UBLK_MINORS
ret = alloc_chrdev_region(&ublk_chr_devt, 0, UBLK_MINORS, "ublk-char");
unregister_chrdev_region(ublk_chr_devt, UBLK_MINORS);
unregister_chrdev_region(ublk_chr_devt, UBLK_MINORS);
#define UBLK_MAX_UBLKS UBLK_MINORS