Interfaces used by disk devices

The classical UNIX device interface

• implement bdevsw
  • methods
    • d_open
    • d_close
    • d_strategy
    • d_ioctl
    • d_psize
    • d_discard
  • constants
    • d_flag
• implement cdevsw
  • methods
    • d_open
    • d_close
    • d_read
    • d_write
    • d_ioctl
    • d_mmap
    • d_discard
  • other methods not used by disk drivers
    • d_stop
    • d_tty
    • d_poll
    • d_kqfilter
  • constants
    • d_flag

Layering is possible, the dk_lookup function is used to get a handle (a vnode) a child device.

• call each other using vnode layer
  • methods VOP_IOCTL VOP_STRATEGY
  • dk_lookup to find vnode
  • vn_close to release vnode

Disk drivers use the kernel disk interface to provide information for iostat and property lists. There are also helper routines to handle partitions and to get a disklabel.

• kernel disk interface
  • kern/subr_disk.c
    • driver calls disk_init() to register
    • instrumentation (iostat)
    • DIOCGDISKINFO
    • partition support (bounds_check_..)
    • disklabel routines
    • low-level routines polluting namespace (disk_read_sectors)
    • registers a callback to strategy and minphys functions, currently there are only one user, dk(4) calls the minphys function.

Almost all disk drivers have a regular driver structure that is created by the autoconf framework. An exception is the ccd(4) driver that hand-crafts its private data without autoconf.

• autoconf device building
  • kern/subr_autoconf.c
    • generic for all device drivers, not only disk drivers
    • provides common data including a proplib dictionary
    • the dictionary is used to register unit data, for example a drive geometry.

The disklabel routines are mostly MD, the sun/sparc routines are not in arch/ but in dev/sun/, the i386/amd64 routines (also used by other archs) are in kern/subr_disk_mbr.c

• disklabel routines
  • really MD, implement readdisklabel, writedisklabel, setdisklabel
  • kern/subr_disk_mbr.c
    • more disklabel routines, MBR, ISO9660
    • writedisklabel
  • sun/disksubr.c
    • like subr_disk_mbr.c but different for sun3/sparc/sparc64
  • arch/*/*/disksubr.c
    • like subr_disk_mbr.c but different

A wedge is a layered device on top of the RAW_PART of a standard disk device. All disk drivers need to implement ioctls to manually attach wedge devices (using dkctl(8)) and need to call dkwedge_discover to implement autodetect

• wedge routines
  • dev/dkwedge/*
  • implement dk device
  • implement scan routines for various labels (MBR,BSD,GPT)
  • IOCTLs to attach dk devices
  • scan routines called by each disk driver

The attempt to refactor disk drivers by collecting common functionality. Hardly used, the dk_lookup routine doesn't really belong here (it's more a vnode routine).

• dksubr interface
  • dk_lookup helper to find vnodes
  • dk_intf switch implements common parts of
    • open
    • close
    • strategy
    • size
    • ioctl
    • dump
  • dk_getdisklabel, dk_makedisklabel. fallback routines ?.
  • used only by cgd, xbd