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New HOWTO: Plug-and-Play-HOWTO - page 3

Table of Contents

  • April 26, 2001
  2.  What PnP Should Do: Allocate "Bus-Resources"

  2.1.  What is Plug-and-Play (PnP)?

  If you don't understand this section, read the next section ``How a
  Computer Finds Devices (and  conversely)''


  Oversimplified, Plug-and-Play automatically tells the software (device
  drivers) where to find various pieces of hardware (devices) such as
  modems, network cards, sound cards, etc.  Plug-and-Play's task is to
  match up physical devices with the software (device drivers) that
  operates them and to establish channels of communication between each
  physical device and its driver.  In order to achieve this, PnP
  allocates the following "bus-resources" to both drivers and hardware:
  I/O addresses, IRQs, DMA channels (ISA bus only), and memory regions.
  These 4 things are sometimes called 1st order resources.  If you don't
  understand what these 4 bus-resources are, read the following
  subsections of this HOWTO: I/O Addresses, IRQs, DMA Channels, Memory
  Regions.  An article in Linux Gazette about 3 of these bus-resources
  is Introduction to IRQs, DMAs and Base Addresses.  Once these bus-
  resources have been assigned (and if the correct driver is installed),
  the "files" for such devices in the /dev directory are ready to use.

  This PnP assignment of bus-resources is sometimes called "configuring"
  but it is only a low level type of configuring.  Even with PnP fully
  utilized, much configuring of devices is done by other than PnP.  For
  example, for modem configuration an "init string" is sent to the modem
  over the I/0 address "channel".  This "init string" has nothing to do
  with PnP although the "channel" used to send it to the modem was
  allocated by PnP.  Setting the speed (and many other parameters) of a
  serial port is done by sending messages to the device driver from
  programs run by the user, application programs, or by start-up scripts
  at boot-time.  This configuring also has nothing to do with PnP.  Thus
  when talking about PnP, "configuring" means only a certain type of
  configuring.  While other documentation (such a for MS Windows) simply
  calls bus-resources "resources",  I have used the term "bus-resources"
  so as to distinguish it from the multitude of other kinds of
  resources.


  2.2.  How a Computer Finds Devices (and conversely)

  A computer consists of a CPU/processor to do the computing and RAM
  memory to store programs and data (for fast access).  In addition,
  there are a number of devices such as various kinds of disk-drives, a
  video card, a keyboard, network cards, modem cards, sound cards, the
  USB bus, serial and parallel ports, etc.  There is also a power supply
  to provide electric energy, various buses on a motherboard to connect
  the devices to the CPU, and a case to put all this into.

  In olden days most all devices had their own plug-in cards (printed
  circuit boards).  Today, in addition to plug-in cards, many "devices"
  are small chips permanently mounted on the "motherboard".  Cards which
  plug into the motherboard may contain more than one device.  Memory
  chips are also sometimes considered to be devices but are not plug-
  and-play in the sense used in this HOWTO.

  For the computer system to work right, each device must be under the
  control of its "device driver".  This is software which is a part of
  the operating system (perhaps loaded as a module) and runs on the CPU.
  Device drivers are associated with "special files" in the /dev
  directory although they are not really files.  They have names such as
  hda1 (first partition on hard drive a), ttyS0 (the first serial port),
  eth1 (the second ethernet card), etc.  To make matters more
  complicated, the particular device driver selected, say for eth1, will
  depend on the type of ethernet card you have.  Thus eth1 can't just be
  assigned to any ethernet driver.  It must be assigned to a certain
  driver that will work for the type of ethernet card you have
  installed.  For modules, some of these assignments might be found in
  /etc/modules.conf (called "alias") while others may reside in an
  internal kernel table.

  To control a device, the CPU (under the control of the device driver)
  sends commands (and data) to and reads info from the various devices.
  In order to do this each device driver must know the address of the
  device it controls.  Knowing such an address is equivalent to setting
  up a communication channel, even though the physical "channel" is
  actually the data bus inside the PC which is shared with almost
  everything else.

  This communication channel is actually a little more complex than
  described above.  An "address" is actually a range of addresses so
  that sometimes the word "range" is used instead of "address".  There
  could even be more that one range (with no overlapping) for a single
  device.  Also, there is a reverse part of the channel (known as
  interrupts) which allows devices to send an urgent "help" request to
  their device driver.


  2.3.  Addresses

  PCs have 3 address spaces: I/O, main memory, and configuration (except
  that the old ISA bus lacks a "configuration" address space).  All of
  these 3 types of addresses share the same bus inside the PC.  But the
  voltage on certain dedicated wires on the PC's bus tells which "space"
  an address is in: I/O, main memory, (see ``Memory Ranges''),  or
  configuration.  See ``Address Details'' for more details.  Only two of
  these 3 address spaces are used for device I/O: I/0 and main memory.


  2.4.  I/O Addresses and Allocating Them

  Devices were originally located in I/O address space but today they
  may use space in main memory.  An I/0 address is sometimes just called
  "I/O", "IO", "i/o" or "io".  The terms "I/O port" or "I/O range" are
  also used.  There are two main steps to allocate the I/O addresses (or
  other bus-resources such as interrupts):


  1. Set the I/O address, etc. on the card (in one of its registers)

  2. Let its device driver know what this I/O address, etc. is


  The two step process above is something like the two part problem of
  finding someone's house number on a street.  Someone must install a
  number on the front of the house so that it may be found and then you
  must obtain (and write down) this house number.  In computers the
  device hardware must first set the address it will use in  a special
  register and then the device driver must obtain this address.  Both of
  these must be done, either automatically by software or by entering
  the data manually into some file.  Problems occur when only one of
  them gets done (or is attempted).

  For manual PnP configuration some people make the mistake of doing
  only one of these and then wonder why the computer can't find the
  device.  For example, they may use "setserial" to assign an address to
  a serial port without realizing that this only tells the driver an
  address.  It doesn't set the address in the serial port hardware
  itself.  If the serial port hardware doesn't have the address you told
  setserial (or doesn't have any address set in it) then you're in
  trouble.

  An obvious requirement is that before the device driver can use an
  address it must be first set on the physical device (such as a card).
  Since device drivers often start up soon after you start the computer,
  they sometimes try to access a card (to see if it's there, etc.)
  before the address has been set in the card by a PnP configuration
  program.  Then you see an error message that they can't find the card
  even though it's there (but doesn't yet have an address).

  What was said in the last few paragraphs regarding I/O addresses
  applies with equal force to other bus-resources: ``Memory Ranges'',
  ``IRQs --Overview'' and ``DMA Channels''.  What these are will be
  explained in the next 3 sections.


  2.5.  Memory Ranges

  Many devices are assigned address space in main memory.  It's
  sometimes called "shared memory" or "memory-mapped I/O".  This memory
  is physically located on the device.  When discussing bus-resources
  it's often just called "memory".  In addition to using such "memory",
  such a device might also use conventional I/O address space.

  When you plug in such a card, you are in effect also plugging in a
  memory module for main memory.  A high address is selected for it by
  PnP so that it doesn't conflict with main memory chips.  This memory
  can either be ROM (Read Only Memory) or shared memory.  Shared memory
  is shared between the device and the CPU (running the device driver)
  just as IO address space is shared between the device and the CPU.
  This shared memory serves as a means of data "transfer" between the
  device and main memory. It's IO but it's not done in IO space.  Both
  the card and the device driver need to know where it is.

  ROM is different.  It is likely a program (perhaps a device driver)
  which will be used with the device.  It could be initialization code
  so that a device driver is still required.  Hopefully, it will work
  with Linux and not just MS Windows ??  It may need to be shadowed
  which means that it is copied to your main memory chips in order to
  run faster.  Once it's shadowed it's no longer "read only".

  2.6.  IRQs --Overview

  After reading this you may read ``Interrupts --Details'' for some more
  details.  The following is intentionally oversimplified:  Besides the
  address, there is also an interrupt number to deal with (such as IRQ
  5).  It's called an IRQ (Interrupt ReQuest) number.  We already
  mentioned above that the device driver must know the address of a card
  in order to be able to communicate with it.  But what about
  communication in the opposite direction?  Suppose the device needs to
  tell its device driver something immediately?  For example, the device
  may have just received a lot of bytes destined for main memory and the
  device needs to tell its driver to fetch these bytes at once and
  transfer them from the device's nearly full buffer into main memory.
  Another example is to signal the driver that the device has finished
  sending out a bunch of bytes and is now waiting for some more bytes
  from the driver so it can send them too.

  How should the device rapidly signal its driver?  It may not be able
  to use the main data bus since it's likely already in use.  Instead it
  puts a voltage on a dedicated interrupt wire (part of the bus) which
  is often reserved for that device alone.  This voltage signal is
  called an Interrupt ReQuest (IRQ) or just an "interrupt" for short.
  There are the equivalent of 16 such wires in a PC and each wire leads
  (indirectly) to a certain device driver.  Each wire has a unique IRQ
  (Interrupt ReQuest) number.  The device must put its interrupt on the
  correct wire and the device driver must listen for the interrupt on
  the correct wire.  Which wire the device sends help requests on is
  determined by the IRQ number stored in the device.  This same IRQ
  number must be known to the device driver so that the device driver
  knows which IRQ line to listen on.

  Once the device driver gets the interrupt from the device it must find
  out why the interrupt was issued and take appropriate action to
  service the interrupt.  On the ISA bus each device usually needs its
  own unique IRQ number.  For the PCI bus and other special cases the
  sharing of IRQs is allowed.


  2.7.  DMA Channels

  DMA channels are only for the ISA bus.  DMA stands for "Direct Memory
  Access".  This is where a device is allowed to take over the main
  computer bus from the CPU and transfer bytes directly to main memory.
  Normally the CPU would make such a transfer in a two step process:

  1. reading from the I/O memory space of the device and putting these
     bytes into the CPU itself

  2. writing these bytes from the CPU to main memory


  1. With DMA it's usually a one step process of sending the bytes
     directly from the device to memory

     The device must have such capabilities built into its hardware and
     thus not all devices can do DMA.  While DMA is going on the CPU
     can't do too much since the main bus is being used by the DMA
     transfer.

  The PCI bus doesn't really have any DMA but instead it has something
  even better: bus mastering.  It works something like DMA and is
  sometimes called DMA (for example, hard disk drives that call
  themselves "UltraDMA").  It allows devices to temporarily become bus
  masters and to transfer bytes almost like the bus master was the CPU.
  It doesn't use any channel numbers since the organization of the PCI
  bus is such that the PCI hardware knows which device is currently the
  bus master and which device is requesting to become a bus master.
  Thus there is no allocation of DMA channels for the PCI bus.

  When a device on the ISA bus wants to do DMA it issues a DMA-request
  using dedicated DMA request wires much like an interrupt request.  DMA
  actually could have been handled by using interrupts but this would
  introduce some delays so it's faster to do it by having a special type
  of interrupt known as a DMA-request.  Like interrupts, DMA-requests
  are numbered so as to identify which device is making the request.
  This number is called a DMA-channel.  Since DMA transfers all use the
  main bus (and only one can run at a time) they all actually use the
  same channel but the "DMA channel" number serves to identify who is
  using the "channel".  Hardware registers exist on the motherboard
  which store the current status of each "channel".  Thus in order to
  issue a DMA-request, the device must know its DMA-channel number which
  must be stored in a special register on the physical device.



  2.8.  "Resources" for both Device and Driver

  Thus device drivers must be "attached" in some way to the hardware
  they control.  This is done by allocating bus-resources (I/O, Memory,
  IRQ's, DMA's) to both the physical device and the device driver
  software.  For example, a serial port uses only 2 (out of 4 possible)
  resources: an IRQ and an I/O address.  Both of these values must be
  supplied to the device driver and the physical device.  The driver
  (and its device) is also given a name in the /dev directory (such as
  ttyS1).  The address and IRQ number is stored by the physical device
  in configuration registers on its card (or in a chip on the
  motherboard).  For the case of jumpers, it's the location of the
  jumpers themselves that store the bus-resource configuration in the
  device hardware (on the card, etc.).  For the case of PnP, the
  configuration register data is usually lost when the PC is powered
  down (turned off) so that the bus-resource data must be supplied to
  each device anew each time the PC is powered on.


  2.9.  The Problem

  The architecture of the PC provides only a limited number of IRQ's,
  DMA channels, I/O address, and memory regions.  If there were only
  several devices and they all had standardized bus-resource (such as
  unique I/O addresses and IRQ numbers) there would be no problem of
  attaching device drivers to devices.  Each device would have a fixed
  resources which would not conflict with any other device on your
  computer.  No two devices would have the same addresses, there would
  be no IRQ conflicts, etc.  Each driver would be programmed with the
  unique addresses, IRQ, etc. hard-coded into the program.  Life would
  be simple.

  But it's not.  Not only are there so many different devices today that
  conflicts are frequent, but one sometimes needs to have more than one
  of the same type of device.  For example, one may want to have a few
  different disk-drives, a few network cards, etc.  For these reasons
  devices need to have some flexibility so that they can be set to
  whatever address, IRQ, etc. is needed to avoid conflicts.  But some
  IRQ's and addresses are pretty standard such as the ones for the clock
  and keyboard.  These don't need such flexibility.

  Besides the problem of conflicting allocation of bus-resources, there
  is a problem of making a mistake in telling the device driver what the
  bus-resources are.  For example, suppose that you enter IRQ 4 in a
  configuration file when the device is actually set at IRQ 5.  This is
  another type of bus-resource allocation error.

  The allocation of bus-resources, if done correctly, establishes
  channels of communication between physical hardware and their device
  drivers.  For example, if a certain I/O address range (resource) is
  allocated to both a device driver and a piece of hardware, then this
  has established a one-way communication channel between them.  The
  driver may send commands and info to the device.  It's actually a
  little more than one-way since the driver may get information from the
  device by reading its registers.  But the device can't initiate any
  communication this way.  To initiate communication the device needs an
  IRQ so it can send interrupts to its driver.  This creates a two-way
  communication channel where both the driver and the physical device
  can initiate communication.


  2.10.  PnP Finds Devices Plugged Into Serial Ports

  External devices that connect to the serial port via a cable (such as
  external modems) can also be called Plug-and-Play.  Since only the
  serial port itself needs bus-resources (an IRQ and I/O address) there
  are no bus-resources to allocate to such plug-in devices.  Thus PnP is
  not really needed for them.  Even so, there is a PnP specification for
  such external serial devices.

  A PnP operating system will find such an external device and read its
  model number, etc.  Then it may be able to find a device driver for it
  so that you don't have to tell an application program that you have a
  certain device on say /dev/ttyS1.  Since you should be able to
  manually inform your application program (via a configuration file,
  etc.) what serial port the device is on (and possibly what model
  number it is) you should not really need this "serial port" feature of
  PnP.  I don't think Linux supports it ??
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