ARM的嵌入式Linux移植體驗(yàn)之設(shè)備驅(qū)動(dòng)

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 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(notifier);
　　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);

　　從上述源代碼中我們發(fā)現(xiàn)，塊設(shè)備也以與字符設(shè)備register_chrdev、unregister_ chrdev 函數(shù)類(lèi)似的方法進(jìn)行設(shè)備的注冊(cè)與釋放：

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使用一個(gè)向 file_operations 結(jié)構(gòu)的指針，而register_blkdev 則使用 block_device_operations 結(jié)構(gòu)的指針，其中定義的open、release 和 ioctl 方法和字符設(shè)備的對(duì)應(yīng)方法相同，但未定義 read 或者 write 操作。這是因?yàn)椋猩婕暗綁K設(shè)備的 I/O 通常由系統(tǒng)進(jìn)行緩沖處理。

　　塊驅(qū)動(dòng)程序最終必須提供完成實(shí)際塊 I/O 操作的機(jī)制，在 Linux 當(dāng)中，用于這些 I/O 操作的方法稱(chēng)為"request（請(qǐng)求）"。在塊設(shè)備的注冊(cè)過(guò)程中，需要初始化request隊(duì)列，這一動(dòng)作通過(guò)blk_init_queue來(lái)完成，blk_init_queue函數(shù)建立隊(duì)列，并將該驅(qū)動(dòng)程序的 request 函數(shù)關(guān)聯(lián)到隊(duì)列。在模塊的清除階段，應(yīng)調(diào)用 blk_cleanup_queue 函數(shù)。

　　本例中相關(guān)的代碼為：

BLK_INIT_QUEUE(BLK_DEFAULT_QUEUE(MAJOR_NR), mtdblock_request, mtdblock_lock);
blk_cleanup_queue(BLK_DEFAULT_QUEUE(MAJOR_NR));

　　每個(gè)設(shè)備有一個(gè)默認(rèn)使用的請(qǐng)求隊(duì)列，必要時(shí)，可使用 BLK_DEFAULT_QUEUE(major) 宏得到該默認(rèn)隊(duì)列。這個(gè)宏在 blk_dev_struct 結(jié)構(gòu)形成的全局?jǐn)?shù)組（該數(shù)組名為 blk_dev）中搜索得到對(duì)應(yīng)的默認(rèn)隊(duì)列。blk_dev 數(shù)組由內(nèi)核維護(hù)，并可通過(guò)主設(shè)備號(hào)索引。blk_dev_struct 接口定義如下：

struct blk_dev_struct {
　/*
　* queue_proc has to be atomic
　*/
　request_queue_t request_queue;
　queue_proc *queue;
　void *data;
};

　　request_queue 成員包含了初始化之后的 I/O 請(qǐng)求隊(duì)列，data 成員可由驅(qū)動(dòng)程序使用，以便保存一些私有數(shù)據(jù)。

　　request_queue定義為：

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;
};

　　下圖表征了blk_dev、blk_dev_struct和request_queue的關(guān)系：

　　下圖則表征了塊設(shè)備的注冊(cè)和釋放過(guò)程：

　　5.小結(jié)

　　本章講述了Linux設(shè)備驅(qū)動(dòng)程序的入口函數(shù)及驅(qū)動(dòng)程序中的內(nèi)存申請(qǐng)、中斷等，并分別以實(shí)例講述了字符設(shè)備及塊設(shè)備的驅(qū)動(dòng)開(kāi)發(fā)方法。