MMC_BLOCK_MAJOR
md->disk->major = MMC_BLOCK_MAJOR;
res = register_blkdev(MMC_BLOCK_MAJOR, "mmc");
unregister_blkdev(MMC_BLOCK_MAJOR, "mmc");
unregister_blkdev(MMC_BLOCK_MAJOR, "mmc");
#define MMC_IOC_CMD _IOWR(MMC_BLOCK_MAJOR, 0, struct mmc_ioc_cmd)
#define MMC_IOC_MULTI_CMD _IOWR(MMC_BLOCK_MAJOR, 1, struct mmc_ioc_multi_cmd)