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New HOWTO: Plug-and-Play-HOWTO - page 4

Table of Contents

  • April 26, 2001
  3.  The Plug-and-Play (PnP) Solution

  3.1.  Introduction to PnP

  The term Plug-and-Play (PnP) has various meanings.  In the broad sense
  it is just auto-configuration where one just plugs in a device and it
  configures itself.  In the sense used in this HOWTO, the configuration
  is only that of configuring PnP bus-resources and letting the device
  drivers know about it.  In a narrower sense it is just setting bus-
  resources in the hardware devices.  For the case of Linux, it is often
  just a driver detecting how the bus-resources have been set in its
  device by the BIOS, etc.  "PnP' may just mean the PnP specifications
  which (among other things).  There are long specs for PnP on the
  resource the ISA bus.  The standard PCI specifications (which are not
  called "PnP") do the same for the PCI bus.

  PnP matches up devices with their device drivers and specifies their
  communication channels.  On the ISA bus before Plug-and-Play the bus-
  resources were formerly set in hardware devices by jumpers.  Software
  drivers were assigned bus-resources by configuration files (or the
  like) or by probing the for the device at addresses where it's
  expected to reside.  The PCI bus was PnP-like from the beginning but
  at first it wasn't called PnP.  While the PCI bus specifications don't
  use the term PnP it supports in hardware what today is called PnP.


  3.2.  How It Works (simplified)

  Here's an oversimplified view of how PnP should work.  The PnP
  configuration program (in Linux only the BIOS  does this) finds all
  PnP devices and asks each what bus-resources it needs.  Then it checks
  what bus-resources (IRQs, etc.) it has to give away.  Of course, if it
  has reserved bus-resources used by non-PnP (legacy) devices (if it
  knows about them) it doesn't give these away.  Then it uses some
  criteria (not specified by PnP specifications) to give out the bus-
  resources so that there are no conflicts and so that all devices get
  what they need (if possible).  It then tells each physical device what
  bus-resources are assigned to it and the devices set themselves up to
  use only the assigned bus-resources.  Then the device drivers somehow
  find out what bus-resources their devices use and are thus able to
  communicate effectively with the devices they control.

  For example, suppose a card needs one interrupt (IRQ number) and 1 MB
  of shared memory.  The PnP program reads this request from the card.
  It then assigns the card IRQ5 and 1 MB of memory addresses space,
  starting at address 0xe9000000.  It's not always this simple as the
  card may specify that it can only use certain IRQ numbers (ISA only)
  or that the 1 MB of memory must lie within a certain range of
  addresses.  The details are different for the PCI and ISA buses with
  more complexity on the ISA bus.

  There are some shortcuts that PnP software may use.  One is to keep
  track of how it assigned bus-resources at the last configuration (when
  the computer was last used) and reuse this.   Windows9x and PnP BIOSs
  do this but standard Linux doesn't.  Windows9x stores this info in its
  "Registry" on the hard disk and a PnP BIOS stores it in non-volatile
  memory in your PC (known as ESCD; see ``The BIOS's ESCD Database'').

  While MS Windows is a PnP OS, Linux isn't.  But PnP still often works
  in Linux due to the BIOS doing the configuring of bus-resources and
  the device drivers finding out (using programs supplied by the Linux
  kernel) what the BIOS has done.  Drivers can also change bus-resource
  assignments using such programs (but they might take a bus-resource
  needed by another device).  Some device drivers store the last
  configuration they used and use it the next time the computer is
  powered on.

  If the device hardware remembered their previous configuration, then
  there wouldn't be any hardware to configure at the next boot-time, but
  they seem to forget their configuration when the power is turned off.
  Some devices contain a default configuration (but not necessarily the
  last one used).  Thus a PnP configuration program needs to be run each
  time the PC is powered on.  Also, if a new device has been added, then
  it too needs to be configured.  Allocating bus-resources to this new
  device might involve taking some bus-resources away from an existing
  device and assigning the existing device alternative bus-resources
  that it can use instead.  At present, Linux can't do this.


  3.3.  Starting Up the PC

  When the PC is first turned on the BIOS chip runs its program to get
  the computer started (the first step is to check out the hardware).
  If the operating system is stored on the hard-drive (as it normally
  is) then the BIOS must know about the hard-drive.  If the hard-drive
  is PnP then the BIOS may use PnP methods to find it.  Also, in order
  to permit the user to manually configure the BIOS's CMOS and respond
  to error messages when the computer starts up, a screen (video card)
  and keyboard are also required.  Thus the BIOS must always PnP-
  configure these devices on its own.

  Once the BIOS has identified the hard-drive, the video card, and the
  keyboard it is ready to start booting (loading the operating system
  into memory from the hard-disk).  If you've told the BIOS that you
  have a PnP operating system (PnP OS), it should start booting the PC
  as above and let the operating system finish the PnP configuring.
  Otherwise, a PnP-BIOS will (prior to booting) likely try to do the
  rest of the PnP configuring of devices (but not informing their
  drivers).  This is what usually happens when running Linux.


  3.4.  Buses

  ISA is the old bus of the old IBM PCs while PCI is a newer and faster
  bus from Intel.  The PCI bus was designed for what is today called
  PnP.  It makes it easy (as compared to the ISA bus) to find out how
  PnP bus-resources have been assigned to hardware devices.  To see what
  has happened use the commands lspci or scanpci (Xwindows) and/or look
  at /proc/pci or /proc/bus/pci.  The boot-up messages on your display
  are useful (use shift-PageUp to back up thru them).  See ``Boot-time
  Messages''


  For the ISA bus there was a real problem with implementing PnP since
  no one had PnP in mind when the ISA bus was designed and there are
  almost no I/O addresses available for PnP to use for sending
  configuration info to physical device.  As a result, the way PnP was
  shoehorned onto the ISA bus is very complicated.  Whole books have
  been written about it.  See ``PnP Book''.  Among other things, it
  requires that each PnP device be assigned a temporary "handle" by the
  PnP program so that one may address it for PnP configuring.  Assigning
  these "handles" is call "isolation".  See ``ISA Isolation'' for the
  complex details.

  Eventually, the ISA bus should become extinct.  When it does, PnP will
  be easier since it will be easy to find out how the BIOS has
  configured the hardware.  There will still be the need to match up
  device drivers with devices and also a need to configure devices that
  are added when the PC is up and running.  These needs would be better
  satisfied if Linux was a PnP operating system.


  3.5.  How Linux Does PnP

  Linux has had a serious problem dealing with PnP and still has a
  problem but it's not as severe as it once was.  Linux still is not
  really a PnP operating system and seems to mainly rely on and device
  drivers and the PnP BIOS to configure bus-resources for devices.  But
  the kernel provides help for the drivers in the form of PnP programs
  they may call on.  In many cases the device driver does all the needed
  configuring.  In other cases the BIOS may configure and then the
  device driver may find out how the BIOS has configured it.  The kernel
  provides the drivers with some functions (program code) that the
  drivers may use to find out if their device exists, how it's been
  configured, and functions to modify the configuration.  Kernel 2.2
  could do this only for the PCI bus but Kernel 2.4 has this feature for
  both the ISA and PCI buses (provided that the PNP options have been
  selected when compiling the kernel).  This by no means guarantees that
  all drivers will fully and correctly use these features.

  In addition, the kernel helps avoid resource conflicts by not allowing
  two devices to use the same bus-resources at the same time.
  Originally this was only for IRQs, and DMAs but now it's for address
  resources as well.

  Prior to Kernel 2.4, the standalone program: isapnp was often run to
  configure and/or get info from PnP devices on the ISA bus.  isapnp is
  still needed for cases where the device driver is not fully PnP for
  the ISA bus..  There was at least one attempt to make Linux a true PnP
  operating system.  See  .  But
  it never was put into the kernel.

  To see what help the kernel may provide to device drivers see the
  kernel documentation.   This documentation (if you have it) is a
  directory /usr/.../.../Documentation where one of the ... contains the
  word "kernel".  Use the "locate" command to find it.  In this
  documentation directory see pci.txt ("How to Write Linux PCI Drivers")
  and the file: /usr/include/linux/pci.h.  Unless you are a driver guru
  and know C Programming, these files are written so tersely that they
  will not actually teach you how to write a driver.  But it will give
  you some idea of what PnP type functions are available for drivers to
  use.  For the ISA bus see isapnp.txt and possibly (for kernel 2.4)
  /usr/include/linux/isapnp.h.

  When the PC starts up you may note from the messages on the screen
  that some Linux device drivers often find their hardware devices (and
  the bus-resources the BIOS has assigned them).  But there are
  situations that a real PnP operating system could handle better:

  �  A shortage of bus-resources

  �  More than one driver for a physical device

  �  Finding a driver for a detected device (instead of making drivers
     do the searching)

  �  Saving a lot of work for the programmers of device drivers

  The "shortage of bus-resources" problem is becoming less of a problem
  for two reasons:  One reason is that the PCI bus is replacing the ISA
  bus.  Under PCI there is no shortage of IRQs since IRQs may be shared
  (even though sharing is less efficient).  Also, PCI doesn't use DMA
  resources (although it does the equivalent of DMA without needing such
  resources).

  The second reason is that more and more physical devices are using
  main memory addresses instead of IO address space.  On 32-bit PCs
  there is 4GB of main memory address space and much of this bus-
  resource is available for device IO (unless you had 4GB of main memory
  installed).  Compare this to the IO address space which is limited to
  64KB.  So the memory space for device IO is not (yet ?) in short
  supply.
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