linux驱动开发----zynq----SPI驱动（总线框架）_zynq linux spi驱动开发

注：本文针对linux-xlnx-xilinx-v2017.4版本进行介绍

一、引言      

        前面我们讲到了SPI的接口标准，以及SPI的工作模式，那么知道这些其实就可以进行SPI驱动的开发了，但是我们这里讲的是linux驱动开发，那么是不是掌握了SPI接口的一些知识就可以进行linux驱动开发呢？当然不是，linux驱动开发对应于总线类型（如：SPI / IIC / USB）的驱动，有一套独立的驱动框架，SPI驱动的开发就需要在SPI驱动框架中去实现。

二、体系结构

        SPI的驱动框架主要包含三个部分：SPI主机控制器驱动、SPI 核心、SPI设备驱动。

        可能说到这里，有人对SPI控制器、SPI核心、SPI设备驱动还不清楚是什么东西？那么我们就以xilinx的zynqmp系列芯片来讲解这几个模块。

        SPI主机控制器是具有特定属性的，这个主要看处理器上搭载的是哪个公司生产的SPI控制器，针对zynq中可以在其datasheet中找到其SPI控制器是cadence公司的，那么在内核中必然会存在cadence控制器的驱动程序（如果某个芯片在linux内核驱动中没有对应的驱动程序，那就说明这个芯片太小众了，估计后续技术支持也跟不上）

       SPI核心是通用文件，一方面对SPI子系统进行初始化工作，注册spi bus，注册spi_master class，同时提供spi设备驱动对spi总线进行操作的API。SPI设备驱动包含的种类较多，可以是FLASH驱动、RTC驱动等等，通常对于应用程序来说，在应用层中直接操作设备的file_operation的接口，根本不需要关心SPI总线是如何工作的，这就能很好的将主机与设备进行隔离。

        那有人要说了，我就想用个SPI去发个数据，那怎么办呢？当然，linux内核也可以将主机控制器实现为一个字符设备spidev，这是一个通用的SPI设备文件，应用程序可直接利用spidev来控制SPI主机控制器来产生时序信号，实现对SPI设备的访问。

在linux内核源码里，SPI核心是由\drivers\spi\spi.c来实现的，主机控制器程序是由\drivers\spi\spi-cadence.c来实现的，字符设备spidev由\drivers\spi\spidev.c实现。下面，我们以spidev.c、spi.c、spi-cadence.c这三个文件来分析SPI的总线驱动模型。

       首先，我们看看这个spidev.c文件，我们可以先猜猜这个文件是干啥的？首先它是一个字符设备，那么必然满足字符设备的框架（注册设备、构造file_operation结构体、提供给虚拟文件系统的open、read、write函数接口）。

#include <linux/init.h>
#include <linux/module.h>
#include <linux/ioctl.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/err.h>
#include <linux/list.h>
#include <linux/errno.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/compat.h>
#include <linux/of.h>
#include <linux/of_device.h>
#include <linux/acpi.h>
 
#include <linux/spi/spi.h>
#include <linux/spi/spidev.h>
 
#include <linux/uaccess.h>
 
 
#define SPIDEV_MAJOR			153	/* assigned */
#define N_SPI_MINORS			32	/* ... up to 256 */
 
static DECLARE_BITMAP(minors, N_SPI_MINORS);
 
#define SPI_MODE_MASK		(SPI_CPHA | SPI_CPOL | SPI_CS_HIGH \
				| SPI_LSB_FIRST | SPI_3WIRE | SPI_LOOP \
				| SPI_NO_CS | SPI_READY | SPI_TX_DUAL \
				| SPI_TX_QUAD | SPI_RX_DUAL | SPI_RX_QUAD)
 
struct spidev_data {                        //spidev的结构体
	dev_t			devt;
	spinlock_t		spi_lock;
	struct spi_device	*spi;
	struct list_head	device_entry;
	struct mutex		buf_lock;
	unsigned		users;
	u8			*tx_buffer;
	u8			*rx_buffer;
	u32			speed_hz;
};
 
static LIST_HEAD(device_list);
static DEFINE_MUTEX(device_list_lock);
 
static unsigned bufsiz = 4096;
module_param(bufsiz, uint, S_IRUGO);
MODULE_PARM_DESC(bufsiz, "data bytes in biggest supported SPI message");
 
/*------------------------spidev的同步操作-----------------------------*/
static ssize_t spidev_sync(struct spidev_data *spidev, struct spi_message *message)
{
	DECLARE_COMPLETION_ONSTACK(done);
	int status;
	struct spi_device *spi;
 
	spin_lock_irq(&spidev->spi_lock);
	spi = spidev->spi;
	spin_unlock_irq(&spidev->spi_lock);
 
	if (spi == NULL)
		status = -ESHUTDOWN;
	else
		status = spi_sync(spi, message);		/*调用spi.c中的函数，进行同步操作*/
 
	if (status == 0)
		status = message->actual_length;
 
	return status;
}
 
/*------------------------spidev同步写操作-----------------------------*/
static inline ssize_t
spidev_sync_write(struct spidev_data *spidev, size_t len)	
{
	struct spi_transfer	t = {
			.tx_buf		= spidev->tx_buffer,
			.len		= len,
			.speed_hz	= spidev->speed_hz,
		};
	struct spi_message	m;
 
	spi_message_init(&m);
	spi_message_add_tail(&t, &m);
	return spidev_sync(spidev, &m);
}
 
/*------------------------spidev同步读操作-----------------------------*/
static inline ssize_t
spidev_sync_read(struct spidev_data *spidev, size_t len)		
{
	struct spi_transfer	t = {
			.rx_buf		= spidev->rx_buffer,
			.len		= len,
			.speed_hz	= spidev->speed_hz,
		};
	struct spi_message	m;
 
	spi_message_init(&m);
	spi_message_add_tail(&t, &m);
	return spidev_sync(spidev, &m);
}
 
 
/*------------------------spidev只读-----------------------------*/
/* Read-only message with current device setup */
static ssize_t
spidev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
/*spidev读操作（只读模式），，对应于用户空间的read函数*/
{
	struct spidev_data	*spidev;
	ssize_t			status = 0;
 
	/* chipselect only toggles at start or end of operation */
	if (count > bufsiz)
		return -EMSGSIZE;
 
	spidev = filp->private_data;
 
	mutex_lock(&spidev->buf_lock);
	status = spidev_sync_read(spidev, count);		/*spidev同步读操作*/
	if (status > 0) {
		unsigned long	missing;
 
		missing = copy_to_user(buf, spidev->rx_buffer, status);			/*将读回来的数返回给用户空间*/
		if (missing == status)
			status = -EFAULT;
		else
			status = status - missing;
	}
	mutex_unlock(&spidev->buf_lock);
 
	return status;
}
 
/*------------------------spidev只写-----------------------------*/
/* Write-only message with current device setup */
static ssize_t
spidev_write(struct file *filp, const char __user *buf,			
/*spidev写操作（只写模式），对应于用户空间的write函数*/
		size_t count, loff_t *f_pos)
{
	struct spidev_data	*spidev;
	ssize_t			status = 0;
	unsigned long		missing;
 
	/* chipselect only toggles at start or end of operation */
	if (count > bufsiz)
		return -EMSGSIZE;
 
	spidev = filp->private_data;
	mutex_lock(&spidev->buf_lock);
	missing = copy_from_user(spidev->tx_buffer, buf, count);	/*将用户空间中写入spidev的数据拷贝到内核空间*/
 
	if (missing == 0)
		status = spidev_sync_write(spidev, count);		/*进行同步写操作*/
	else
		status = -EFAULT;
	mutex_unlock(&spidev->buf_lock);
 
	return status;
}
 
/*------------------------spidev读写操作-----------------------------*/
static int spidev_message(struct spidev_data *spidev,		/*启动spidev的数据传输，相当于写一次读一次*/
		struct spi_ioc_transfer *u_xfers, unsigned n_xfers)
{
	struct spi_message	msg;
	struct spi_transfer	*k_xfers;
	struct spi_transfer	*k_tmp;
	struct spi_ioc_transfer *u_tmp;
	unsigned		n, total, tx_total, rx_total;
	u8			*tx_buf, *rx_buf;
	int			status = -EFAULT;
 
	spi_message_init(&msg);
	k_xfers = kcalloc(n_xfers, sizeof(*k_tmp), GFP_KERNEL);
	if (k_xfers == NULL)
		return -ENOMEM;
 
	/* Construct spi_message, copying any tx data to bounce buffer.
	 * We walk the array of user-provided transfers, using each one
	 * to initialize a kernel version of the same transfer.
	 */
	tx_buf = spidev->tx_buffer;
	rx_buf = spidev->rx_buffer;
	total = 0;
	tx_total = 0;
	rx_total = 0;
	for (n = n_xfers, k_tmp = k_xfers, u_tmp = u_xfers;
			n;
			n--, k_tmp++, u_tmp++) {
		k_tmp->len = u_tmp->len;
 
		total += k_tmp->len;
		/* Since the function returns the total length of transfers
		 * on success, restrict the total to positive int values to
		 * avoid the return value looking like an error.  Also check
		 * each transfer length to avoid arithmetic overflow.
		 */
		if (total > INT_MAX || k_tmp->len > INT_MAX) {
			status = -EMSGSIZE;
			goto done;
		}
 
		if (u_tmp->rx_buf) {
			/* this transfer needs space in RX bounce buffer */
			rx_total += k_tmp->len;
			if (rx_total > bufsiz) {
				status = -EMSGSIZE;
				goto done;
			}
			k_tmp->rx_buf = rx_buf;
			if (!access_ok(VERIFY_WRITE, (u8 __user *)
						(uintptr_t) u_tmp->rx_buf,
						u_tmp->len))
				goto done;
			rx_buf += k_tmp->len;
		}
		if (u_tmp->tx_buf) {
			/* this transfer needs space in TX bounce buffer */
			tx_total += k_tmp->len;
			if (tx_total > bufsiz) {
				status = -EMSGSIZE;
				goto done;
			}
			k_tmp->tx_buf = tx_buf;
			if (copy_from_user(tx_buf, (const u8 __user *)
						(uintptr_t) u_tmp->tx_buf,
					u_tmp->len))
				goto done;
			tx_buf += k_tmp->len;
		}
 
		k_tmp->cs_change = !!u_tmp->cs_change;
		k_tmp->tx_nbits = u_tmp->tx_nbits;
		k_tmp->rx_nbits = u_tmp->rx_nbits;
		k_tmp->bits_per_word = u_tmp->bits_per_word;
		k_tmp->delay_usecs = u_tmp->delay_usecs;
		k_tmp->speed_hz = u_tmp->speed_hz;
		if (!k_tmp->speed_hz)
			k_tmp->speed_hz = spidev->speed_hz;
#ifdef VERBOSE
		dev_dbg(&spidev->spi->dev,
			"  xfer len %u %s%s%s%dbits %u usec %uHz\n",
			u_tmp->len,
			u_tmp->rx_buf ? "rx " : "",
			u_tmp->tx_buf ? "tx " : "",
			u_tmp->cs_change ? "cs " : "",
			u_tmp->bits_per_word ? : spidev->spi->bits_per_word,
			u_tmp->delay_usecs,
			u_tmp->speed_hz ? : spidev->spi->max_speed_hz);
#endif
		spi_message_add_tail(k_tmp, &msg);
	}
 
	status = spidev_sync(spidev, &msg);
	if (status < 0)
		goto done;
 
	/* copy any rx data out of bounce buffer */
	rx_buf = spidev->rx_buffer;
	for (n = n_xfers, u_tmp = u_xfers; n; n--, u_tmp++) {
		if (u_tmp->rx_buf) {
			if (__copy_to_user((u8 __user *)
					(uintptr_t) u_tmp->rx_buf, rx_buf,
					u_tmp->len)) {
				status = -EFAULT;
				goto done;
			}
			rx_buf += u_tmp->len;
		}
	}
	status = total;
 
done:
	kfree(k_xfers);
	return status;
}
 
/*------------------------获取用户空间的ioc消息体-----------------------------*/
static struct spi_ioc_transfer *
spidev_get_ioc_message(unsigned int cmd, struct spi_ioc_transfer __user *u_ioc,
		unsigned *n_ioc)
{
	struct spi_ioc_transfer	*ioc;
	u32	tmp;
 
	/* Check type, command number and direction */
	if (_IOC_TYPE(cmd) != SPI_IOC_MAGIC
			|| _IOC_NR(cmd) != _IOC_NR(SPI_IOC_MESSAGE(0))
			|| _IOC_DIR(cmd) != _IOC_WRITE)
		return ERR_PTR(-ENOTTY);
 
	tmp = _IOC_SIZE(cmd);
	if ((tmp % sizeof(struct spi_ioc_transfer)) != 0)
		return ERR_PTR(-EINVAL);
	*n_ioc = tmp / sizeof(struct spi_ioc_transfer);
	if (*n_ioc == 0)
		return NULL;
 
	/* copy into scratch area */
	ioc = kmalloc(tmp, GFP_KERNEL);
	if (!ioc)
		return ERR_PTR(-ENOMEM);
	if (__copy_from_user(ioc, u_ioc, tmp)) {
		kfree(ioc);
		return ERR_PTR(-EFAULT);
	}
	return ioc;
}
 
/*------------------------spi_ioctl函数，对应于用户空间的ioctl函数-----------------------------*/
static long
spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
	int			err = 0;
	int			retval = 0;
	struct spidev_data	*spidev;
	struct spi_device	*spi;
	u32			tmp;
	unsigned		n_ioc;
	struct spi_ioc_transfer	*ioc;
 
	/* Check type and command number */
	if (_IOC_TYPE(cmd) != SPI_IOC_MAGIC)
		return -ENOTTY;
 
	/* Check access direction once here; don't repeat below.
	 * IOC_DIR is from the user perspective, while access_ok is
	 * from the kernel perspective; so they look reversed.
	 */
	if (_IOC_DIR(cmd) & _IOC_READ)
		err = !access_ok(VERIFY_WRITE,
				(void __user *)arg, _IOC_SIZE(cmd));
	if (err == 0 && _IOC_DIR(cmd) & _IOC_WRITE)
		err = !access_ok(VERIFY_READ,
				(void __user *)arg, _IOC_SIZE(cmd));
	if (err)
		return -EFAULT;
 
	/* guard against device removal before, or while,
	 * we issue this ioctl.
	 */
	spidev = filp->private_data;
	spin_lock_irq(&spidev->spi_lock);
	spi = spi_dev_get(spidev->spi);
	spin_unlock_irq(&spidev->spi_lock);
 
	if (spi == NULL)
		return -ESHUTDOWN;
 
	/* use the buffer lock here for triple duty:
	 *  - prevent I/O (from us) so calling spi_setup() is safe;
	 *  - prevent concurrent SPI_IOC_WR_* from morphing
	 *    data fields while SPI_IOC_RD_* reads them;
	 *  - SPI_IOC_MESSAGE needs the buffer locked "normally".
	 */
	mutex_lock(&spidev->buf_lock);
 
	switch (cmd) {			/*判断ioctl传入的命令*/
	/* read requests */
	case SPI_IOC_RD_MODE:
		retval = __put_user(spi->mode & SPI_MODE_MASK,
					(__u8 __user *)arg);
		break;
	case SPI_IOC_RD_MODE32:
		retval = __put_user(spi->mode & SPI_MODE_MASK,
					(__u32 __user *)arg);
		break;
	case SPI_IOC_RD_LSB_FIRST:
		retval = __put_user((spi->mode & SPI_LSB_FIRST) ?  1 : 0,
					(__u8 __user *)arg);
		break;
	case SPI_IOC_RD_BITS_PER_WORD:
		retval = __put_user(spi->bits_per_word, (__u8 __user *)arg);
		break;
	case SPI_IOC_RD_MAX_SPEED_HZ:
		retval = __put_user(spidev->speed_hz, (__u32 __user *)arg);
		break;
 
	/* write requests */
	case SPI_IOC_WR_MODE:
	case SPI_IOC_WR_MODE32:
		if (cmd == SPI_IOC_WR_MODE)
			retval = __get_user(tmp, (u8 __user *)arg);
		else
			retval = __get_user(tmp, (u32 __user *)arg);
		if (retval == 0) {
			u32	save = spi->mode;
 
			if (tmp & ~SPI_MODE_MASK) {
				retval = -EINVAL;
				break;
			}
 
			tmp |= spi->mode & ~SPI_MODE_MASK;
			spi->mode = (u16)tmp;
			retval = spi_setup(spi);
			if (retval < 0)
				spi->mode = save;
			else
				dev_dbg(&spi->dev, "spi mode %x\n", tmp);
		}
		break;
	case SPI_IOC_WR_LSB_FIRST:
		retval = __get_user(tmp, (__u8 __user *)arg);
		if (retval == 0) {
			u32	save = spi->mode;
 
			if (tmp)
				spi->mode |= SPI_LSB_FIRST;
			else
				spi->mode &= ~SPI_LSB_FIRST;
			retval = spi_setup(spi);
			if (retval < 0)
				spi->mode = save;
			else
				dev_dbg(&spi->dev, "%csb first\n",
						tmp ? 'l' : 'm');
		}
		break;
	case SPI_IOC_WR_BITS_PER_WORD:
		retval = __get_user(tmp, (__u8 __user *)arg);
		if (retval == 0) {
			u8	save = spi->bits_per_word;
 
			spi->bits_per_word = tmp;
			retval = spi_setup(spi);
			if (retval < 0)
				spi->bits_per_word = save;
			else
				dev_dbg(&spi->dev, "%d bits per word\n", tmp);
		}
		break;
	case SPI_IOC_WR_MAX_SPEED_HZ:
		retval = __get_user(tmp, (__u32 __user *)arg);
		if (retval == 0) {
			u32	save = spi->max_speed_hz;
 
			spi->max_speed_hz = tmp;
			retval = spi_setup(spi);
			if (retval >= 0)
				spidev->speed_hz = tmp;
			else
				dev_dbg(&spi->dev, "%d Hz (max)\n", tmp);
			spi->max_speed_hz = save;
		}
		break;
 
	default:					/*执行一次发送*/
		/* segmented and/or full-duplex I/O request */
		/* Check message and copy into scratch area */
		ioc = spidev_get_ioc_message(cmd,
				(struct spi_ioc_transfer __user *)arg, &n_ioc);
		if (IS_ERR(ioc)) {
			retval = PTR_ERR(ioc);
			break;
		}
		if (!ioc)
			break;	/* n_ioc is also 0 */
 
		/* translate to spi_message, execute */
		retval = spidev_message(spidev, ioc, n_ioc);
		kfree(ioc);
		break;
	}
 
	mutex_unlock(&spidev->buf_lock);
	spi_dev_put(spi);
	return retval;
}
 
#ifdef CONFIG_COMPAT
static long
spidev_compat_ioc_message(struct file *filp, unsigned int cmd,
		unsigned long arg)
{
	struct spi_ioc_transfer __user	*u_ioc;
	int				retval = 0;
	struct spidev_data		*spidev;
	struct spi_device		*spi;
	unsigned			n_ioc, n;
	struct spi_ioc_transfer		*ioc;
 
	u_ioc = (struct spi_ioc_transfer __user *) compat_ptr(arg);
	if (!access_ok(VERIFY_READ, u_ioc, _IOC_SIZE(cmd)))
		return -EFAULT;
 
	/* guard against device removal before, or while,
	 * we issue this ioctl.
	 */
	spidev = filp->private_data;
	spin_lock_irq(&spidev->spi_lock);
	spi = spi_dev_get(spidev->spi);
	spin_unlock_irq(&spidev->spi_lock);
 
	if (spi == NULL)
		return -ESHUTDOWN;
 
	/* SPI_IOC_MESSAGE needs the buffer locked "normally" */
	mutex_lock(&spidev->buf_lock);
 
	/* Check message and copy into scratch area */
	ioc = spidev_get_ioc_message(cmd, u_ioc, &n_ioc);
	if (IS_ERR(ioc)) {
		retval = PTR_ERR(ioc);
		goto done;
	}
	if (!ioc)
		goto done;	/* n_ioc is also 0 */
 
	/* Convert buffer pointers */
	for (n = 0; n < n_ioc; n++) {
		ioc[n].rx_buf = (uintptr_t) compat_ptr(ioc[n].rx_buf);
		ioc[n].tx_buf = (uintptr_t) compat_ptr(ioc[n].tx_buf);
	}
 
	/* translate to spi_message, execute */
	retval = spidev_message(spidev, ioc, n_ioc);
	kfree(ioc);
 
done:
	mutex_unlock(&spidev->buf_lock);
	spi_dev_put(spi);
	return retval;
}
 
static long
spidev_compat_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
	if (_IOC_TYPE(cmd) == SPI_IOC_MAGIC
			&& _IOC_NR(cmd) == _IOC_NR(SPI_IOC_MESSAGE(0))
			&& _IOC_DIR(cmd) == _IOC_WRITE)
		return spidev_compat_ioc_message(filp, cmd, arg);
 
	return spidev_ioctl(filp, cmd, (unsigned long)compat_ptr(arg));
}
#else
#define spidev_compat_ioctl NULL
#endif /* CONFIG_COMPAT */
 
/*------------------------打开spidev设备-----------------------------*/
static int spidev_open(struct inode *inode, struct file *filp)		
{
	struct spidev_data	*spidev;
	int			status = -ENXIO;
 
	mutex_lock(&device_list_lock);
 
	list_for_each_entry(spidev, &device_list, device_entry) {
		if (spidev->devt == inode->i_rdev) {
			status = 0;
			break;
		}
	}
 
	if (status) {
		pr_debug("spidev: nothing for minor %d\n", iminor(inode));
		goto err_find_dev;
	}
 
	if (!spidev->tx_buffer) {
		spidev->tx_buffer = kmalloc(bufsiz, GFP_KERNEL);/*从内核中分配一块内存给tx_buffer*/
		if (!spidev->tx_buffer) {
			dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
			status = -ENOMEM;
			goto err_find_dev;
		}
	}
 
	if (!spidev->rx_buffer) {
		spidev->rx_buffer = kmalloc(bufsiz, GFP_KERNEL);/*从内核中分配一块内存给rx_buffer*/
		if (!spidev->rx_buffer) {
			dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
			status = -ENOMEM;
			goto err_alloc_rx_buf;
		}
	}
 
	spidev->users++;
	filp->private_data = spidev;
	nonseekable_open(inode, filp);		/*不需要可搜索文件描述符的子系统使用它*/
 
	mutex_unlock(&device_list_lock);
	return 0;
 
err_alloc_rx_buf:
	kfree(spidev->tx_buffer);
	spidev->tx_buffer = NULL;
err_find_dev:
	mutex_unlock(&device_list_lock);
	return status;
}
 
/*------------------------释放spidev设备-----------------------------*/
static int spidev_release(struct inode *inode, struct file *filp)				
{
	struct spidev_data	*spidev;
 
	mutex_lock(&device_list_lock);
	spidev = filp->private_data;
	filp->private_data = NULL;
 
	/* last close? */
	spidev->users--;
	if (!spidev->users) {
		int		dofree;
 
		kfree(spidev->tx_buffer);
		spidev->tx_buffer = NULL;
 
		kfree(spidev->rx_buffer);
		spidev->rx_buffer = NULL;
 
		spin_lock_irq(&spidev->spi_lock);
		if (spidev->spi)
			spidev->speed_hz = spidev->spi->max_speed_hz;
 
		/* ... after we unbound from the underlying device? */
		dofree = (spidev->spi == NULL);
		spin_unlock_irq(&spidev->spi_lock);
 
		if (dofree)
			kfree(spidev);
	}
	mutex_unlock(&device_list_lock);
 
	return 0;
}
 
/*-------------------------------------构造file_operation结构体------------------------------------*/
static const struct file_operations spidev_fops = {			
	.owner =	THIS_MODULE,
	/* REVISIT switch to aio primitives, so that userspace
	 * gets more complete API coverage.  It'll simplify things
	 * too, except for the locking.
	 */
	.write =	spidev_write,
	.read =		spidev_read,
	.unlocked_ioctl = spidev_ioctl,
	.compat_ioctl = spidev_compat_ioctl,
	.open =		spidev_open,
	.release =	spidev_release,
	.llseek =	no_llseek,
};
 
/*-------------------------------------------------------------------------*/
 
/* The main reason to have this class is to make mdev/udev create the
 * /dev/spidevB.C character device nodes exposing our userspace API.
 * It also simplifies memory management.
 */
 
static struct class *spidev_class;
 
#ifdef CONFIG_OF
static const struct of_device_id spidev_dt_ids[] = {	//驱动程序的可匹配的设备列表
	{ .compatible = "rohm,dh2228fv" },
	{ .compatible = "lineartechnology,ltc2488" },
	{ .compatible = "foocorp,modem" },
	{},
};
MODULE_DEVICE_TABLE(of, spidev_dt_ids);
#endif
 
#ifdef CONFIG_ACPI
 
/* Dummy SPI devices not to be used in production systems */
#define SPIDEV_ACPI_DUMMY	1
 
static const struct acpi_device_id spidev_acpi_ids[] = {
	/*
	 * The ACPI SPT000* devices are only meant for development and
	 * testing. Systems used in production should have a proper ACPI
	 * description of the connected peripheral and they should also use
	 * a proper driver instead of poking directly to the SPI bus.
	 */
	{ "SPT0001", SPIDEV_ACPI_DUMMY },
	{ "SPT0002", SPIDEV_ACPI_DUMMY },
	{ "SPT0003", SPIDEV_ACPI_DUMMY },
	{},
};
MODULE_DEVICE_TABLE(acpi, spidev_acpi_ids);
 
static void spidev_probe_acpi(struct spi_device *spi)
{
	const struct acpi_device_id *id;
 
	if (!has_acpi_companion(&spi->dev))
		return;
 
	id = acpi_match_device(spidev_acpi_ids, &spi->dev);
	if (WARN_ON(!id))
		return;
 
	if (id->driver_data == SPIDEV_ACPI_DUMMY)
		dev_warn(&spi->dev, "do not use this driver in production systems!\n");
}
#else
static inline void spidev_probe_acpi(struct spi_device *spi) {}
#endif
 
/*-------------------------------------------------------------------------*/
static int spidev_probe(struct spi_device *spi)		/*spidev初始化函数*/
{
	struct spidev_data	*spidev;
	int			status;
	unsigned long		minor;
 
	/*
	 * spidev should never be referenced in DT without a specific
	 * compatible string, it is a Linux implementation thing
	 * rather than a description of the hardware.
	 */
	if (spi->dev.of_node && !of_match_device(spidev_dt_ids, &spi->dev)) {			/*判断设备树中有没有匹配的字符串*/
		dev_err(&spi->dev, "buggy DT: spidev listed directly in DT\n");
		WARN_ON(spi->dev.of_node &&
			!of_match_device(spidev_dt_ids, &spi->dev));
	}
 
	spidev_probe_acpi(spi);			/*高级配置和电源管理接口*/
 
	/* Allocate driver data */
	spidev = kzalloc(sizeof(*spidev), GFP_KERNEL);	/*从内核中分配一个spidev_data结构体*/
	if (!spidev)
		return -ENOMEM;
 
	/* Initialize the driver data */
	spidev->spi = spi;
	spin_lock_init(&spidev->spi_lock);
	mutex_init(&spidev->buf_lock);
 
	INIT_LIST_HEAD(&spidev->device_entry);
 
	/* If we can allocate a minor number, hook up this device.
	 * Reusing minors is fine so long as udev or mdev is working.
	 */
	mutex_lock(&device_list_lock);
	minor = find_first_zero_bit(minors, N_SPI_MINORS);	/*查找一个可用的次设备号*/
	if (minor < N_SPI_MINORS) {
		struct device *dev;
 
		spidev->devt = MKDEV(SPIDEV_MAJOR, minor);
		dev = device_create(spidev_class, &spi->dev, spidev->devt,	/*创建spidev设备*/
				    spidev, "spidev%d.%d",
				    spi->master->bus_num, spi->chip_select);
		status = PTR_ERR_OR_ZERO(dev);
	} else {
		dev_dbg(&spi->dev, "no minor number available!\n");
		status = -ENODEV;
	}
	if (status == 0) {
		set_bit(minor, minors);
		list_add(&spidev->device_entry, &device_list);
	}
	mutex_unlock(&device_list_lock);
 
	spidev->speed_hz = spi->max_speed_hz;
	if (status == 0)
		spi_set_drvdata(spi, spidev);
	else
		kfree(spidev);
 
	return status;
}
 
static int spidev_remove(struct spi_device *spi)	/*spidev移除函数*/
{
	struct spidev_data	*spidev = spi_get_drvdata(spi);
 
	/* make sure ops on existing fds can abort cleanly */
	spin_lock_irq(&spidev->spi_lock);
	spidev->spi = NULL;
	spin_unlock_irq(&spidev->spi_lock);
 
	/* prevent new opens */
	mutex_lock(&device_list_lock);
	list_del(&spidev->device_entry);
	device_destroy(spidev_class, spidev->devt);	/*从spidev_class删除spidev*/
	clear_bit(MINOR(spidev->devt), minors);		/*清除当前spidev的次设备号*/
	if (spidev->users == 0)
		kfree(spidev);
	mutex_unlock(&device_list_lock);
 
	return 0;
}
 
static struct spi_driver spidev_spi_driver = {
	.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.
	 */
};
 
/*-------------------------------------------------------------------------*/
 
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);/*注册spidev字符设备*/
	if (status < 0)
		return status;
 
	spidev_class = class_create(THIS_MODULE, "spidev");	/*创建spidev_class，并将spidev注册到内核中*/
	if (IS_ERR(spidev_class)) {
		unregister_chrdev(SPIDEV_MAJOR, spidev_spi_driver.driver.name);
		return PTR_ERR(spidev_class);
	}
 
	status = spi_register_driver(&spidev_spi_driver);	/*注册spi驱动*/
	if (status < 0) {
		class_destroy(spidev_class);
		unregister_chrdev(SPIDEV_MAJOR, spidev_spi_driver.driver.name);
	}
	return status;
}
module_init(spidev_init);			//作为模块加载进内核
 
static void __exit spidev_exit(void)
{
	spi_unregister_driver(&spidev_spi_driver);
	class_destroy(spidev_class);
	unregister_chrdev(SPIDEV_MAJOR, spidev_spi_driver.driver.name);
}
module_exit(spidev_exit);			//从内核卸载该模块
 
MODULE_AUTHOR("Andrea Paterniani, <a.paterniani@swapp-eng.it>");//模块声明
MODULE_DESCRIPTION("User mode SPI device interface");
MODULE_LICENSE("GPL");
MODULE_ALIAS("spi:spidev");

        spidev.c文件中包含有841行代码，那么我们应该怎么看这个代码呢？其实要想初步了解下各个函数的意思还是比较简单的，我们顺着内核模块加载的思路去分析下代码。

       当驱动编译好要insmod进内核时，执行的就是module_init(spidev_init)，有加载必然有卸载函数，卸载时就执行module_exit(spidev_exit)。那么我们从module_init(spidev_init)来分析：

下面我们看看spi-cadence.c

/*
 * Cadence SPI controller driver (master mode only)
 *
 * Copyright (C) 2008 - 2014 Xilinx, Inc.
 *
 * based on Blackfin On-Chip SPI Driver (spi_bfin5xx.c)
 *
 * This program is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License version 2 as published by the
 * Free Software Foundation; either version 2 of the License, or (at your
 * option) any later version.
 */
 
#include <linux/clk.h>
#include <linux/delay.h>
#include <linux/interrupt.h>
#include <linux/io.h>
#include <linux/module.h>
#include <linux/of_irq.h>
#include <linux/of_address.h>
#include <linux/platform_device.h>
#include <linux/pm_runtime.h>
#include <linux/spi/spi.h>
 
/* Name of this driver */
#define CDNS_SPI_NAME		"cdns-spi"
 
/* Register offset definitions */
#define CDNS_SPI_CR	0x00 /* Configuration  Register, RW */
#define CDNS_SPI_ISR	0x04 /* Interrupt Status Register, RO */
#define CDNS_SPI_IER	0x08 /* Interrupt Enable Register, WO */
#define CDNS_SPI_IDR	0x0c /* Interrupt Disable Register, WO */
#define CDNS_SPI_IMR	0x10 /* Interrupt Enabled Mask Register, RO */
#define CDNS_SPI_ER	0x14 /* Enable/Disable Register, RW */
#define CDNS_SPI_DR	0x18 /* Delay Register, RW */
#define CDNS_SPI_TXD	0x1C /* Data Transmit Register, WO */
#define CDNS_SPI_RXD	0x20 /* Data Receive Register, RO */
#define CDNS_SPI_SICR	0x24 /* Slave Idle Count Register, RW */
#define CDNS_SPI_THLD	0x28 /* Transmit FIFO Watermark Register,RW */
 
#define SPI_AUTOSUSPEND_TIMEOUT		3000
/*
 * SPI Configuration Register bit Masks
 *
 * This register contains various control bits that affect the operation
 * of the SPI controller
 */
#define CDNS_SPI_CR_MANSTRT	0x00010000 /* Manual TX Start */
#define CDNS_SPI_CR_CPHA		0x00000004 /* Clock Phase Control */
#define CDNS_SPI_CR_CPOL		0x00000002 /* Clock Polarity Control */
#define CDNS_SPI_CR_SSCTRL		0x00003C00 /* Slave Select Mask */
#define CDNS_SPI_CR_PERI_SEL	0x00000200 /* Peripheral Select Decode */
#define CDNS_SPI_CR_BAUD_DIV	0x00000038 /* Baud Rate Divisor Mask */
#define CDNS_SPI_CR_MSTREN		0x00000001 /* Master Enable Mask */
#define CDNS_SPI_CR_MANSTRTEN	0x00008000 /* Manual TX Enable Mask */
#define CDNS_SPI_CR_SSFORCE	0x00004000 /* Manual SS Enable Mask */
#define CDNS_SPI_CR_BAUD_DIV_4	0x00000008 /* Default Baud Div Mask */
#define CDNS_SPI_CR_DEFAULT	(CDNS_SPI_CR_MSTREN | \
					CDNS_SPI_CR_SSCTRL | \
					CDNS_SPI_CR_BAUD_DIV_4)
//	CDNS_SPI_CR_SSFORCE | \
/*
 * SPI Configuration Register - Baud rate and slave select
 *
 * These are the values used in the calculation of baud rate divisor and
 * setting the slave select.
 */
 
#define CDNS_SPI_BAUD_DIV_MAX		7 /* Baud rate divisor maximum */
#define CDNS_SPI_BAUD_DIV_MIN		1 /* Baud rate divisor minimum */
#define CDNS_SPI_BAUD_DIV_SHIFT		3 /* Baud rate divisor shift in CR */
#define CDNS_SPI_SS_SHIFT		10 /* Slave Select field shift in CR */
#define CDNS_SPI_SS0			0x1 /* Slave Select zero */
 
/*
 * SPI Interrupt Registers bit Masks
 *
 * All the four interrupt registers (Status/Mask/Enable/Disable) have the same
 * bit definitions.
 */
#define CDNS_SPI_IXR_TXOW	0x00000004 /* SPI TX FIFO Overwater */
#define CDNS_SPI_IXR_MODF	0x00000002 /* SPI Mode Fault */
#define CDNS_SPI_IXR_RXNEMTY 0x00000010 /* SPI RX FIFO Not Empty */
#define CDNS_SPI_IXR_DEFAULT	(CDNS_SPI_IXR_TXOW | \
					CDNS_SPI_IXR_MODF)
#define CDNS_SPI_IXR_TXFULL	0x00000008 /* SPI TX Full */
#define CDNS_SPI_IXR_ALL	0x0000007F /* SPI all interrupts */
 
/*
 * SPI Enable Register bit Masks
 *
 * This register is used to enable or disable the SPI controller
 */
#define CDNS_SPI_ER_ENABLE	0x00000001 /* SPI Enable Bit Mask */
#define CDNS_SPI_ER_DISABLE	0x0 /* SPI Disable Bit Mask */
 
/* SPI FIFO depth in bytes */
#define CDNS_SPI_FIFO_DEPTH	128
 
/* Default number of chip select lines */
#define CDNS_SPI_DEFAULT_NUM_CS		4
 
/**
 * struct cdns_spi - This definition defines spi driver instance
 * @regs:		Virtual address of the SPI controller registers
 * @ref_clk:		Pointer to the peripheral clock
 * @pclk:		Pointer to the APB clock
 * @speed_hz:		Current SPI bus clock speed in Hz
 * @txbuf:		Pointer	to the TX buffer
 * @rxbuf:		Pointer to the RX buffer
 * @tx_bytes:		Number of bytes left to transfer
 * @rx_bytes:		Number of bytes requested
 * @dev_busy:		Device busy flag
 * @is_decoded_cs:	Flag for decoder property set or not
 */
struct cdns_spi {			/*定义cadence_spi驱动结构体，一个结构体就是一个对象*/
	void __iomem *regs;
	struct clk *ref_clk;
	struct clk *pclk;
	u32 speed_hz;
	const u8 *txbuf;
	u8 *rxbuf;
	int tx_bytes;
	int rx_bytes;
	u8 dev_busy;
	u32 is_decoded_cs;
};
 
/* Macros for the SPI controller read/write */
static inline u32 cdns_spi_read(struct cdns_spi *xspi, u32 offset)/*cadence_spi读寄存器*/
{
	return readl_relaxed(xspi->regs + offset);
}
 
static inline void cdns_spi_write(struct cdns_spi *xspi, u32 offset, u32 val)/*cadence_spi写寄存器*/
{
	writel_relaxed(val, xspi->regs + offset);
}
 
/**
 * cdns_spi_init_hw - Initialize the hardware and configure the SPI controller
 * @xspi:	Pointer to the cdns_spi structure
 *
 * On reset the SPI controller is configured to be in master mode, baud rate
 * divisor is set to 4, threshold value for TX FIFO not full interrupt is set
 * to 1 and size of the word to be transferred as 8 bit.
 * This function initializes the SPI controller to disable and clear all the
 * interrupts, enable manual slave select and manual start, deselect all the
 * chip select lines, and enable the SPI controller.
 */
static void cdns_spi_init_hw(struct cdns_spi *xspi)/*初始化cadence spi控制器*/
{
	u32 ctrl_reg = CDNS_SPI_CR_DEFAULT;	/*控制寄存器默认为：主机模式使能、无外设被片选、手动片选使能、4分频*/
 
	if (xspi->is_decoded_cs)
		ctrl_reg |= CDNS_SPI_CR_PERI_SEL;								/*外设片选3-8译码*/
 
	cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_DISABLE);				/*SPI模块去使能*/
	cdns_spi_write(xspi, CDNS_SPI_IDR, CDNS_SPI_IXR_ALL);				/*去使能中断寄存器*/
 
	/* Clear the RX FIFO */
	while (cdns_spi_read(xspi, CDNS_SPI_ISR) & CDNS_SPI_IXR_RXNEMTY)	/*等待中断状态寄存器和rx_fifo被清空*/
		cdns_spi_read(xspi, CDNS_SPI_RXD);
 
	cdns_spi_write(xspi, CDNS_SPI_ISR, CDNS_SPI_IXR_ALL);				/*清空spi中断控制器的状态*/
	cdns_spi_write(xspi, CDNS_SPI_CR, ctrl_reg);						/*配置控制寄存器*/
	cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_ENABLE);				/*使能SPI*/
}
 
/**
 * cdns_spi_chipselect - Select or deselect the chip select line
 * @spi:	Pointer to the spi_device structure
 * @is_high:	Select(0) or deselect (1) the chip select line
 */
static void cdns_spi_chipselect(struct spi_device *spi, bool is_high)/*片选操作*/
{
	struct cdns_spi *xspi = spi_master_get_devdata(spi->master);	/*获取spi->master的相关信息*/
	u32 ctrl_reg;
 
	ctrl_reg = cdns_spi_read(xspi, CDNS_SPI_CR);	/*读spi控制寄存器的值*/
 
	if (is_high) {
		/* Deselect the slave */
		ctrl_reg |= CDNS_SPI_CR_SSCTRL;		/*不选择该从机*/
	} else {
		/* Select the slave */
		ctrl_reg &= ~CDNS_SPI_CR_SSCTRL;
		if (!(xspi->is_decoded_cs))			/*是否用3-8译码器来片选*/
			ctrl_reg |= ((~(CDNS_SPI_SS0 << spi->chip_select)) <<
				     CDNS_SPI_SS_SHIFT) &
				     CDNS_SPI_CR_SSCTRL;
		else
			ctrl_reg |= (spi->chip_select << CDNS_SPI_SS_SHIFT) &
				     CDNS_SPI_CR_SSCTRL;
	}
 
	cdns_spi_write(xspi, CDNS_SPI_CR, ctrl_reg);		/*重新写控制寄存器的片选位*/
}
 
/**
 * cdns_spi_config_clock_mode - Sets clock polarity and phase
 * @spi:	Pointer to the spi_device structure
 *
 * Sets the requested clock polarity and phase.
 */
static void cdns_spi_config_clock_mode(struct spi_device *spi)/*配置时钟相位和极性*/
{
	struct cdns_spi *xspi = spi_master_get_devdata(spi->master);/*获取spi->master的相关信息*/
	u32 ctrl_reg, new_ctrl_reg;
 
	new_ctrl_reg = cdns_spi_read(xspi, CDNS_SPI_CR);
	ctrl_reg = new_ctrl_reg;
 
	/* Set the SPI clock phase and clock polarity */
	new_ctrl_reg &= ~(CDNS_SPI_CR_CPHA | CDNS_SPI_CR_CPOL);
	if (spi->mode & SPI_CPHA)
		new_ctrl_reg |= CDNS_SPI_CR_CPHA;
	if (spi->mode & SPI_CPOL)
		new_ctrl_reg |= CDNS_SPI_CR_CPOL;
 
	if (new_ctrl_reg != ctrl_reg) {
		/*
		 * Just writing the CR register does not seem to apply the clock
		 * setting changes. This is problematic when changing the clock
		 * polarity as it will cause the SPI slave to see spurious clock
		 * transitions. To workaround the issue toggle the ER register.
		 */
		cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_DISABLE);
		cdns_spi_write(xspi, CDNS_SPI_CR, new_ctrl_reg);	/*重新写控制寄存器的时钟模式*/
		cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_ENABLE);
	}
}
 
/**
 * cdns_spi_config_clock_freq - Sets clock frequency
 * @spi:	Pointer to the spi_device structure
 * @transfer:	Pointer to the spi_transfer structure which provides
 *		information about next transfer setup parameters
 *
 * Sets the requested clock frequency.
 * Note: If the requested frequency is not an exact match with what can be
 * obtained using the prescalar value the driver sets the clock frequency which
 * is lower than the requested frequency (maximum lower) for the transfer. If
 * the requested frequency is higher or lower than that is supported by the SPI
 * controller the driver will set the highest or lowest frequency supported by
 * controller.
 */
static void cdns_spi_config_clock_freq(struct spi_device *spi,/*设置SPI时钟频率*/
				       struct spi_transfer *transfer)
{
	struct cdns_spi *xspi = spi_master_get_devdata(spi->master);
	u32 ctrl_reg, baud_rate_val;
	unsigned long frequency;
 
	frequency = clk_get_rate(xspi->ref_clk);
 
	ctrl_reg = cdns_spi_read(xspi, CDNS_SPI_CR);
	/* Set the clock frequency */
	if (xspi->speed_hz != transfer->speed_hz) {
		/* first valid value is 1 */
		baud_rate_val = CDNS_SPI_BAUD_DIV_MIN;
		while ((baud_rate_val < CDNS_SPI_BAUD_DIV_MAX) &&
		       (frequency / (2 << baud_rate_val)) > transfer->speed_hz)
			baud_rate_val++;
 
		ctrl_reg &= ~CDNS_SPI_CR_BAUD_DIV;
		ctrl_reg |= baud_rate_val << CDNS_SPI_BAUD_DIV_SHIFT;
 
		xspi->speed_hz = frequency / (2 << baud_rate_val);
	}
	cdns_spi_write(xspi, CDNS_SPI_CR, ctrl_reg);
}
 
/**
 * cdns_spi_setup_transfer - Configure SPI controller for specified transfer
 * @spi:	Pointer to the spi_device structure
 * @transfer:	Pointer to the spi_transfer structure which provides
 *		information about next transfer setup parameters
 *
 * Sets the operational mode of SPI controller for the next SPI transfer and
 * sets the requested clock frequency.
 *
 * Return:	Always 0
 */
static int cdns_spi_setup_transfer(struct spi_device *spi,/*为指定的发送配置SPI控制器*/
				   struct spi_transfer *transfer)
{
	struct cdns_spi *xspi = spi_master_get_devdata(spi->master);
 
	cdns_spi_config_clock_freq(spi, transfer);
 
	dev_dbg(&spi->dev, "%s, mode %d, %u bits/w, %u clock speed\n",
		__func__, spi->mode, spi->bits_per_word,
		xspi->speed_hz);
 
	return 0;
}
 
/**
 * cdns_spi_fill_tx_fifo - Fills the TX FIFO with as many bytes as possible
 * @xspi:	Pointer to the cdns_spi structure
 */
static void cdns_spi_fill_tx_fifo(struct cdns_spi *xspi)/*向tx_buf中填充数据*/
{
	unsigned long trans_cnt = 0;
 
	while ((trans_cnt < CDNS_SPI_FIFO_DEPTH) &&
	       (xspi->tx_bytes > 0)) {
		if (xspi->txbuf)
			cdns_spi_write(xspi, CDNS_SPI_TXD, *xspi->txbuf++);
		else
			cdns_spi_write(xspi, CDNS_SPI_TXD, 0);
 
		xspi->tx_bytes--;
		trans_cnt++;
	}
}
 
/**
 * cdns_spi_irq - Interrupt service routine of the SPI controller
 * @irq:	IRQ number
 * @dev_id:	Pointer to the xspi structure
 *
 * This function handles TX empty and Mode Fault interrupts only.
 * On TX empty interrupt this function reads the received data from RX FIFO and
 * fills the TX FIFO if there is any data remaining to be transferred.
 * On Mode Fault interrupt this function indicates that transfer is completed,
 * the SPI subsystem will identify the error as the remaining bytes to be
 * transferred is non-zero.
 *
 * Return:	IRQ_HANDLED when handled; IRQ_NONE otherwise.
 */
static irqreturn_t cdns_spi_irq(int irq, void *dev_id)/*SPI控制器中断服务*/
{
	struct spi_master *master = dev_id;
	struct cdns_spi *xspi = spi_master_get_devdata(master);
	u32 intr_status, status;
 
	status = IRQ_NONE;
	intr_status = cdns_spi_read(xspi, CDNS_SPI_ISR);
	cdns_spi_write(xspi, CDNS_SPI_ISR, intr_status);
 
	if (intr_status & CDNS_SPI_IXR_MODF) {
		/* Indicate that transfer is completed, the SPI subsystem will
		 * identify the error as the remaining bytes to be
		 * transferred is non-zero
		 */
		cdns_spi_write(xspi, CDNS_SPI_IDR, CDNS_SPI_IXR_DEFAULT);
		spi_finalize_current_transfer(master);
		status = IRQ_HANDLED;
	} else if (intr_status & CDNS_SPI_IXR_TXOW) {
		unsigned long trans_cnt;
 
		trans_cnt = xspi->rx_bytes - xspi->tx_bytes;
 
		/* Read out the data from the RX FIFO */
		while (trans_cnt) {
			u8 data;
 
			data = cdns_spi_read(xspi, CDNS_SPI_RXD);
			if (xspi->rxbuf)
				*xspi->rxbuf++ = data;
 
			xspi->rx_bytes--;
			trans_cnt--;
		}
 
		if (xspi->tx_bytes) {
			/* There is more data to send */
			cdns_spi_fill_tx_fifo(xspi);
		} else {
			/* Transfer is completed */
			cdns_spi_write(xspi, CDNS_SPI_IDR,
				       CDNS_SPI_IXR_DEFAULT);
			spi_finalize_current_transfer(master);
		}
		status = IRQ_HANDLED;
	}
 
	return status;
}
 
static int cdns_prepare_message(struct spi_master *master,/*准备发送*/
				struct spi_message *msg)
{
	cdns_spi_config_clock_mode(msg->spi);
	return 0;
}
 
/**
 * cdns_transfer_one - Initiates the SPI transfer
 * @master:	Pointer to spi_master structure
 * @spi:	Pointer to the spi_device structure
 * @transfer:	Pointer to the spi_transfer structure which provides
 *		information about next transfer parameters
 *
 * This function fills the TX FIFO, starts the SPI transfer and
 * returns a positive transfer count so that core will wait for completion.
 *
 * Return:	Number of bytes transferred in the last transfer
 */
static int cdns_transfer_one(struct spi_master *master,/*初始化SPI发送*/
			     struct spi_device *spi,
			     struct spi_transfer *transfer)
{
	struct cdns_spi *xspi = spi_master_get_devdata(master);
 
	xspi->txbuf = transfer->tx_buf;
	xspi->rxbuf = transfer->rx_buf;
	xspi->tx_bytes = transfer->len;
	xspi->rx_bytes = transfer->len;
	cdns_spi_setup_transfer(spi, transfer);/*设置SPI时钟频率*/
 
	cdns_spi_fill_tx_fifo(xspi);/*向tx_buf中填充数据*/
 
	cdns_spi_write(xspi, CDNS_SPI_IER, CDNS_SPI_IXR_DEFAULT);
	return transfer->len;
}
 
/**
 * cdns_prepare_transfer_hardware - Prepares hardware for transfer.
 * @master:	Pointer to the spi_master structure which provides
 *		information about the controller.
 *
 * This function enables SPI master controller.
 *
 * Return:	0 always
 */
static int cdns_prepare_transfer_hardware(struct spi_master *master)/*准备硬件去发送*/
{
	struct cdns_spi *xspi = spi_master_get_devdata(master);
 
	cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_ENABLE);
 
	return 0;
}
 
/**
 * cdns_unprepare_transfer_hardware - Relaxes hardware after transfer
 * @master:	Pointer to the spi_master structure which provides
 *		information about the controller.
 *
 * This function disables the SPI master controller.
 *
 * Return:	0 always
 */
static int cdns_unprepare_transfer_hardware(struct spi_master *master)/*发送完成后释放硬件*/
{
	struct cdns_spi *xspi = spi_master_get_devdata(master);
 
	cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_DISABLE);
 
	return 0;
}
 
/**
 * cdns_spi_probe - Probe method for the SPI driver
 * @pdev:	Pointer to the platform_device structure
 *
 * This function initializes the driver data structures and the hardware.
 *
 * Return:	0 on success and error value on error
 */
static int cdns_spi_probe(struct platform_device *pdev)/*cadence_spi驱动探针函数*/
{
	int ret = 0, irq;
	struct spi_master *master;
	struct cdns_spi *xspi;
	struct resource *res;
	u32 num_cs;
	master = spi_alloc_master(&pdev->dev, sizeof(*xspi));//分配一个SPI主机控制器
	if (!master)
		return -ENOMEM;
 
	xspi = spi_master_get_devdata(master);
	master->dev.of_node = pdev->dev.of_node;
	platform_set_drvdata(pdev, master);
 
	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);	//获取设备树中SPI的IO资源
	xspi->regs = devm_ioremap_resource(&pdev->dev, res);	//对寄存器进行映射
	if (IS_ERR(xspi->regs)) {
		ret = PTR_ERR(xspi->regs);
		goto remove_master;
	}
	
	xspi->pclk = devm_clk_get(&pdev->dev, "pclk");		//获取ARB时钟，用作配置寄存器
	if (IS_ERR(xspi->pclk)) {
		dev_err(&pdev->dev, "pclk clock not found.\n");
		ret = PTR_ERR(xspi->pclk);
		goto remove_master;
	}
 
	xspi->ref_clk = devm_clk_get(&pdev->dev, "ref_clk");	//获取参考时钟，用作波特率
	if (IS_ERR(xspi->ref_clk)) {
		dev_err(&pdev->dev, "ref_clk clock not found.\n");
		ret = PTR_ERR(xspi->ref_clk);
		goto remove_master;
	}
 
	ret = clk_prepare_enable(xspi->pclk);			//使能APB时钟
	if (ret) {
		dev_err(&pdev->dev, "Unable to enable APB clock.\n");
		goto remove_master;
	}
 
	ret = clk_prepare_enable(xspi->ref_clk);		//使能参考时钟
	if (ret) {
		dev_err(&pdev->dev, "Unable to enable device clock.\n");
		goto clk_dis_apb;
	}
 
	pm_runtime_use_autosuspend(&pdev->dev);
	pm_runtime_set_autosuspend_delay(&pdev->dev, SPI_AUTOSUSPEND_TIMEOUT);
	pm_runtime_set_active(&pdev->dev);
	pm_runtime_enable(&pdev->dev);
 
	ret = of_property_read_u32(pdev->dev.of_node, "num-cs", &num_cs);//获取设备树中num-cs资源
	if (ret < 0)
		master->num_chipselect = CDNS_SPI_DEFAULT_NUM_CS;
	else
		master->num_chipselect = num_cs;
 
	ret = of_property_read_u32(pdev->dev.of_node, "is-decoded-cs",//获取设备树中is-decoded-cs资源
				   &xspi->is_decoded_cs);
	if (ret < 0)
		xspi->is_decoded_cs = 0;
 
	/* SPI controller initializations */
	cdns_spi_init_hw(xspi);
 
	pm_runtime_mark_last_busy(&pdev->dev);
	pm_runtime_put_autosuspend(&pdev->dev);
 
	irq = platform_get_irq(pdev, 0);//获取设备树中中断资源
 
	if (irq <= 0) {
		ret = -ENXIO;
		dev_err(&pdev->dev, "irq number is invalid\n");
		goto clk_dis_all;
	}
 
	ret = devm_request_irq(&pdev->dev, irq, cdns_spi_irq,//向系统申请中断
			       0, pdev->name, master);
	if (ret != 0) {
		ret = -ENXIO;
		dev_err(&pdev->dev, "request_irq failed\n");
		goto clk_dis_all;
	}
 
	master->prepare_transfer_hardware = cdns_prepare_transfer_hardware;		//使能SPI寄存器
	master->prepare_message = cdns_prepare_message;							//设置SPI的时钟和相位
	master->transfer_one = cdns_transfer_one;								//设置波特率
	master->unprepare_transfer_hardware = cdns_unprepare_transfer_hardware;	//关闭SPI寄存器
	master->set_cs = cdns_spi_chipselect;									//片选
	master->auto_runtime_pm = true;
	master->mode_bits = SPI_CPOL | SPI_CPHA;
 
	/* Set to default valid value */
	master->max_speed_hz = clk_get_rate(xspi->ref_clk) / 4;	//	设置波特率、字长默认值
	xspi->speed_hz = master->max_speed_hz;
 
	master->bits_per_word_mask = SPI_BPW_MASK(8);
 
	ret = spi_register_master(master);		//向系统注册SPI主机控制器
	if (ret) {
		dev_err(&pdev->dev, "spi_register_master failed\n");
		goto clk_dis_all;
	}
 
	return ret;
 
clk_dis_all:
	pm_runtime_set_suspended(&pdev->dev);
	pm_runtime_disable(&pdev->dev);
	clk_disable_unprepare(xspi->ref_clk);
clk_dis_apb:
	clk_disable_unprepare(xspi->pclk);
remove_master:
	spi_master_put(master);
	return ret;
}
 
/**
 * cdns_spi_remove - Remove method for the SPI driver
 * @pdev:	Pointer to the platform_device structure
 *
 * This function is called if a device is physically removed from the system or
 * if the driver module is being unloaded. It frees all resources allocated to
 * the device.
 *
 * Return:	0 on success and error value on error
 */
static int cdns_spi_remove(struct platform_device *pdev)/*cadence_spi驱动移除*/
{
	struct spi_master *master = platform_get_drvdata(pdev);
	struct cdns_spi *xspi = spi_master_get_devdata(master);
 
	cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_DISABLE);
 
	clk_disable_unprepare(xspi->ref_clk);
	clk_disable_unprepare(xspi->pclk);
	pm_runtime_set_suspended(&pdev->dev);
	pm_runtime_disable(&pdev->dev);
 
	spi_unregister_master(master);
 
	return 0;
}
 
/**
 * cdns_spi_suspend - Suspend method for the SPI driver
 * @dev:	Address of the platform_device structure
 *
 * This function disables the SPI controller and
 * changes the driver state to "suspend"
 *
 * Return:	0 on success and error value on error
 */
static int __maybe_unused cdns_spi_suspend(struct device *dev)/*cadence_spi驱动暂停*/
{
	struct platform_device *pdev = to_platform_device(dev);
	struct spi_master *master = platform_get_drvdata(pdev);
 
	return spi_master_suspend(master);
}
 
/**
 * cdns_spi_resume - Resume method for the SPI driver
 * @dev:	Address of the platform_device structure
 *
 * This function changes the driver state to "ready"
 *
 * Return:	0 on success and error value on error
 */
static int __maybe_unused cdns_spi_resume(struct device *dev)/*cadence_spi驱动恢复*/
{
	struct platform_device *pdev = to_platform_device(dev);
	struct spi_master *master = platform_get_drvdata(pdev);
	struct cdns_spi *xspi = spi_master_get_devdata(master);
 
	cdns_spi_init_hw(xspi);
	return spi_master_resume(master);
}
 
/**
 * cdns_spi_runtime_resume - Runtime resume method for the SPI driver
 * @dev:	Address of the platform_device structure
 *
 * This function enables the clocks
 *
 * Return:	0 on success and error value on error
 */
static int __maybe_unused cnds_runtime_resume(struct device *dev)/*SPI驱动程序的运行时恢复*/
{
	struct spi_master *master = dev_get_drvdata(dev);
	struct cdns_spi *xspi = spi_master_get_devdata(master);
	int ret;
 
	ret = clk_prepare_enable(xspi->pclk);
	if (ret) {
		dev_err(dev, "Cannot enable APB clock.\n");
		return ret;
	}
 
	ret = clk_prepare_enable(xspi->ref_clk);
	if (ret) {
		dev_err(dev, "Cannot enable device clock.\n");
		clk_disable(xspi->pclk);
		return ret;
	}
	return 0;
}
 
/**
 * cdns_spi_runtime_suspend - Runtime suspend method for the SPI driver
 * @dev:	Address of the platform_device structure
 *
 * This function disables the clocks
 *
 * Return:	Always 0
 */
static int __maybe_unused cnds_runtime_suspend(struct device *dev)/*SPI驱动程序的运行时挂起*/
{
	struct spi_master *master = dev_get_drvdata(dev);
	struct cdns_spi *xspi = spi_master_get_devdata(master);
 
	clk_disable_unprepare(xspi->ref_clk);
	clk_disable_unprepare(xspi->pclk);
 
	return 0;
}
 
static const struct dev_pm_ops cdns_spi_dev_pm_ops = {
	SET_RUNTIME_PM_OPS(cnds_runtime_suspend,
			   cnds_runtime_resume, NULL)
	SET_SYSTEM_SLEEP_PM_OPS(cdns_spi_suspend, cdns_spi_resume)
};
 
static const struct of_device_id cdns_spi_of_match[] = {
	{ .compatible = "xlnx,zynq-spi-r1p6" },
	{ .compatible = "cdns,spi-r1p6" },
	{ /* end of table */ }
};
MODULE_DEVICE_TABLE(of, cdns_spi_of_match);
 
/* cdns_spi_driver - This structure defines the SPI subsystem platform driver */
static struct platform_driver cdns_spi_driver = {
	.probe	= cdns_spi_probe,
	.remove	= cdns_spi_remove,
	.driver = {
		.name = CDNS_SPI_NAME,
		.of_match_table = cdns_spi_of_match,
		.pm = &cdns_spi_dev_pm_ops,
	},
};
 
module_platform_driver(cdns_spi_driver);
 
MODULE_AUTHOR("Xilinx, Inc.");
MODULE_DESCRIPTION("Cadence SPI driver");
MODULE_LICENSE("GPL");

spi-cadence.c文件中包含有548行代码，主要作用就是配置SPI主机控制器的，我们可以顺着module_platform_driver(cdns_spi_driver)往下看：

       在我们看完上面两个c文件后，明显能看到这两个文件并没有直接调用或者交互的关系，但都跟spi.c有调用关系，所以很显然，spi.c的作用就是让spidev和spi-cadence能够关联起来。

       我们可以想一想，这个spi.c需要做哪些工作呢？

       联想下平台系统驱动的框架：设备-驱动-总线模型，因为SPI总线也是由平台总线派生出来的，所以必然也会遵循这个架构。那么是不是就可以猜测spi.c的作用就是注册SPI总线呢？当然spi.c还有一个作用就是怎么将spidev和spi-cadence连接起来，我们来看看spi.c到底干了啥？

static int __init spi_init(void)
{
	int	status;
 
	buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
	if (!buf) {
		status = -ENOMEM;
		goto err0;
	}
 
	status = bus_register(&spi_bus_type);	/*注册spi总线*/
	if (status < 0)
		goto err1;
 
	status = class_register(&spi_master_class);		/*将spi_master注册到内核中*/
	if (status < 0)
		goto err2;
 
	if (IS_ENABLED(CONFIG_OF_DYNAMIC))
		WARN_ON(of_reconfig_notifier_register(&spi_of_notifier));
	if (IS_ENABLED(CONFIG_ACPI))
		WARN_ON(acpi_reconfig_notifier_register(&spi_acpi_notifier));
 
	return 0;
 
err2:
	bus_unregister(&spi_bus_type);
err1:
	kfree(buf);
	buf = NULL;
err0:
	return status;
}
 
/* board_info is normally registered in arch_initcall(),
 * but even essential drivers wait till later
 *
 * REVISIT only boardinfo really needs static linking. the rest (device and
 * driver registration) _could_ be dynamically linked (modular) ... costs
 * include needing to have boardinfo data structures be much more public.
 */
postcore_initcall(spi_init);//在moudule_init之前加载

        上面这段代码的作用就是向内核注册SPI总线，以及向内核注册spi的主机控制器，只有在spi.c中先注册了主机控制器，在spi-cadence.c中才可以向内核申请一个SPI主机控制器，以及向内核注册；显然，spi-cadence.c与spi.c的联系就建立起来了。而spidev.c与spi.c的联系比较复杂，这块内容留着后面再分析吧。

那么，在我们看完这三个文件后，基本上可以梳理下SPI的驱动是怎么实现的了。

第一步：向内核注册SPI总线以及SPI主机控制器；

第二步：向内核申请一个SPI主机控制器的空间，注册我们要用的主机控制器；

第三步：向内核注册SPI设备，以及构造file_operation结构体；

有了这三步，用户空间就可以通过open、write、read、ioctl函数来操作字符设备spidev了。