12.块设备驱动程序（磁盘）

1.小结

分配gendisk结构
分配设置队列，队列提供实际的读写函数，这个会赋值到gendisk
注册块设备驱动（对比注册字符设备驱动，并没有提供一个file_operations）
设置gendisk参数，包括先前的队列
注册这个gendisk

2.关键函数解析

register_blkdev
注册块设备

int register_blkdev(unsigned int major, const char *name)            //如果major为0则为其自动分配主设备号

gendisk
磁盘设备

struct gendisk {
  int major;			/* major number of driver */
  int first_minor;
  int minors;                     /* maximum number of minors, =1 for
                                       * disks that can't be partitioned. 分区此设备数，在alloc_disk中分配*/
  char disk_name[DISK_NAME_LEN];	/* name of major driver */
  char *(*devnode)(struct gendisk *gd, umode_t *mode);

  unsigned int events;		/* supported events */
  unsigned int async_events;	/* async events, subset of all */

  /* Array of pointers to partitions indexed by partno.
   * Protected with matching bdev lock but stat and other
   * non-critical accesses use RCU.  Always access through
   * helpers.
   */
  struct disk_part_tbl __rcu *part_tbl;
  struct hd_struct part0;                  //分区表信息

  const struct block_device_operations *fops;      //块设备的操作函数
  struct request_queue *queue;                  //请求队列，用于管理设备IO请求队列的指针
  void *private_data;                           //私有数据

  int flags;
  struct device *driverfs_dev;  // FIXME: remove
  struct kobject *slave_dir;

  struct timer_rand_state *random;
  atomic_t sync_io;		/* RAID */
  struct disk_events *ev;
#ifdef  CONFIG_BLK_DEV_INTEGRITY
  struct blk_integrity *integrity;
#endif
  int node_id;
};

void set_capacity(struct gendisk *disk, sector_t size)
设置扇区数。512位一个扇区，描述设备容量
```
static inline void set_capacity(struct gendisk *disk, sector_t size)
{
    disk->part0.nr_sects = size;
}
```

3.代码解析

static struct gendisk *ramblock_disk;
static request_queue_t *ramblock_queue;

static int major;

static DEFINE_SPINLOCK(ramblock_lock);

  #define RAMBLOCK_SIZE (1024*1024)
static unsigned char *ramblock_buf;

static int ramblock_getgeo(struct block_device *bdev, struct hd_geometry *geo)
{
	/* 容量=heads*cylinders*sectors*512 */
	geo->heads     = 2;
	geo->cylinders = 32;
	geo->sectors   = RAMBLOCK_SIZE/2/32/512;
	return 0;
}


static struct block_device_operations ramblock_fops = {
	.owner	= THIS_MODULE,
	.getgeo	= ramblock_getgeo,
};

static void do_ramblock_request(request_queue_t * q)
{
	static int r_cnt = 0;
	static int w_cnt = 0;
	struct request *req;
	
	//printk("do_ramblock_request %d
", ++cnt);

	while ((req = elv_next_request(q)) != NULL) {            //电梯算法
		/* 数据传输三要素: 源,目的,长度 */
		/* 源/目的: */
		unsigned long offset = req->sector * 512;

		/* 目的/源: */
		// req->buffer

		/* 长度: */		
		unsigned long len = req->current_nr_sectors * 512;

		if (rq_data_dir(req) == READ)
		{
			//printk("do_ramblock_request read %d
", ++r_cnt);
			memcpy(req->buffer, ramblock_buf+offset, len);
		}
		else
		{
			//printk("do_ramblock_request write %d
", ++w_cnt);
			memcpy(ramblock_buf+offset, req->buffer, len);
		}		
		
		end_request(req, 1);
	}
}

static int ramblock_init(void)
{
	/* 1. 分配一个gendisk结构体 */
	ramblock_disk = alloc_disk(16); /* 次设备号个数: 分区个数+1 */

	/* 2. 设置 */
	/* 2.1 分配/设置队列: 提供读写能力 */
	ramblock_queue = blk_init_queue(do_ramblock_request, &ramblock_lock);            //do_ramblock_request具体的读写函数
	ramblock_disk->queue = ramblock_queue;
	
	/* 2.2 设置其他属性: 比如容量 */
	major = register_blkdev(0, "ramblock");  /* cat /proc/devices */	
	ramblock_disk->major       = major;
	ramblock_disk->first_minor = 0;
	sprintf(ramblock_disk->disk_name, "ramblock");
	ramblock_disk->fops        = &ramblock_fops;
	set_capacity(ramblock_disk, RAMBLOCK_SIZE / 512);

	/* 3. 硬件相关操作 */
	ramblock_buf = kzalloc(RAMBLOCK_SIZE, GFP_KERNEL);            //kzalloc申请内存外还会清零

	/* 4. 注册 */
	add_disk(ramblock_disk);

	return 0;
}

static void ramblock_exit(void)
{
	unregister_blkdev(major, "ramblock");
	del_gendisk(ramblock_disk);
	put_disk(ramblock_disk);
	blk_cleanup_queue(ramblock_queue);

	kfree(ramblock_buf);
}

module_init(ramblock_init);
module_exit(ramblock_exit);

MODULE_LICENSE("GPL");