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 resources and letting the device drivers know about it. In a more narrow sense it just setting resources in the hardware devices. It may also mean the PnP specifications which (among other things) specify how PnP resource data is to be read and written to devices (often cards) on the ISA bus. The standard PCI (and not PnP) specifications 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 resources were set in hardware devices by jumpers. Software drivers were assigned 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 so it was trivial to implement PnP for this bus. Since the PCI bus specifications don't use the term PnP it's not clear whether or not the PCI bus should be called PnP (but it supports in hardware what today is called PnP).
Here's an oversimplified view of how PnP works. The PnP configuration program (perhaps a program in the BIOS) finds all PnP devices and asks each what resources it needs. Then it checks what resources (IRQs, etc.) it has to give away. Of course if it has reserved 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 resources so that there are no conflicts and so that all devices get what they need. It then tells each physical device what resources are assigned to it and the devices set themselves up to use only the assigned resources. Then the device drivers somehow find out what 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 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" and a PnP BIOS stores it in non-volatile memory in your PC (known as ESCD; see The BIOS's ESCD Database).
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 resources to this new device might involve taking some resources away from an existing device and assigning the existing device alternative resources.
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 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 "boot" (load the operating system from the hard-disk). If you've told the BIOS that you have a PnP operating system, it should boot the PC and let the operating system finish the PnP configuring. Otherwise, a PnP-BIOS will likely try to do the rest of the PnP configuring of devices (but not their drivers).
ISA is the old bus of the old IBM PC's while PCI is a newer and faster bus from Intel. Eventually, the ISA bus should become extinct. When it does, there should be almost no PnP problem in Linux provided: 1. the only bus in use is PCI. 2. The BIOS does a good job of PnP configuring it.
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 resources have been assigned to hardware devices. To see what has happened look at the /proc/pci "file" (/proc/bus/pnp/devices for kernel 2.2+), the boot-up messages on your display (use shift-PageUp to back up), or use PCI Utilities (for kernel 2.2+).
For the ISA bus there is 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 configure info to physical device. As a result, the way PnP was shoehorned onto the ISA bus is very complicated. A whole book has 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 Isolation for the complex details.
PnP was invented by Compaq, Intel, and Phoenix. Microsoft has been a leading promoter of it. Linux would have been better off if PnP had never been "invented". Eventually the ISA bus would have become extinct and the PnP-like PCI bus would prevail so that we would have in effect gotten an easy-to-implement PnP. But like it or not, most all new ISA hardware today is PnP and Linux has no choice but to deal effectively with PnP. But standard Linux (as of early 1999) makes dealing with PnP complicated (especially on the ISA bus) while the purpose of PnP was to make it simple.
In a sense, Linux is already somewhat PnP for the PCI bus. 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 resources the BIOS has assigned them). But there still could be conflicts with the ISA bus.
Linux users should not need to delve into the details of PnP to configure ISA PnP devices as they now need to. One solution would be a standardized version of the Linux kernel that supports Plug-and-Play on both the ISA and PCI buses. A patch to the kernel has been written although most drivers don't support it. It's not part of standard Linux. See Patch Kernel.
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