通过用户态驱动来读写spi 设备

当有一个spi device的时候，但是不想再内核态写spi_driver的话，可以使能CONFIG_SPI_SPIDEV
这样的话会生成一个spidev.ko ,插入这个ko后就可以在用户态open(（“/dev/spidev0.0”）, O_RDWR)的方式pen 这个设备
然后通过SPI_IOC_WR_MODE/SPI_IOC_RD_MODE 设置SPI_MODE_0,设置SPI_IOC_WR_BITS_PER_WORD/SPI_IOC_RD_BITS_PER_WORD为25000000，设置SPI_IOC_WR_MAX_SPEED_HZ/SPI_IOC_RD_MAX_SPEED_HZ 为25000000，最后就可以通过io(fd,SPI_IOC_MESSAGE(len),&spi),其中spi的类型为spi_ioc_transfer spi[len].来读写spi device。
这个驱动的实现在driver/spi/spidev.c中，这是个内核模块，其入口函数为
static int __init spidev_init(void)
{
	int status;

	/* Claim our 256 reserved device numbers.  Then register a class
	 * that will key udev/mdev to add/remove /dev nodes.  Last, register
	 * the driver which manages those device numbers.
	 */
	BUILD_BUG_ON(N_SPI_MINORS > 256);
	status = register_chrdev(SPIDEV_MAJOR, "spi", &spidev_fops);
	if (status < 0)
		return status;

	spidev_class = class_create(THIS_MODULE, "spidev");
	if (IS_ERR(spidev_class)) {
		unregister_chrdev(SPIDEV_MAJOR, spidev_spi_driver.driver.name);
		return PTR_ERR(spidev_class);
	}
#注册一个spi 驱动
	status = spi_register_driver(&spidev_spi_driver);
	if (status < 0) {
		class_destroy(spidev_class);
		unregister_chrdev(SPIDEV_MAJOR, spidev_spi_driver.driver.name);
	}
	return status;
}
module_init(spidev_init);

static struct spi_driver spidev_spi_driver = {
#在spidev_init 中注册的驱动的name为spidev，也就是我们我们在用户态需要open的device，可以先通过ls  /dev/spidev0.0
	.driver = {
		.name =		"spidev",
		.of_match_table = of_match_ptr(spidev_dt_ids),
		.acpi_match_table = ACPI_PTR(spidev_acpi_ids),
	},
	.probe =	spidev_probe,
	.remove =	spidev_remove,

	/* NOTE:  suspend/resume methods are not necessary here.
	 * We don't do anything except pass the requests to/from
	 * the underlying controller.  The refrigerator handles
	 * most issues; the controller driver handles the rest.
	 */
};