块设备驱动系统架构和简单设计

1.块设备概念
块设备是指只能以块(512Byte)为单位进行访问的设备，块大小一般是512个字节的整数倍。常见的块设备包括硬件，SD卡，光盘等。

2.快速体验

insmod simple-blk.ko
ls /dev/simp_blkdev0
mkfs.ext3 /dev/simp_blk0
mkdir –p /mnt/blk
mount /dev/simp_blk0 /mnt/blk
cp /etc/init.d/* /mnt/blk
ls /mnt/blk
umount /mnt/blk
ls /mnt/blk

3.块设备驱动系统架构 

VFS是对各种具体文件系统的一种封装 ，为用户程序访问文件提供统一的接口。

4.系统架构-Cache

当用户发起文件访问请求的时候，首先会到Disk Cache中寻找文件是否被缓存了，如果在cache中，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。

5.Mapping Layer（映射层、FS文件系统）

1. 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。

2. 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。

6. Generic Block Layer
Linux内核把把块设备看作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。

7. 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。


8. I/O Scheduler Layer
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。

9.块设备驱动
在块系统架构的最底层，由块设备驱动根据排序好的请求，对硬件进行数据访问。

10.块设备驱动实例分析

#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)                                                                                    //扇区访问函数
{
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);                                                                                       //对内存读写
else
   memcpy(buffer, dev->data + offset, nbytes);
}

/*
* The simple form of the request function.
*/
static void blk_request(struct request_queue *q)                                                                                      //实现读写请求处理函数
{
struct request *req;

req = blk_fetch_request(q);                                                                                                           //从队列中取出一个请求
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))                                                                                                 //判断请求队列是否为空
   {
    req = blk_fetch_request(q);                                                                                                       //嵌套处理
   }
}
}

/*
* The device operations structure.
*/
static struct block_device_operations blk_ops = {
.owner = THIS_MODULE,
};


/*
* Set up our internal device.
*/
static void setup_device()
{
/*
* Get some memory.
*/
dev->size = nsectors*sect_size;                                                                                  //获取设备大小
dev->data = vmalloc(dev->size);                                                                                  //获取数据指针
if (dev->data == NULL) {
   printk (KERN_NOTICE "vmalloc failure.\n");
   return;
}

   dev->queue = blk_init_queue(blk_request, NULL);                                                               //请求队列初始化，blk_request是处理上层传下来的请求的
   if (dev->queue == NULL)
    goto out_vfree;

blk_queue_logical_block_size(dev->queue, sect_size);                                                             //指明扇区大小
dev->queue->queuedata = dev;
/*
* And the gendisk structure.
*/
dev->gd = alloc_disk(1);                                                                                         //为块设备分配gendisk结构，并初始化
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));
add_disk(dev->gd);                                                                                               //注册块设备
return;

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

static int __init blk_init(void)
{
/*
* Get registered.
*/
major = register_blkdev(major, "blk");                                                                  //注册块设备驱动，major若为0会自动分配
if (major <= 0) {
   printk(KERN_WARNING "blk: unable to get major number\n");
   return -EBUSY;
}

dev = kmalloc(sizeof(struct blk_dev), GFP_KERNEL);                                                      //分配一个blk_dev空间
if (dev == NULL)
   goto out_unregister;

   setup_device();                                                                                      //调用函数

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);

11、简单块设备驱动设计

#include <linux/module.h>
#include <linux/init.h>
#include <linux/errno.h>
#include <linux/blkdev.h>
#include <linux/bio.h>
#include <linux/string.h>
#include <asm/uaccess.h>
#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/genhd.h>

MODULE_LICENSE("GPL");

static int major = 0;

static int sect_size = 512;
static int nsectors = 1024;

struct blk_dev {
    int size;
    u8 *data;
    struct request_queue *queue;
    struct gendisk *gd;
};

struct blk_dev *dev;

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 nbyte = nsect * sect_size;

    if((offset + nbyte) > dev->size)
    {
        printk(KERN_NOTICE"Beyond-end write (%ld %ld)\n", offset, nbyte);
    return;
    }

    if(write)
    {
        memcpy(dev->data + offset, buffer, nbyte);
    }
    else
    {
        memcpy(buffer, dev->data + offset, nbyte);
    }
}

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;
        //处理该请求
    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);
    }

}

static struct block_device_operations blk_ops = {
    .owner = THIS_MODULE,
};

static void setup_device(void)
{
    dev->size = nsectors * sect_size;
    dev->data = vmalloc(dev->size);
    if(dev->data == NULL) {
        printk(KERN_NOTICE "vmalloc failure.\n");
    return;
    }

    dev->queue = blk_init_queue(blk_request, NULL);
    if(dev->queue == NULL)
    {
        goto out_vfree;
    }

    blk_queue_logical_block_size(dev->queue, sect_size);
    dev->queue->queuedata = dev;

    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);
    add_disk(dev->gd);
    return;

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

static int __init blk_init(void)
{
    major = register_blkdev(0, "blk");
    if(major <= 0)
    {
        printk(KERN_WARNING "register blk dev fail!\n");
    return -EBUSY;
    }

    dev = kmalloc(sizeof(struct blk_dev), GFP_KERNEL);
    if(dev == NULL)
        goto out_unregister;

    setup_device();

    return 0;
out_unregister:
    unregister_blkdev(major, "sbd");
    return -ENOMEM;
}

static void blk_exit(void)
{
    del_gendisk(dev->gd);
    blk_cleanup_queue(dev->queue);
    vfree(dev->data);
    unregister_blkdev(major, "blk");
    kfree(dev);
}

module_init(blk_init);
module_exit(blk_exit);

错误总结：一开始写的时候，编译完加载进内核。直接死机，因为代码有点长。就从逻辑处一点一点注释掉来分析，果然有好多错误。
第一次遇到的问题，内核中常用goto跳转处理错误情况。如果跳转处前面没有return就坑爹了，第一次遇到找了好久