2.12 How Pnp Works (simplified)
Here's how PnP should work in theory. The hypothetical PnP configuration program 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 indirectly 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 configuration registers on the card. It then assigns the card IRQ5 and 1 MB of memory addresses space, starting at address 0xe9000000. The PnP program also reads identifying information from the card telling what type of device it is, its ID number, etc. Then it directly or indirectly tells the appropriate device driver what it's done. If it's the driver itself that is doing the PnP, then there's no need to find a driver for the device (since it's driver is already running). Otherwise a suitable device driver needs to be found and sooner or later told how it's device is configured.
It's not always this simple since the card (or routing table for PCI) may specify that it can only use certain IRQ numbers 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.
One way commonly used to allocate resources is to start with one device and allocate it bus-resources. Then do the same for the next device, etc. Then if finally all devices get allocated resources without conflicts, then all is OK. But if allocating a needed resource would create a conflict, then it's necessary to go back and try to make some changes in previous allocations so as to obtain the needed bus-resource. This is called rebalancing. Linux doesn't do rebalancing but MS Windows does in some cases. For Linux, all this is done by the BIOS and/or kernel and/or device drivers. In Linux, the device driver doesn't get it's final allocation of resources until the driver starts up, so one way to avoid conflicts is just not to start any device that might cause a conflict. However, the BIOS often allocates resources to the physical device before Linux is even booted and the kernel checks PCI devices for addresses conflicts at boot-time.
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. BIOSs do this as does MS Windows and this but standard Linux doesn't. But in a way it does since it often uses what the BIOS has done. Windows stores this info in its "Registry" on the hard disk and a PnP/PCI BIOS stores it in non-volatile memory in your PC (known as ESCD; see The BIOS's ESCD Database). Some say that not having a registry (like Linux) is better since with Windows, the registry may get corrupted and is difficult to edit. But PnP in Linux has problems too.
While MS Windows (except for Windows 3.x and NT4) were PnP, Linux was not originally a PnP OS but has been gradually becoming a PnP OS. PnP originally worked for Linux because a PnP BIOS would configure the bus-resources and the device drivers would find out (using programs supplied by the Linux kernel) what the BIOS has done. Today, most drivers can issue commands to do their own bus-resource configuring and don't need to always rely on the BIOS. Unfortunately a driver could grab a bus-resource which another device will need later on. Some device drivers may store the last configuration they used in a configuration file and use it the next time the computer is
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