init spi_init(void) 2: { 3: int status; 4: 5: buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL); /* 初始化缓存 */ 6: if (!buf) { 7: status = -ENOMEM; 8: goto err0; 9: } 10: 11: status = bus_register(&spi_bus_type); /* 注册spi总线,此步骤之后就会在/sys/bus目录下生成spi子目录 */ 12: if (status < 0) 13: goto err1; 14: 15: status = class_register(&spi_master_class);;/* 注册spi类,此步骤之后就会在/sys/class目录下生成spi_master子目录 */ 16: if (status < 0) 17: goto err2; 18: return 0; 19: 20: err2: 21: bus_unregister(&spi_bus_type); 22: err1: 23: kfree(buf); 24: buf = NULL; 25: err0: 26: return status; 27: }
1: struct bus_type spi_bus_type = { 2: .name = "spi", 3: .dev_attrs = spi_dev_attrs, 4: .match = spi_match_device, 5: .uevent = spi_uevent, 6: .pm = &spi_pm, 7: };
1: static struct class spi_master_class = { 2: .name = "spi_master", 3: .owner = THIS_MODULE, 4: .dev_release = spi_master_release, 5: };
1: postcore_initcall(spi_init); /* 注册 */
说明：
1) 由postcore_initcall(spi_init);可以看出，此宏在系统初始化时是先于module_init()执行的。
2) 申请的buf空间用于在spi数据传输中。
3) 接下来是总线注册和类注册。
二：
此函数是半双工的形式写then读
1: int spi_write_then_read(struct spi_device *spi,
2: const void *txbuf, unsigned n_tx,
3: void *rxbuf, unsigned n_rx)
4: {
5: static DEFINE_MUTEX(lock);
6:
7: int status;
8: struct spi_message message;
9: struct spi_transfer x[2];
10: u8 *local_buf;
11:
12: /* Use preallocated DMA-safe buffer. We can't avoid copying here,
13: * (as a pure convenience thing), but we can keep heap costs
14: * out of the hot path ...
15: */
16: if ((n_tx + n_rx) > SPI_BUFSIZ)
17: return -EINVAL;
18:
19: spi_message_init(&message);
20: memset(x, 0, sizeof x);
21: if (n_tx) {
22: x[0].len = n_tx;
23: spi_message_add_tail(&x[0], &message);
24: }
25: if (n_rx) {
26: x[1].len = n_rx;
27: spi_message_add_tail(&x[1], &message);
28: }
29:
30: /* ... unless someone else is using the pre-allocated buffer */
31: if (!mutex_trylock(&lock)) {
32: local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
33: if (!local_buf)
34: return -ENOMEM;
35: } else
36: local_buf = buf;
37:
38: memcpy(local_buf, txbuf, n_tx);
39: x[0].tx_buf = local_buf;
40: x[1].rx_buf = local_buf + n_tx;
41:
42: /* do the i/o */
43: status = spi_sync(spi, &message);
44: if (status == 0)
45: memcpy(rxbuf, x[1].rx_buf, n_rx);
46:
47: if (x[0].tx_buf == buf)
48: mutex_unlock(&lock);
49: else
50: kfree(local_buf);
51:
52: return status;
53: }
1: 对master操作的加锁与解锁
2: int spi_bus_lock(struct spi_master *master)
3: {
4: unsigned long flags;
5:
6: mutex_lock(&master->bus_lock_mutex);
7:
8: spin_lock_irqsave(&master->bus_lock_spinlock, flags);
9: master->bus_lock_flag = 1;
10: spin_unlock_irqrestore(&master->bus_lock_spinlock, flags);
11:
12: /* mutex remains locked until spi_bus_unlock is called */
13:
14: return 0;
15: }
16: int spi_bus_unlock(struct spi_master *master)
17: {
18: master->bus_lock_flag = 0;
19:
20: mutex_unlock(&master->bus_lock_mutex);
21:
22: return 0;
23: }
同步数据交互
1: int spi_sync(struct spi_device *spi, struct spi_message *message) 2: { 3: return __spi_sync(spi, message, 0); 4: } 5: int spi_sync_locked(struct spi_device *spi, struct spi_message *message) 6: { 7: return __spi_sync(spi, message, 1); 8: }
异步数据交互
1: int spi_async(struct spi_device *spi, struct spi_message *message) 2: { 3: struct spi_master *master = spi->master; 4: int ret; 5: unsigned long flags; 6: 7: spin_lock_irqsave(&master->bus_lock_spinlock, flags); 8: 9: if (master->bus_lock_flag) 10: ret = -EBUSY; 11: else 12: ret = __spi_async(spi, message); 13: 14: spin_unlock_irqrestore(&master->bus_lock_spinlock, flags); 15: 16: return ret; 17: } 18: 19: int spi_async_locked(struct spi_device *spi, struct spi_message *message) 20: { 21: struct spi_master *master = spi->master; 22: int ret; 23: unsigned long flags; 24: 25: spin_lock_irqsave(&master->bus_lock_spinlock, flags); 26: 27: ret = __spi_async(spi, message); 28: 29: spin_unlock_irqrestore(&master->bus_lock_spinlock, flags); 30: 31: return ret; 32: 33: }
//创建master
1: struct spi_master *spi_alloc_master(struct device *dev, unsigned size) 2: { 3: struct spi_master *master; 4: 5: if (!dev) 6: return NULL; 7: 8: master = kzalloc(size + sizeof *master, GFP_KERNEL); 9: if (!master) 10: return NULL; 11: 12: device_initialize(&master->dev); 13: master->dev.class = &spi_master_class; 14: master->dev.parent = get_device(dev); 15: spi_master_set_devdata(master, &master[1]); 16: 17: return master; 18: }
//spi_register_master
1: int spi_register_master(struct spi_master *master) 2: { 3: static atomic_t dyn_bus_id = ATOMIC_INIT((1<<15) - 1); 4: struct device *dev = master->dev.parent; 5: struct boardinfo *bi; 6: int status = -ENODEV; 7: int dynamic = 0; 8: 9: if (!dev) 10: return -ENODEV; 11: 12: /* even if it's just one always-selected device, there must 13: * be at least one chipselect 14: */ 15: if (master->num_chipselect == 0) 16: return -EINVAL; 17: 18: /* convention: dynamically assigned bus IDs count down from the max */ 19: if (master->bus_num < 0) { 20: /* FIXME switch to an IDR based scheme, something like 21: * I2C now uses, so we can't run out of "dynamic" IDs 22: */ 23: master->bus_num = atomic_dec_return(&dyn_bus_id); 24: dynamic = 1; 25: } 26: 27: spin_lock_init(&master->bus_lock_spinlock); 28: mutex_init(&master->bus_lock_mutex); 29: master->bus_lock_flag = 0; 30: 31: /* register the device, then userspace will see it. 32: * registration fails if the bus ID is in use. 33: */ 34: dev_set_name(&master->dev, "spi%u", master->bus_num); 35: status = device_add(&master->dev); 36: if (status < 0) 37: goto done; 38: dev_dbg(dev, "registered master %s%s
", dev_name(&master->dev), 39: dynamic ? " (dynamic)" : ""); 40: 41: mutex_lock(&board_lock); 42: list_add_tail(&master->list, &spi_master_list); 43: list_for_each_entry(bi, &board_list, list) 44: spi_match_master_to_boardinfo(master, &bi->board_info); 45: mutex_unlock(&board_lock); 46: 47: status = 0; 48: 49: /* Register devices from the device tree */ 50: of_register_spi_devices(master); 51: done: 52: return status; 53: }
分析以上spi_register_master代码：
1. spi_match_master_to_boardinfo会将master和每个注册进来的device board联系起来