2.15 How Linux Does PnP
Linux has had serious problems in the past in dealing with PnP but most of those problems have now been solved (as of mid 2004). Linux has gone from a non-PnP system originally, to one that can be PnP if certain options are selected when compiling the kernel. The BIOS may assign IRQs but Linux may also assign some of them or even reassign what the BIOS did. The configuration part of ACPI (Advance Configuration and Power Interface) is designed to make it easy for operating systems to do their own configuring. Linux can use ACPI if it's selected when the kernel is compiled.
In Linux, it's traditional for each device driver to do it's own low level configuring. This was difficult until Linux supplied software in the kernel that the drivers could use to make it easier on them. Today (2005) it has reached the point where the driver simply calls the kernel function: pci_enable_device() and the device gets configured by being enabled and having both an irq (if needed) and addresses assigned to the device. This assignment could be what was previously assigned by the BIOS or what the kernel had previously reserved for it when the pci or isapnp device was detected by the kernel. There's even an acpi option for Linux to assign all devices IRQs at boot-time.
So today, in a sense, the drivers are still doing the configuring but they can do it just by telling Linux to do it (and Linux may not need to do much since it sometimes is able to use what has already been set by the BIOS or Linux). So it's really the non-device-driver part of the Linux kernel that is doing most of the configuring. Thus, it may be correct to call Linux a PnP operating system, at least for common computer architectures.
Then when a device driver finds its device, it asks to see what addresses and IRQ have been assigned (by the BIOS and/or Linux) and normally just accepts them. But if the driver wants to do so, it can try to change the addresses, using functions supplied by the kernel. But the kernel will not accept addresses that conflict with other devices or ones that the hardware can't support. When the PC starts up, you may note messages on the screen showing that some Linux device drivers have found their hardware devices and what the IRQ and address ranges are.
Thus, the kernel provides the drivers with 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 if needed. Kernel 2.2 could do this only for the PCI bus but Kernel 2.4 had this feature for both the ISA and PCI buses (provided that the appropriate PNP and PCI options have been selected when compiling the kernel). Kernel 2.6 came out with better utilization of ACPI. This by no means guarantees that all drivers will fully and correctly use these features. And legacy devices that the BIOS doesn't know about, may not get configured until you (or some configuration utility) puts its address, irq, etc. into a configuration file.
In addition, the kernel helps avoid resource conflicts by not allowing two devices that it knows about 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.
If your have an old ISA bus, the program isapnp is run to find and configure pnp devices on the ISA bus.
To see what help the kernel may provide to device drivers see the directory /usr/.../.../Documentation where one of the ... contains the word "kernel-doc" or the like. Warning: documentation here tends to be out-of-date so to get the latest info you would need to read messages on mailing lists sent by kernel developers and possibly the computer code that they write including comments. In this kernel 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 enable you to write a driver. But it will give you some idea of what PnP type functions are available for drivers to use.
For kernel 2.4 see isapnp.txt. For kernel 2.6, isapnp.txt is replaced by pnp.txt which is totally different than isapnp.txt
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