Devices and Drivers (Chapter 7)

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Devices and Drivers (Chapter 7)

• Khattab Alhabashi
• UNIX System Administration

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Introduction

• Definition A device driver is a program that
  manages the systems interaction with a
  particular piece of hardware.
• The driver translates between the hardware
  commands understood by the device and the
  stylized programming interface used by the
  kernel.
• The existence of the driver layer helps to keep
  UNIX reasonably device-independent.
• Device drivers are part of the kernel they are
  not user processes. However, a driver can be
  accessed both from within the kernel and from
  user space.
• User-level access to devices is provided through
  special device file that live in the /dev
  directory.

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Introduction Contd.

• Most devices in the system will fall into three
  categories
• 1) SCSI
• Devices that attach to a SCSI bus are easy to
  configure.
• Most systems have one driver that allows direct
  access to the SCSI bus, plus additional drivers
  for special types of devices such as disks and
  tapes.
• 2) Vendor
• Most devices you can buy from your hardware
  vendor will already be supported by the operating
  system.
• Unless you have explicitly removed drivers, the
  kernel will usually recognize a new device as
  soon as you install it.
• The appropriate device files in /dev will already
  have been created for you, or the OS will create
  them on the fly.

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Introduction Contd.

• 3) Third-party
• When you purchase a piece of hardware from a
  third party (someone other than your hardware
  vendor), it will usually come with an
  installation script that installs the device,
  configures the kernel if necessary, and makes
  devices entries.
• On systems with loadable drivers, kernel
  configuration may not be necessary.

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Device Numbers and Jump Tables

• Device files are mapped to devices via their
  major and minor device numbers, values that are
  stored in the files inode structure.
• The major device number identifies the driver
  that the file is associated with.
• The minor device number identifies which
  particular device of a given type is to be
  addressed. It is often called the unit number or
  instance of the device.
• There are two types of device files
• Block device files it is read or written a block
  (a group of bytes, usually multiple of 512) at a
  time.
• Character device files it can be read or written
  one byte at a time.

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Device Numbers and Jump Tables Contd.

• Some devices support access via both block and
  character device files.
• Each driver has routines for performing some or
  all of the following functions
• probe attach open
  close read reset stop
• select strategy dump
  psize write timeout
• process a transmit interrupt
• process a receive interrupt
• ioctl (input / output control)
• Inside the kernel, the addresses of these
  functions for each driver are stored in a
  structure called a jump table.

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Device Numbers and Jump Tables Contd.

• There are actually two tables
• - One for character device and
• - One for block devices.
• The jump tables are indexed by major device
  numbers.
• When a program performs an operation on a device
  file, the kernel automatically catches the
  reference, looks up the appropriate function name
  in the jump table, and transfers control to it.
• To perform an unusual operation that doesnt have
  a direct analog in filesystem model (for example,
  ejecting floppy disk), the ioctl system call is
  used to pass a message directly from user space
  into the driver.

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Device Numbers and Jump Tables Contd.

• Three examples were discussed in this chapter
  about adding device drivers
• - Adding a BSD Device Driver Adding a
  completely new device driver to BSD machine
  involves adding it to a couple of configuration
  files and editing the kernel source code to
  include references to the drivers routine.
• - Adding an HP-UX Device Driver On HP-UX
  systems, the jump tables are constructed from
  specifications in a text file called master.
• - Adding an IRIX Device Driver IRIX uses a
  directory of files (/var/sysgen/master.d) to
  perform the same function as the single master
  file in HP-UX.

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Device Numbers and Jump Tables Contd.

• There are some common steps in all three examples
  that have to be done after adding and modifying
  the files specified for each example
• - Building a new kernel.
• - Copying the old kernel aside and installing
  the new kernel.
• - Rebooting and testing the new kernel.
• - Creating device files and test the device
  itself.

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Device Files

• By convention, device files are kept in the /dev
  directory.
• ATT systems handle device files quite nicely by
  using separate subdirectory of /dev for each
  type of device
• disk, tape, terminal, etc.
• Device files are created with the mknod command,
  which has the syntax
• mknod filename type major minor
• where
• - filename is the device file to be created.
• - type is c for a character device or b for a
  block device.
• - major and minor are the major and minor
  device numbers.

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Device Files Contd.

• A shell script called MAKEDEV is sometimes
  provided (in /dev) to automatically supply
  default values to mknod.
• You need to scan through the script to find the
  arguments needed for your device. For example,
  to make PTY entries on a SunOS system, you would
  use the following command
• cd /dev
• ./MAKEDEV pty

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Naming Conventions For Devices

• Devices that have both block and character
  identities usually
• - preface the character device name with the
  letter r for row (e.g.,
  /dev/sd0 and /dev/rsd0), or
• - place it in a subdirectory with a name that
  starts with r
• (e.g., /dev/dsk/dks0d3s0 vs. /dev/rdsk/dks0d3s0)
  .
• Serial devices are usually named tty followed by
  a sequence of letters that identify the interface
  the port is attached to (See chapter 8 for more
  information
• about serial ports).

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Naming Conventions For Devices Contd.

• BSD disk names often begin with a two-letter
  abbreviation for either the drive of the
  controller, followed by the drive number and
  partition name.
• - Example sd0a is the block device
  representing the partition of the first disk
  drive on a SCSI controller rsd0a is the
  corresponding character device.
• The names of tapes and devices often include not
  only a reference to the drive itself, but also an
  indication of whether the device rewinds after
  each tape operation and the density at which it
  reads and writes.

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Loadable Kernel Modules

• Loadable modules allow device drivers to be
  linked into and removed from the kernel while it
  is running.
• - This makes the installation of drivers much
  easier, since the kernel binary does not have to
  be changed.
• - It also allows the kernel to be smaller,
  because drivers are not loaded unless they are
  needed.
• Loadable modules are implemented by providing one
  or more documents hooks into the kernel where
  additional device drivers can grab on.
• A user-level command communicates with the kernel
  and tells it to load new modules into memory and
  to make entries for them in the systems jump
  tables.

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Loadable kernel Modules Contd.

• Although loadable drivers are convenient, they
  are not entirely safe.
• Any time you load or unload a module, you risk
  causing a kernel panic.
• Like other aspects of device and driver
  management, the implementation of loadable
  modules is operating system dependent.
• There are four of the example systems that
  support loadable modules
• - Solaris In Solaris, virtually everything is
  a loadable module. The modinfo command lists the
  currently-loaded modules. You can add a driver
  with the add_drv command.

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Loadable Kernel Modules Contd.

• - HP-UX Generic HP-UX does not support
  loadable modules, but there is an option that
  allows the loading of STREAMS modules.
• - IRIX IRIX 5.2 supports loadable modules
  earlier versions did not. Modules are
  manipulated with the ml command. ml with the
  list option catalogs the modules that the kernel
  is currently aware of.
• - SunOS SunOS versions 4.1.2 and later
  contain support for loadable modules.
  Currently-loaded modules can be listed with
  modstat.

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• END OF CHAPTER
• QUESTIONS???