Linux-Flash驱动(2)-块设备驱动实例分析

在上一节课中，我们在内存中划分出512kB作为一个块设备，并对它实现读写的操作。现在我们来具体分析这段代码。

#include <linux/module.h>#include <linux/moduleparam.h>#include <linux/init.h>#include <linux/sched.h>#include <linux/kernel.h> /* printk() */#include <linux/slab.h>   /* kmalloc() */#include <linux/fs.h>   /* everything... */#include <linux/errno.h> /* error codes */#include <linux/timer.h>#include <linux/types.h> /* size_t */#include <linux/fcntl.h> /* O_ACCMODE */#include <linux/hdreg.h> /* HDIO_GETGEO */#include <linux/kdev_t.h>#include <linux/vmalloc.h>#include <linux/genhd.h>#include <linux/blkdev.h>#include <linux/buffer_head.h> /* invalidate_bdev */#include <linux/bio.h>MODULE_LICENSE("Dual BSD/GPL");static int major = 0;static int sect_size = 512;static int nsectors = 1024; /** The internal representation of our device.*/struct blk_dev{         int size;                        /* Device size in sectors */         u8 *data;                        /* The data array */         struct request_queue *queue;     /* The device request queue */         struct gendisk *gd;              /* The gendisk structure */};struct blk_dev *dev;/** Handle an I/O request, in sectors.*/static void blk_transfer(struct blk_dev *dev, unsigned long sector,   unsigned long nsect, char *buffer, int write){unsigned long offset = sector*sect_size;unsigned long nbytes = nsect*sect_size;if ((offset + nbytes) > dev->size) {printk (KERN_NOTICE "Beyond-end write (%ld %ld)\n", offset, nbytes);return;}if (write)memcpy(dev->data + offset, buffer, nbytes);elsememcpy(buffer, dev->data + offset, nbytes);}/** 读写请求处理函数*/static void blk_request(struct request_queue *q){struct request *req;//从队列中取出要处理的一个请求req = blk_fetch_request(q);while (req != NULL) {struct blk_dev *dev = req->rq_disk->private_data;blk_transfer(dev, blk_rq_pos(req), blk_rq_cur_sectors(req), req->buffer, rq_data_dir(req));   if(!__blk_end_request_cur(req, 0)) {req = blk_fetch_request(q);}}}/** Transfer a single BIO.*/static int blk_xfer_bio(struct blk_dev *dev, struct bio *bio){int i;struct bio_vec *bvec;sector_t sector = bio->bi_sector;/* Do each segment independently. */bio_for_each_segment(bvec, bio, i) {char *buffer = __bio_kmap_atomic(bio, i, KM_USER0);blk_transfer(dev, sector, bio_cur_bytes(bio)>>9 /* in sectors */, buffer, bio_data_dir(bio) == WRITE);sector += bio_cur_bytes(bio)>>9; /* in sectors */__bio_kunmap_atomic(bio, KM_USER0);}return 0; /* Always "succeed" */}/** Transfer a full request.*/static int blk_xfer_request(struct blk_dev *dev, struct request *req){struct bio *bio;int nsect = 0;     __rq_for_each_bio(bio, req) {blk_xfer_bio(dev, bio);nsect += bio->bi_size/sect_size;}return nsect;}/** The device operations structure.*/static struct block_device_operations blk_ops = {.owner            = THIS_MODULE,};/** Set up our internal device.*/static void setup_device(){//计算设备大小dev->size = nsectors*sect_size;dev->data = vmalloc(dev->size);if (dev->data == NULL) {printk (KERN_NOTICE "vmalloc failure.\n");return;}//把块设备放入请求队列中，blk_request用于指明处理这个请求的函数dev->queue = blk_init_queue(blk_request, NULL);if (dev->queue == NULL)goto out_vfree;//指明设备的扇区大小blk_queue_logical_block_size(dev->queue, sect_size);dev->queue->queuedata = dev;//分配gendisk结构dev->gd = alloc_disk(1);if (! dev->gd) {printk (KERN_NOTICE "alloc_disk failure\n");goto out_vfree;}/*初始化alloc_disk*/dev->gd->major = major;//主设备号dev->gd->first_minor = 0;//次设备号dev->gd->fops = &blk_ops;//操作函数集dev->gd->queue = dev->queue;//请求队列dev->gd->private_data = dev;//私有数据sprintf (dev->gd->disk_name, "simp_blk%d", 0);//磁盘名字set_capacity(dev->gd, nsectors*(sect_size/sect_size));//扇区数//注册块设备add_disk(dev->gd);return;out_vfree:if (dev->data)vfree(dev->data);}static int __init blk_init(void){/** 注册块设备，申请主设备号*/major = register_blkdev(major, "blk");if (major <= 0) {printk(KERN_WARNING "blk: unable to get major number\n");return -EBUSY;}//申请一个描述结构（不是每个块设备都有）dev = kmalloc(sizeof(struct blk_dev), GFP_KERNEL);if (dev == NULL)goto out_unregister;//安装这个设备setup_device();     return 0;out_unregister:unregister_blkdev(major, "sbd");return -ENOMEM;}static void blk_exit(void){if (dev->gd) {del_gendisk(dev->gd);put_disk(dev->gd);}if (dev->queue)blk_cleanup_queue(dev->queue);if (dev->data)vfree(dev->data);nregister_blkdev(major, "blk");kfree(dev);}module_init(blk_init);module_exit(blk_exit);

这是一个模块的程序，先看看模块初始化里面的blk_init函数，里面做了这几件事：

1、注册一个快设备register_blkdev，如果没有分配到主设备号则打印错误信息

2、然后是申请一个结构，这个结构是用来保存这个块设备信息的，不是每个块设备都有

3、再安装这个设备setup_device，这个函数是自定义的

    3.1完成块设备大小的计算

    3.2把快设备放入请求队列中（IO调度层把请求排序后放入请求队列中，里面的参数blk_request是一个函数，用于指明使用哪个函数对这个请求进行处理）

    3.3指明设备的扇区大小

    3.4然后用alloc_disk函数分配一个gendisk结构（一个驱动可能对于几个块设备，用gendisk来区分）

    3.5紧接着需要对这个结构进行初始化，如下：

/*初始化alloc_disk*/dev->gd->major = major;//主设备号dev->gd->first_minor = 0;//次设备号dev->gd->fops = &blk_ops;//操作函数集dev->gd->queue = dev->queue;//请求队列dev->gd->private_data = dev;//私有数据sprintf (dev->gd->disk_name, "simp_blk%d", 0);//磁盘名字set_capacity(dev->gd, nsectors*(sect_size/sect_size));//扇区数

3.6注册这个块设备

第二个重要的函数是实现读写请求处理，读写请求通过blk_request函数来实现：

1、使用blk_fetch_request从队列中取出要处理的一个请求

2、使用blk_transfer实现对对应扇区的硬件操作，比如读和写，应该这里的块设备是内存模拟的，所以使用的是memcpy函数

3、使用__blk_end_request_cur判断请求队列是否还有请求要处理，如果有则继续处理，没有退出。