Block Drivers

Chapter 16

Block Drivers

  • Accessed by fixed sized blocks
  • Primarily used for disks drivers
  • Performance is more important than with char devices
  • Need to include in the driver:
#include <linux/genhd.h>
#include <linux/blkdev.h>
#include <linux/fs.h>

Data types

  • Blocks are fixed size chunks of data, with size determined by the Kernel - usually 4096 bytes
  • Sectors are smaller fixed size chunks of data, with size determined by the attached hardware (i.e. disk) - usually 512 bytes

Registration

  • Block devices are registered and unregistered using the following functions included from linux/fs.h
int register_blkdev(unsigned int major, const char *name);
int unregister_blkdev(unsigned int major, const char *name);
  • As with char devices, passing 0 as the major number will make the kernel assign an available major number

Block Drivers Operations

  • Some of the operations work exactly like they do in char drivers:
int (*open)(struct inode *inode, struct file *filp);
int (*release)(struct inode *inode, struct file *filp);
int (*ioctl)(struct inode *inode, struct file *filp, unsigned int cmd, unsigned long arg);

Block Driver Operations

  • Block driver specific methods:
int (*media_changed) (struct gendisk *gd);
  • Checks if the user made a change to the removable media: returns non-zero value on change
int (*revalidate_disk) (struct gendisk *gd);
  • Called when a user makes a change in the media, performs required work according to the change.
  • struct gendisk represents a single disk in the kernel

struct gendisk

  • See linux/genhd.h
  • Initialized by the Block driver
  • Arguments:
int major; int first_minor; int minors; # each minor represents a partition
char disk_name[32]; # cat /proc/partitions
struct block_device_operations *fops;
struct request_queue *queue; # used by kernel to manage I/O requests
int flags; # bit fields flags describing the device (i.e. GENHD_FL_REMOVABLE)
sector_t capacity; # the capacity of the device in 512-byte long sectors unit
void *private_data; # to be used at the discretion of the block driver

struct gendisk

  • To allocate a struct gendisk:
struct gendisk *alloc_disk(int minors);
  • Free using:
void del_gendisk(struct gendisk *gd);
  • Finally, add the gendisk to the system using:
void add_disk(struct gendisk *gd);
  • Set the capacity for the gendisk using:
void set_capacity(struct gendisk *gd, sector_t sectors)