内容简介:
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Linux下的设备驱动程序被组织为一组完成不同任务的函数的集合，通过这些函数使得Windows的设备操作犹如文件一般.在应用程序看来，硬件设备 只是一个设备文件，应用程序可以象操作普通文件一样对硬件设备进行操作，如open ()、close ()、read ()、write () 等。
设备驱动程序是操作系统内核和机器硬件之间的接口，它为应用程序屏蔽硬件的细节，一般来说，Linux的设备驱动程序需要完成如下功能：
设备初始化、释放；
·提供各类设备服务；
·负责内核和设备之间的数据交换；
·检测和处理设备工作过程中出现的错误。
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4.块设备驱动
块设备驱动程序的编写是一个浩繁的工程，其难度远超过字符设备，上千行的代码往往只能搞定一个简单 的块设备，而数十行代码就可能搞定一个字符设备。因此，非得有相当的基本功才能完成此项工作。下面先给出一个实例，即mtdblock块设备的驱动。我们 通过分析此实例中的代码来说明块设备驱动程序的写法（由于篇幅的关系，大量的代码被省略，只保留了必要的主干）：
#include <linux/config.h>
#include <linux/devfs_fs_kernel.h>
static void mtd_notify_add(struct mtd_info* mtd);
static void mtd_notify_remove(struct mtd_info* mtd);
static struct mtd_notifier notifier = {
mtd_notify_add,
mtd_notify_remove,
NULL
};
static devfs_handle_t devfs_dir_handle = NULL;
static devfs_handle_t devfs_rw_handle[MAX_MTD_DEVICES];
static struct mtdblk_dev {
struct mtd_info *mtd; /* Locked */
int count;
struct semaphore cache_sem;
unsigned char *cache_data;
unsigned long cache_offset;
unsigned int cache_size;
enum { STATE_EMPTY, STATE_CLEAN, STATE_DIRTY } cache_state;
} *mtdblks[MAX_MTD_DEVICES];
static spinlock_t mtdblks_lock;
/* this lock is used just in kernels >= 2.5.x */
static spinlock_t mtdblock_lock;
static int mtd_sizes[MAX_MTD_DEVICES];
static int mtd_blksizes[MAX_MTD_DEVICES];
static void erase_callback(struct erase_info *done)
{
wait_queue_head_t *wait_q = (wait_queue_head_t *)done->priv;
wake_up(wait_q);
}
static int erase_write (struct mtd_info *mtd, unsigned long pos,
int len, const char *buf)
{
struct erase_info erase;
DECLARE_WAITQUEUE(wait, current);
wait_queue_head_t wait_q;
size_t retlen;
int ret;
/*
* First, let's erase the flash block.
*/
init_waitqueue_head(&wait_q);
erase.mtd = mtd;
erase.callback = erase_callback;
erase.addr = pos;
erase.len = len;
erase.priv = (u_long)&wait_q;
set_current_state(TASK_INTERRUPTIBLE);
add_wait_queue(&wait_q, &wait);
ret = MTD_ERASE(mtd, &erase);
if (ret) {
set_current_state(TASK_RUNNING);
remove_wait_queue(&wait_q, &wait);
printk (KERN_WARNING "mtdblock: erase of region [0x%lx, 0x%x] " "on \"%s\" failed\n",
pos, len, mtd->name);
return ret;
}
schedule(); /* Wait for erase to finish. */
remove_wait_queue(&wait_q, &wait);
/*
* Next, writhe data to flash.
*/
ret = MTD_WRITE (mtd, pos, len, &retlen, buf);
if (ret)
return ret;
if (retlen != len)
return -EIO;
return 0;
}
static int write_cached_data (struct mtdblk_dev *mtdblk)
{
struct mtd_info *mtd = mtdblk->mtd;
int ret;
if (mtdblk->cache_state != STATE_DIRTY)
return 0;
DEBUG(MTD_DEBUG_LEVEL2, "mtdblock: writing cached data for \"%s\" "
"at 0x%lx, size 0x%x\n", mtd->name,
mtdblk->cache_offset, mtdblk->cache_size);
ret = erase_write (mtd, mtdblk->cache_offset,
mtdblk->cache_size, mtdblk->cache_data);
if (ret)
return ret;
mtdblk->cache_state = STATE_EMPTY;
return 0;
}
static int do_cached_write (struct mtdblk_dev *mtdblk, unsigned long pos,
int len, const char *buf)
{
…
}
static int do_cached_read (struct mtdblk_dev *mtdblk, unsigned long pos,
int len, char *buf)
{
…
}
static int mtdblock_open(struct inode *inode, struct file *file)
{
…
}
static release_t mtdblock_release(struct inode *inode, struct file *file)
{
int dev;
struct mtdblk_dev *mtdblk;
DEBUG(MTD_DEBUG_LEVEL1, "mtdblock_release\n");
if (inode == NULL)
release_return(-ENODEV);
dev = minor(inode->i_rdev);
mtdblk = mtdblks[dev];
down(&mtdblk->cache_sem);
write_cached_data(mtdblk);
up(&mtdblk->cache_sem);
spin_lock(&mtdblks_lock);
if (!--mtdblk->count) {
/* It was the last usage. Free the device */
mtdblks[dev] = NULL;
spin_unlock(&mtdblks_lock);
if (mtdblk->mtd->sync)
mtdblk->mtd->sync(mtdblk->mtd);
put_mtd_device(mtdblk->mtd);
vfree(mtdblk->cache_data);
kfree(mtdblk);
} else {
spin_unlock(&mtdblks_lock);
}
DEBUG(MTD_DEBUG_LEVEL1, "ok\n");
BLK_DEC_USE_COUNT;
release_return(0);
}
/*
* This is a special request_fn because it is executed in a process context
* to be able to sleep independently of the caller. The
* io_request_lock (for <2.5) or queue_lock (for >=2.5) is held upon entry
* and exit. The head of our request queue is considered active so there is
* no need to dequeue requests before we are done.
*/
static void handle_mtdblock_request(void)
{
struct request *req;
struct mtdblk_dev *mtdblk;
unsigned int res;
for (;;) {
INIT_REQUEST;
req = CURRENT;
spin_unlock_irq(QUEUE_LOCK(QUEUE));
mtdblk = mtdblks[minor(req->rq_dev)];
res = 0;
if (minor(req->rq_dev) >= MAX_MTD_DEVICES)
panic("%s : minor out of bound", __FUNCTION__);
if (!IS_REQ_CMD(req))
goto end_req;
if ((req->sector + req->current_nr_sectors) > (mtdblk->mtd->size >> 9))
goto end_req;
// Handle the request
switch (rq_data_dir(req))
{
int err;
case READ:
down(&mtdblk->cache_sem);
err = do_cached_read (mtdblk, req->sector << 9,
req->current_nr_sectors << 9,
req->buffer);
up(&mtdblk->cache_sem);
if (!err)
res = 1;
break;
case WRITE:
// Read only device
if ( !(mtdblk->mtd->flags & MTD_WRITEABLE) )
break;
// Do the write
down(&mtdblk->cache_sem);
err = do_cached_write (mtdblk, req->sector << 9,req->current_nr_sectors << 9, req->buffer);
up(&mtdblk->cache_sem);
if (!err)
res = 1;
break;
}
end_req:
spin_lock_irq(QUEUE_LOCK(QUEUE));
end_request(res);
}
}
static volatile int leaving = 0;
static DECLARE_MUTEX_LOCKED(thread_sem);
static DECLARE_WAIT_QUEUE_HEAD(thr_wq);
int mtdblock_thread(void *dummy)
{
…
}
#define RQFUNC_ARG request_queue_t *q
static void mtdblock_request(RQFUNC_ARG)
{
/* Don't do anything, except wake the thread if necessary */
wake_up(&thr_wq);
}
static int mtdblock_ioctl(struct inode * inode, struct file * file,
unsigned int cmd, unsigned long arg)
{
struct mtdblk_dev *mtdblk;
mtdblk = mtdblks[minor(inode->i_rdev)];
switch (cmd) {
case BLKGETSIZE: /* Return device size */
return put_user((mtdblk->mtd->size >> 9), (unsigned long *) arg);
case BLKFLSBUF:
if(!capable(CAP_SYS_ADMIN))
return -EACCES;
fsync_dev(inode->i_rdev);
invalidate_buffers(inode->i_rdev);
down(&mtdblk->cache_sem);
write_cached_data(mtdblk);
up(&mtdblk->cache_sem);
if (mtdblk->mtd->sync)
mtdblk->mtd->sync(mtdblk->mtd);
return 0;
default:
return -EINVAL;
}
}
static struct block_device_operations mtd_fops =
{
owner: THIS_MODULE,
open: mtdblock_open,
release: mtdblock_release,
ioctl: mtdblock_ioctl
};
static void mtd_notify_add(struct mtd_info* mtd)
{
…
}
static void mtd_notify_remove(struct mtd_info* mtd)
{
if (!mtd || mtd->type == MTD_ABSENT)
return;
devfs_unregister(devfs_rw_handle[mtd->index]);
}
int __init init_mtdblock(void)
{
int i;
spin_lock_init(&mtdblks_lock);
/* this lock is used just in kernels >= 2.5.x */
spin_lock_init(&mtdblock_lock);
#ifdef CONFIG_DEVFS_FS
if (devfs_register_blkdev(MTD_BLOCK_MAJOR, DEVICE_NAME, &mtd_fops))
{
printk(KERN_NOTICE "Can't allocate major number %d for Memory Technology Devices.\n",
MTD_BLOCK_MAJOR);
return -EAGAIN;
}
devfs_dir_handle = devfs_mk_dir(NULL, DEVICE_NAME, NULL);
register_mtd_user(¬ifier);
#else
if (register_blkdev(MAJOR_NR,DEVICE_NAME,&mtd_fops)) {
printk(KERN_NOTICE "Can't allocate major number %d for Memory Technology Devices.\n",
MTD_BLOCK_MAJOR);
return -EAGAIN;
}
#endif
/* We fill it in at open() time. */
for (i=0; i< MAX_MTD_DEVICES; i++) {
mtd_sizes[i] = 0;
mtd_blksizes[i] = BLOCK_SIZE;
}
init_waitqueue_head(&thr_wq);
/* Allow the block size to default to BLOCK_SIZE. */
blksize_size[MAJOR_NR] = mtd_blksizes;
blk_size[MAJOR_NR] = mtd_sizes;
BLK_INIT_QUEUE(BLK_DEFAULT_QUEUE(MAJOR_NR), &mtdblock_request, &mtdblock_lock);
kernel_thread (mtdblock_thread, NULL, CLONE_FS|CLONE_FILES|CLONE_SIGHAND);
return 0;
}
static void __exit cleanup_mtdblock(void)
{
leaving = 1;
wake_up(&thr_wq);
down(&thread_sem);
#ifdef CONFIG_DEVFS_FS
unregister_mtd_user(¬ifier);
devfs_unregister(devfs_dir_handle);
devfs_unregister_blkdev(MTD_BLOCK_MAJOR, DEVICE_NAME);
#else
unregister_blkdev(MAJOR_NR,DEVICE_NAME);
#endif
blk_cleanup_queue(BLK_DEFAULT_QUEUE(MAJOR_NR));
blksize_size[MAJOR_NR] = NULL;
blk_size[MAJOR_NR] = NULL;
}
module_init(init_mtdblock);
module_exit(cleanup_mtdblock); |
从上述源代码中我们发现，块设备也以与字符设备register_chrdev、unregister_ chrdev 函数类似的方法进行设备的注册与释放：
int register_blkdev(unsigned int major, const char *name, struct block_device_operations *bdops);
int unregister_blkdev(unsigned int major, const char *name); |
但是，register_chrdev使用一个向 file_operations 结构的指针，而register_blkdev 则使用 block_device_operations 结构的指针，其中定义的open、release 和 ioctl 方法和字符设备的对应方法相同，但未定义 read 或者 write 操作。这是因为，所有涉及到块设备的 I/O 通常由系统进行缓冲处理。
块驱动程序最终必须提供完成实际块 I/O 操作的机制，在 Linux 当中，用于这些 I/O 操作的方法称为"request（请求）"。在块设备的注册过程中，需要初始化request队列，这一动作通过blk_init_queue来完成， blk_init_queue函数建立队列，并将该驱动程序的 request 函数关联到队列。在模块的清除阶段，应调用 blk_cleanup_queue 函数。
本例中相关的代码为：
BLK_INIT_QUEUE(BLK_DEFAULT_QUEUE(MAJOR_NR), &mtdblock_request, &mtdblock_lock);
blk_cleanup_queue(BLK_DEFAULT_QUEUE(MAJOR_NR)); |
每个设备有一个默认使用的请求队列，必要时，可使用 BLK_DEFAULT_QUEUE(major) 宏得到该默认队列。这个宏在 blk_dev_struct 结构形成的全局数组（该数组名为 blk_dev）中搜索得到对应的默认队列。blk_dev 数组由内核维护，并可通过主设备号索引。blk_dev_struct 接口定义如下：
struct blk_dev_struct {
/*
* queue_proc has to be atomic
*/
request_queue_t request_queue;
queue_proc *queue;
void *data;
}; |
request_queue 成员包含了初始化之后的 I/O 请求队列，data 成员可由驱动程序使用，以便保存一些私有数据。
request_queue定义为：
struct request_queue
{
/*
* the queue request freelist, one for reads and one for writes
*/
struct request_list rq[2];
/*
* Together with queue_head for cacheline sharing
*/
struct list_head queue_head;
elevator_t elevator;
request_fn_proc * request_fn;
merge_request_fn * back_merge_fn;
merge_request_fn * front_merge_fn;
merge_requests_fn * merge_requests_fn;
make_request_fn * make_request_fn;
plug_device_fn * plug_device_fn;
/*
* The queue owner gets to use this for whatever they like.
* ll_rw_blk doesn't touch it.
*/
void * queuedata;
/*
* This is used to remove the plug when tq_disk runs.
*/
struct tq_struct plug_tq;
/*
* Boolean that indicates whether this queue is plugged or not.
*/
char plugged;
/*
* Boolean that indicates whether current_request is active or
* not.
*/
char head_active;
/*
* Is meant to protect the queue in the future instead of
* io_request_lock
*/
spinlock_t queue_lock;
/*
* Tasks wait here for free request
*/
wait_queue_head_t wait_for_request;
}; |
下图表征了blk_dev、blk_dev_struct和request_queue的关系：
下图则表征了块设备的注册和释放过程：
5.小结
本章讲述了Linux设备驱动程序的入口函数及驱动程序中的内存申请、中断等，并分别以实例讲述了字符设备及块设备的驱动开发方法。
作者:宋宝华 更新日期:2006-11-21
来源:dev.yesky.com
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