ARM的嵌入式Linux移植体验之设备驱动

liyf

  
         
        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。

　　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。

　　Linux主要将设备分为二类：字符设备和块设备。字符设备是指设备发送和接收数据以字符的形式进行；而块设备则以整个数据缓冲区的形式进行。在对字符设备发出读/写请求时，实际的硬件I/O一般就紧接着发生了；而块设备则不然，它利用一块系统内存作缓冲区，当用户进程对设备请求能满足用户的要求，就返回请求的数据，如果不能，就调用请求函数来进行实际的I/O操作。块设备主要针对磁盘等慢速设备。

　　1.内存分配

　　由于Linux驱动程序在内核中运行，因此在设备驱动程序需要申请/释放内存时，不能使用用户级的malloc/free函数，而需由内核级的函数kmalloc/kfree () 来实现，kmalloc()函数的原型为：


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[tr]
[td]void kmalloc (size_t size ,int priority);[/td][/tr]
  
         
   
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）：


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[tr]
[td]#include
#include
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) 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 current_nr_sectors 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 current_nr_sectors 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(&notifier);
　#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