基于S3C2440的嵌入式Linux驱动——SPI子系统解读（三）

该系列文章将分为四个部分：

第一部分，将对SPI子系统整体进行描述，同时给出SPI的相关数据结构，最后描述SPI总线的注册。基于S3C2440的嵌入式Linux驱动——SPI子系统解读（一）

第二部分，该文将对SPI的主控制器(master)驱动进行描述。基于S3C2440的嵌入式Linux驱动——SPI子系统解读（二）

第三部分，即本篇文章，该文将对SPI设备驱动，也称protocol 驱动，进行讲解。

第四部分，通过SPI设备驱动留给用户层的API，我们将从上到下描述数据是如何通过SPI的protocol 驱动，由bitbang中转，最后由master驱动将数据传输出

去。 基于S3C2440的嵌入式Linux驱动——SPI子系统解读（四）

本文属于第三部分。

5. SPI设备驱动

在主控制器驱动中，spi_device已经注册了，在设备驱动中，首先要做的就是注册spi_driver，并提供用户层相应的API。

5.1 SPI设备驱动的注册

下列数据结构及函数位于drivers/spi/spidev.c。

[cpp]
view plaincopyprint?

static 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,
.open = spidev_open,
.release = spidev_release,
};

/* 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;

static struct spi_driver spidev_spi = {
.driver = {
.name = "spidev",
.owner = THIS_MODULE,
},
.probe = spidev_probe,
.remove = __devexit_p(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); /*检查次设备号*/
status = register_chrdev(SPIDEV_MAJOR, "spi", &spidev_fops); /*注册字符设备，major=153*/
if (status < 0)
return status;

spidev_class = class_create(THIS_MODULE, "spidev"); /*创建spidev类*/
if (IS_ERR(spidev_class)) {
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
return PTR_ERR(spidev_class);
}

status = spi_register_driver(&spidev_spi); /*注册spi_driver,并调用probe方法*/
if (status < 0) {
class_destroy(spidev_class);
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
}
return status;
}
module_init(spidev_init);

static void __exit spidev_exit(void)
{
spi_unregister_driver(&spidev_spi); /*注销spi_driver*/
class_destroy(spidev_class); /*注销类*/
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);/*注销字符设备*/
}
module_exit(spidev_exit);

static 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,
.open =        spidev_open,
.release =    spidev_release,
};

/* 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;

static struct spi_driver spidev_spi = {
.driver = {
.name =        "spidev",
.owner =    THIS_MODULE,
},
.probe =    spidev_probe,
.remove =    __devexit_p(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);	/*检查次设备号*/
status = register_chrdev(SPIDEV_MAJOR, "spi", &spidev_fops); /*注册字符设备，major=153*/
if (status < 0)
return status;

spidev_class = class_create(THIS_MODULE, "spidev");		/*创建spidev类*/
if (IS_ERR(spidev_class)) {
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
return PTR_ERR(spidev_class);
}

status = spi_register_driver(&spidev_spi);		/*注册spi_driver,并调用probe方法*/
if (status < 0) {
class_destroy(spidev_class);
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
}
return status;
}
module_init(spidev_init);

static void __exit spidev_exit(void)
{
spi_unregister_driver(&spidev_spi);			/*注销spi_driver*/
class_destroy(spidev_class);				/*注销类*/
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);/*注销字符设备*/
}
module_exit(spidev_exit);

该函数中，创建了一个字符设备以提供API给用户层，同时创建了一个spidev类，最后注册spi_driver到内核中。

在这里我们看到了SPI设备驱动是如何提供API给用户层的，那就是通过再熟悉不过的字符设备。通过字符设备，给用户层提供了5个API：open，release，write，read和ioctl。本文在后面将介绍open和close，剩余3个将在本系列的第四篇文章中介绍。

接着看下spi_register_driver函数， 该函数位于drivers/spi/spidev.c。

[cpp]
view plaincopyprint?

/** * spi_register_driver - register a SPI driver * @sdrv: the driver to register * Context: can sleep */ int spi_register_driver(struct spi_driver *sdrv) { sdrv->driver.bus = &spi_bus_type; if (sdrv->probe) sdrv->driver.probe = spi_drv_probe; if (sdrv->remove) sdrv->driver.remove = spi_drv_remove; if (sdrv->shutdown) sdrv->driver.shutdown = spi_drv_shutdown; return driver_register(&sdrv->driver); } EXPORT_SYMBOL_GPL(spi_register_driver);

/**
* spi_register_driver - register a SPI driver
* @sdrv: the driver to register
* Context: can sleep
*/
int spi_register_driver(struct spi_driver *sdrv)
{
sdrv->driver.bus = &spi_bus_type;
if (sdrv->probe)
sdrv->driver.probe = spi_drv_probe;
if (sdrv->remove)
sdrv->driver.remove = spi_drv_remove;
if (sdrv->shutdown)
sdrv->driver.shutdown = spi_drv_shutdown;
return driver_register(&sdrv->driver);
}
EXPORT_SYMBOL_GPL(spi_register_driver);

在调用driver_register的过程中，将用driver.name和spi_device的modalias字段进行比较，两者相等则将该spi_driver和spi_device进行绑定。

当spi_driver注册成功以后，将调用probe方法：spidev_probe函数。

5.2 probe方法

我们来看看spidev_probe这个函数，该函数位于drivers/spi/spidev.c。

[cpp]
view plaincopyprint?

#define SPIDEV_MAJOR 153 /* assigned */

#define N_SPI_MINORS 32 /* ... up to 256 */

static unsigned long minors[N_SPI_MINORS / BITS_PER_LONG]; /**/

static LIST_HEAD(device_list);
static DEFINE_MUTEX(device_list_lock);

static int spidev_probe(struct spi_device *spi)
{
struct spidev_data *spidev;
int status;
unsigned long minor;

/* Allocate driver data */
spidev = kzalloc(sizeof(*spidev), GFP_KERNEL); /*以kmalloc分配内存，并清0*/
if (!spidev)
return -ENOMEM;

/* Initialize the driver data */
spidev->spi = spi; /*保存spi_device*/
spin_lock_init(&spidev->spi_lock); /*初始化自旋锁*/
mutex_init(&spidev->buf_lock); /*初始化互斥体*/

INIT_LIST_HEAD(&spidev->device_entry); /*初始化链表头，链表为双向循环链表*/

/* 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); /*上锁*/
minor = find_first_zero_bit(minors, N_SPI_MINORS); /*分配次设备号*/
if (minor < N_SPI_MINORS) {
struct device *dev;

spidev->devt = MKDEV(SPIDEV_MAJOR, minor); /*根据主次设备号来获取设备号*/
dev = device_create(spidev_class, &spi->dev, spidev->devt, /*创建设备节点*/
spidev, "spidev%d.%d",
spi->master->bus_num, spi->chip_select);
status = IS_ERR(dev) ? PTR_ERR(dev) : 0;
} else {
dev_dbg(&spi->dev, "no minor number available!\n");
status = -ENODEV;
}
if (status == 0) {
set_bit(minor, minors); /*保存已使用的次设备号*/
list_add(&spidev->device_entry, &device_list);/*在链表头list后面添加entry*/
}
mutex_unlock(&device_list_lock);/*解锁互斥体*/

if (status == 0)
spi_set_drvdata(spi, spidev);/*spi->dev.driver_data=spidev*/
else
kfree(spidev);

return status;
}

#define SPIDEV_MAJOR            153    /* assigned */
#define N_SPI_MINORS            32    /* ... up to 256 */

static unsigned long    minors[N_SPI_MINORS / BITS_PER_LONG];    /**/

static LIST_HEAD(device_list);
static DEFINE_MUTEX(device_list_lock);

static int spidev_probe(struct spi_device *spi)
{
struct spidev_data	*spidev;
int			status;
unsigned long		minor;

/* Allocate driver data */
spidev = kzalloc(sizeof(*spidev), GFP_KERNEL);	/*以kmalloc分配内存，并清0*/
if (!spidev)
return -ENOMEM;

/* Initialize the driver data */
spidev->spi = spi; 					/*保存spi_device*/
spin_lock_init(&spidev->spi_lock);	/*初始化自旋锁*/
mutex_init(&spidev->buf_lock);		/*初始化互斥体*/

INIT_LIST_HEAD(&spidev->device_entry);	/*初始化链表头，链表为双向循环链表*/

/* 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);		/*上锁*/
minor = find_first_zero_bit(minors, N_SPI_MINORS);	/*分配次设备号*/
if (minor < N_SPI_MINORS) {
struct device *dev;

spidev->devt = MKDEV(SPIDEV_MAJOR, minor);	/*根据主次设备号来获取设备号*/
dev = device_create(spidev_class, &spi->dev, spidev->devt,	/*创建设备节点*/
spidev, "spidev%d.%d",
spi->master->bus_num, spi->chip_select);
status = IS_ERR(dev) ? PTR_ERR(dev) : 0;
} else {
dev_dbg(&spi->dev, "no minor number available!\n");
status = -ENODEV;
}
if (status == 0) {
set_bit(minor, minors);		/*保存已使用的次设备号*/
list_add(&spidev->device_entry, &device_list);/*在链表头list后面添加entry*/
}
mutex_unlock(&device_list_lock);/*解锁互斥体*/

if (status == 0)
spi_set_drvdata(spi, spidev);/*spi->dev.driver_data=spidev*/
else
kfree(spidev);

return status;
}

其中用到的的struct spidev_data结构如下：

[cpp]
view plaincopyprint?

struct spidev_data {
dev_t devt;
spinlock_t spi_lock;
struct spi_device *spi;
struct list_head device_entry;

/* buffer is NULL unless this device is open (users > 0) */
struct mutex buf_lock;
unsigned users;
u8 *buffer;
};
<SPAN style="FONT-SIZE: 12px">
</SPAN>

struct spidev_data {
dev_t            devt;
spinlock_t        spi_lock;
struct spi_device    *spi;
struct list_head    device_entry;

/* buffer is NULL unless this device is open (users > 0) */
struct mutex        buf_lock;
unsigned        users;
u8            *buffer;
};

这个函数中，分配了spidev_data和次设备号，随后根据主次设备号创建了设备节点。设备节点的名字是spidev“bus_num””.chip_select",意思就是该设备是在第几个SPI接口上的第几个设备。

此外，将spidev添加到device_list中，这样做就方便查找该spidev。

5.3 remove方法

下列函数位于drivers/spi/spidev.c。

[cpp]
view plaincopyprint?

static int spidev_remove(struct spi_device *spi)
{
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;
spi_set_drvdata(spi, NULL);
spin_unlock_irq(&spidev->spi_lock);

/* prevent new opens */
mutex_lock(&device_list_lock);
list_del(&spidev->device_entry); /*删除entry*/
device_destroy(spidev_class, spidev->devt); /*删除设备节点*/
clear_bit(MINOR(spidev->devt), minors);/*删除使用的次设备号信息*/
if (spidev->users == 0)
kfree(spidev);
mutex_unlock(&device_list_lock);

return 0;
}

static int spidev_remove(struct spi_device *spi)
{
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;
spi_set_drvdata(spi, NULL);
spin_unlock_irq(&spidev->spi_lock);

/* prevent new opens */
mutex_lock(&device_list_lock);
list_del(&spidev->device_entry);	/*删除entry*/
device_destroy(spidev_class, spidev->devt);	/*删除设备节点*/
clear_bit(MINOR(spidev->devt), minors);/*删除使用的次设备号信息*/
if (spidev->users == 0)
kfree(spidev);
mutex_unlock(&device_list_lock);

return 0;
}

6. open和release

接着来看下open和release系统调用的API接口，其余3个接口将在本系列的第四篇文章中给出。

6.1 open方法

下列函数位于drivers/spi/spidev.c。

[cpp]
view plaincopyprint?

static int spidev_open(struct inode *inode, struct file *filp)
{
struct spidev_data *spidev;
int status = -ENXIO;

lock_kernel(); /*加锁大内核锁，可以睡眠，只能在进程上下文使用*/
mutex_lock(&device_list_lock); /*加锁互斥体*/

list_for_each_entry(spidev, &device_list, device_entry) {/*从list开始遍历entry，即遍历所有的spidev*/
if (spidev->devt == inode->i_rdev) { /*判断设备号是否相等*/
status = 0; /*找到匹配的spi设备*/
break;
}
}
if (status == 0) {
/*NOTE:多个程序调用open方法，但他们共享一个buffer，因此对buufer需要进行互斥保护*/
if (!spidev->buffer) { /*buffer为空*/
spidev->buffer = kmalloc(bufsiz, GFP_KERNEL);/*分配buffer缓冲区,默认4KB*/
if (!spidev->buffer) {
dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
status = -ENOMEM;
}
}
if (status == 0) {
spidev->users++; /*成功open以后，增加用户计数*/
filp->private_data = spidev; /*保存spidev指针*/
nonseekable_open(inode, filp); /*禁用lseek*/
}
} else
pr_debug("spidev: nothing for minor %d\n", iminor(inode));

mutex_unlock(&device_list_lock);/*释放互斥体*/
unlock_kernel(); /*释放大内核锁*/
return status;
}

static int spidev_open(struct inode *inode, struct file *filp)
{
struct spidev_data	*spidev;
int			status = -ENXIO;

lock_kernel();			/*加锁大内核锁，可以睡眠，只能在进程上下文使用*/
mutex_lock(&device_list_lock);	/*加锁互斥体*/

list_for_each_entry(spidev, &device_list, device_entry) {/*从list开始遍历entry，即遍历所有的spidev*/
if (spidev->devt == inode->i_rdev) {	/*判断设备号是否相等*/
status = 0;							/*找到匹配的spi设备*/
break;
}
}
if (status == 0) {
/*NOTE:多个程序调用open方法，但他们共享一个buffer，因此对buufer需要进行互斥保护*/
if (!spidev->buffer) {			/*buffer为空*/
spidev->buffer = kmalloc(bufsiz, GFP_KERNEL);/*分配buffer缓冲区,默认4KB*/
if (!spidev->buffer) {
dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
status = -ENOMEM;
}
}
if (status == 0) {
spidev->users++;		/*成功open以后，增加用户计数*/
filp->private_data = spidev;	/*保存spidev指针*/
nonseekable_open(inode, filp);  /*禁用lseek*/
}
} else
pr_debug("spidev: nothing for minor %d\n", iminor(inode));

mutex_unlock(&device_list_lock);/*释放互斥体*/
unlock_kernel();				/*释放大内核锁*/
return status;
}

在这里，以device_list为链表头，遍历所有的spidev_data结构，通过设备节点的设备号和spidev_data中保存的设备号进行匹配，来找到属于该设备节点的spi设备。随后，分配了spi设备驱动层所使用的缓冲区，最后增加打开计数。

6.2 release方法

下列函数位于drivers/spi/spidev.c。

[cpp]
view plaincopyprint?

static int spidev_release(struct inode *inode, struct file *filp)
{
struct spidev_data *spidev;
int status = 0;

mutex_lock(&device_list_lock); /*加锁互斥体*/
spidev = filp->private_data; /*获取spidev*/
filp->private_data = NULL;

/* last close? */
spidev->users--; /*关闭设备文件，减少用户计数*/
if (!spidev->users) { /*如果用户数为0*/
int dofree;

kfree(spidev->buffer); /*释放缓冲区*/
spidev->buffer = NULL;

/* ... after we unbound from the underlying device? */
spin_lock_irq(&spidev->spi_lock); /*加锁互斥体*/
dofree = (spidev->spi == NULL); /*？？？？*/
spin_unlock_irq(&spidev->spi_lock); /*释放互斥体*/

if (dofree)
kfree(spidev); /*释放spidev，在probe中分配*/
}
mutex_unlock(&device_list_lock);/*释放互斥体*/

return status;
}

static int spidev_release(struct inode *inode, struct file *filp)
{
struct spidev_data	*spidev;
int			status = 0;

mutex_lock(&device_list_lock);	/*加锁互斥体*/
spidev = filp->private_data;	/*获取spidev*/
filp->private_data = NULL;

/* last close? */
spidev->users--;				/*关闭设备文件，减少用户计数*/
if (!spidev->users) {			/*如果用户数为0*/
int		dofree;

kfree(spidev->buffer);		/*释放缓冲区*/
spidev->buffer = NULL;

/* ... after we unbound from the underlying device? */
spin_lock_irq(&spidev->spi_lock);	/*加锁互斥体*/
dofree = (spidev->spi == NULL);		/*？？？？*/
spin_unlock_irq(&spidev->spi_lock);	/*释放互斥体*/

if (dofree)
kfree(spidev);			/*释放spidev，在probe中分配*/
}
mutex_unlock(&device_list_lock);/*释放互斥体*/

return status;
}

至此，对于protocol驱动层的框架进行了简单的分析，在下一篇将对该驱动层很多未分析的函数进行一一讲解。下一篇的内容非常的重要哦。