linux驱动之块设备驱动

块设备驱动的系统架构

VFS：
是对各种具体文件系统的一种封装，用户程序访问文件提供统一的接口。例如：
EXT2，FAT，NFS等
系统架构—Cache：
当用户发起文件访问请求的时候，首先回到Cache中寻址文件是否被缓存了，如果在Cache,则直接从cache中读取。如果数据不在缓存中，就必须要到具体的文件系统中读取数据了。
Mapping Layer：
首先确定文件系统的block size，然后计算所请求的数据包含多少个block.
调用具体文件系统的函数来访问文件的inode结构，确定所请求的数据在磁盘上的地址。
Generic Block Layer：
Linux内核把块设备看做是由若干个扇区组成的数据空间，上层的读写请求在通用块层被构造成一个或多个bio结构。
I/O Scheduler Layer，I/O调度层负责采用某种算法（如：电梯调度算法）将I/O操作进行排序。
电梯调度算法的基本原则：如果电梯现在朝上运动，如果当前楼层的上方和下方都有请求，则先响应所有上方的请求，然后才向下响应下方的请求；如果电梯向下运动，则刚好相反。

块设备注册过程：
1， 注册设备块驱动程序 register_blkdev
2， 初始化请求队列 blk_init_queue
3， 指明扇区的大小 blk_queue_logical_block_size(dev->queue, sect_size);
4， 申请一个gendisk结构，初始化
5， 注册块设备
block_device_operations —— 操作函数接口
blk_init_queue —— 请求块设备队列操作
gendisk —— 磁盘信息结构

#include 
#include 
#include 

#include 
#include  /* printk() */
#include    /* kmalloc() */
#include    /* everything... */
#include  /* error codes */
#include 
#include  /* size_t */
#include  /* O_ACCMODE */
#include  /* HDIO_GETGEO */
#include 
#include 
#include 
#include 
#include  /* invalidate_bdev */
#include 

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);
else
   memcpy(buffer, dev->data + offset, nbytes);
}

/*
* The simple form of the request function.
*/
static void blk_request(struct request_queue *q)
{
struct request *req;

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,
};

static int __init blk_init(void)
{
    //注册设备块驱动程序
    major = register_blkdev(major, "blk");
    if (major <= 0) {
       printk(KERN_WARNING "blk: unable to get major number\n");
       return -EBUSY;
    }
    dev = kmalloc(sizeof(struct blk_dev), GFP_KERNEL);
    if (dev == NULL)
       goto out_unregister;

    /*
    * Get some memory.
    */
    dev->size = nsectors * sect_size;
    dev->data = vmalloc(dev->size);
    if (dev->data == NULL) {
       printk (KERN_NOTICE "vmalloc failure.\n");
       return;
    }

    //初始化请求队列
    dev->queue = blk_init_queue(blk_request, NULL);
    if (dev->queue == NULL)
        goto out_vfree;

    //指明扇区的大小
    blk_queue_logical_block_size(dev->queue, sect_size);
    dev->queue->queuedata = dev;


   //申请一个gendisk结构，初始化
    dev->gd = alloc_disk(1);
    if (! dev->gd) {
       printk (KERN_NOTICE "alloc_disk failure\n");
       goto out_vfree;
    }
    dev->gd->major = major;
    dev->gd->first_minor = 0;
    dev->gd->fops = &blk_ops;
    dev->gd->queue = dev->queue;
    dev->gd->private_data = dev;
    sprintf (dev->gd->disk_name, "simp_blk%d", 0);
    set_capacity(dev->gd, nsectors*(sect_size/sect_size));

    //注册块设备
    add_disk(dev->gd);

out_vfree:
    if (dev->data)
       vfree(dev->data);

    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);

    unregister_blkdev(major, "blk");
    kfree(dev);
}

module_init(blk_init);
module_exit(blk_exit);