Plug-and-Play-HOWTO (for Linux) David S.Lawyer v0.01, March 1999 Help with understanding and dealing with Plug-and-Play devices. How to get your Linux system to support Plug-and-Play. This early version is incomplete but should be of some help. ______________________________________________________________________ Table of Contents 1. Introduction 1.1 Copyright, Trademarks, Disclaimer, & Credits 1.1.1 Copyright 1.1.2 Trademarks 1.1.3 Disclaimer 1.2 Future Plans: You Can Help 1.3 New Versions of this HOWTO 2. What PnP Should Do: Allocate "Resources" 2.1 What is Plug-and-Play (PnP)? 2.2 How a Computer Finds Devices (and conversely) 2.3 I/O Addresses, etc. 2.4 IRQ's --Overview 2.5 DMA Channels 2.6 Memory Regions 2.7 "Resources" in Two Places 2.8 The Problem 2.9 PnP Finds Devices Plugged Into Serial Ports 3. The Plug-and-Play (PnP) Solution 3.1 Introduction 3.2 Linux Needs PnP 3.3 Buses 3.4 Configuring a PnP BIOS 3.4.1 Do you have a PnP operating system? 3.4.2 How are resources to be controlled? 3.4.3 Reset the configuration? 4. How To Deal With PnP Cards 4.1 Introduction 4.2 Disable PnP ? 4.3 BIOS Configures PnP 4.4 Isapnp 4.5 Patch the Kernel to Make Linux PnP 4.6 PnP Software and Documents 5. Appendix 5.1 Addresses 5.1.1 ISA Bus Configuration Address (Read-Port etc.) 5.1.2 Address ranges 5.1.3 Address space 5.1.4 Communicating Directly via Memory 5.2 Interrupts --Details 5.3 PCI Interrupts 5.4 Isolation ______________________________________________________________________ 1. Introduction 1.1. Copyright, Trademarks, Disclaimer, & Credits 1.1.1. Copyright Copyright (c) 1998 by David S. Lawyer. Please freely copy and distribute (sell or give away) this document. For corrections and minor changes contact the maintainer. Otherwise you may create derivative works and distribute them provided you: 1. Discuss it with the maintainer (if there is one). 2. Put the derivative work at the mirrored LDP Internet site for free downloading. 3. License the work in the spirit of this license or use GPL. 4. Give due credit to previous authors and major contributors. 1.1.2. Trademarks If certain words are trademarks, the context should make it clear to whom they belong. For example "MS Windows" (or just "Windows") implies that "Windows" belongs to Microsoft (Micro$oft). 1.1.3. Disclaimer Much of the info in this HOWTO was obtained from Serial-HOWTO, the Internet, etc. and may be unreliable. While I haven't intentionally tried to mislead you, there are likely a number of errors in this document. Please let me know about them. Since this is free documentation, it should be obvious that neither I nor previous authors can be held legally responsible for any errors. 1.2. Future Plans: You Can Help Please let me know of any errors in facts, opinions, logic, spelling, grammar, clarity, links, etc. But first, if the date is over a months old, check to see that you have the latest version. Please send me any info that you think belongs in this document. I haven't studied in detail either isapnptools nor David Howells patches to the kernel. Nor do I fully understand how PnP is configured by the BIOS. Thus this HOWTO is incomplete and may be inaccurate (let me know where I'm wrong). In this HOWTO I've used ?? to indicate that I don't really know the answer. Would you like to improve on (rewrite) and maintain this HOWTO? I'm looking for someone to turn it over to. 1.3. New Versions of this HOWTO New versions of the Plug-and-Play-HOWTO will be available to browse and/or download at LDP mirror sites. For a list of mirror sites see: . Various formats are available. If you only want to quickly check the date of the latest version look at: . 2. What PnP Should Do: Allocate "Resources" 2.1. What is Plug-and-Play (PnP)? Oversimplified, Plug-and-Play is a way to automatically tell 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 device and its driver. More precisely, PnP allocates the following "resources" to both drivers and hardware: I/O addresses, IRQ's, DMA channels (ISA bus only), and memory regions. If you don't understand what these 4 items are read the following subsections. Once these resources have been assigned, the names for such devices in the /dev directory are ready to use. This PnP assignment of certain 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 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 (often automatically boot-time). This configuring also has nothing to do with PnP. Thus when talking about PnP, "resources" means only a limited subset of resources and "configuring" means only a certain type of configuring. 2.2. How a Computer Finds Devices (and conversely) A computer consists of a CPU/processor to do the computing and memory to store programs and data. 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, serial and parallel ports, etc. There is also a power supply to provide electric energy, various buses on a motherboard to connect the devices and the CPU together, 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 a single board called 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" (software which is a part of the operating system and which 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. 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 used for such communication. 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. The communication channel is actually a little more complex than described above. An "address" is actually a range of addresses and 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. I/O Addresses, etc. PC's have 3 address spaces: I/O, main memory, and configuration (only on the PCI bus). Only the first two (memory and I/O) are configured by PnP. All of these 3 types of addresses share the same bus inside the PC. But the voltage on certain dedicated wires tells all devices which "space" an address is in: I/O, main memory, or configuration. Devices are normally located in I/O address space (although in some cases they are also allocated space in main memory). An I/0 address is sometimes just called "I/O", "IO", "i/o" or "io". The term "I/O port" also used. There are two main steps to allocate the I/O addresses (or other 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. You must obtain (and write down) the house number and and someone must install a number on the front of the house so that it may be found. In computers, the device driver must obtain the address and the device hardware must install the same address in one of its registers. Both of these must be done, but 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 will use "setserial" to assign an address to a serial port without realizing that this only tells the driver that this is the address. It doesn't set the address in the serial port hardware itself and if you told the driver the wrong address you're in trouble. Now to explain (in the next 3 sections) some other "resources" which may be assigned: ``IRQ's'', ``DMA Channels'', and ``Memory Regions''. 2.4. IRQ's --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 IRQ5). 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 the device needs to tell its device driver something immediately? Thus the device needs to know the address (or the like) of it's device driver so that it can call it for help. For example, the device may have just received a lot of bytes destined for main memory and the device needs to call its driver to fetch these bytes at once and transfer them from the device's nearly full buffer into main memory. The device calls for such help by putting a voltage on an interrupt wire (part of the bus). There are the equivalent of 16 wires 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 it's put 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 to. On the ISA bus each device needs its own unique IRQ number (except that two or more serial ports may share IRQs starting with kernel 2.2). For the PCI bus, sharing of IRQs is allowed. 2.5. DMA Channels This is not used much since true DMA is only for the ISA bus and the equivalent of DMA on the PCI bus is both much faster and doesn't require a "DMA channel". 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, putting these bytes into the CPU itself 2. writing these byte from the CPU to main memory. 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 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-request 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 register on the physical device. 2.6. Memory Regions Some devices are assigned address space in main memory as well as in I/O address space. When you plug in such a card, you are in effect also plugging in a memory module (for main memory, not just I/O memory). This memory is shared between the device and the CPU (running the device driver). This memory can serve as a means of direct data "transfer" between the device and main memory. It's not really a transfer since the device puts data into its own memory which also happens to be main memory. Both the card and the device driver need to know where it is. The memory address are likely to be very high so that they do not conflict with the lower addresses of the memory chips in your computer. 2.7. "Resources" in Two Places Thus device drivers must be "attached" in some way to the hardware they control. This is done by supplying "resources" (I/O, Memory, IRQ's, DMA's) to both the physical device and the device driver software. For example, in the case of a serial port there are 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 then given a name in the /dev directory (such as ttyS1). The address and IRQ number is stored by the physical device in a register on the card (or in a chip on the motherboard). 2.8. The Problem The architecture of the PC provides only a limited number of IRQ's, DMA channels, I/O address, etc. If there were only several devices and they all had standardized 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 I/O address, there would be no IRQ conflicts, etc. Each driver would be programmed with the I/O address, 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 likely, but 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 serial ports, 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 resources, there is a problem of making a mistake in telling the device driver what the 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 resource allocation error. The allocation of resources, if done correctly, also 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 it's registers. But the device can't initiate any communication this way. Allocating an IRQ makes it a two-way communication channel where both the driver and the device can initiate communication. 2.9. PnP Finds Devices Plugged Into Serial Ports Devices that plug into the serial port can also be Plug-and-Play. Since only the serial port itself needs resources (an IRQ and I/O address) there are no resources to allocate to such plug-in devices. Thus PnP is not really needed for them. However, a PnP operating system will find them and know what model of device is there. 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 are using say /dev/ttyS1. Since you should be able to manually tell your computer (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" feature of PnP. 3. The Plug-and-Play (PnP) Solution 3.1. Introduction Plug-and_Play (PnP) is a way to automate the assignment of PnP resources to hardware and the corresponding software. That is, to match up devices and their device drivers and specify their communication channels. Before Plug-and-Play the resources were set on hardware devices by jumpers and software drivers were assigned resources by configuration files (or the like) or by probing the for the device (which didn't always work right). With full PnP (which doesn't always work right either), both a PnP BIOS and the operating system participate. When the computer is first turned on the BIOS chip runs its program to get the computer started. 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 configure the BIOS when the computer starts up, a screen (video card) and keyboard is required so the BIOS must PnP-configure these devices if needed. 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 do this and let the operating system finish the PnP configuring. Otherwise, a PnP-BIOS will likely try to do the rest of the PnP configuring itself. 3.2. Linux Needs PnP PnP was invented by Wintel (Microsoft and Intel). Partly for this reason, and partly due to dislike of the way it was implemented there has been some justified prejudice against PnP in the Linux community. But like it or not, most all hardware today is PnP and Linux has no choice but to deal effectively with PnP. Linux users should not need to delve into the details of PnP to configure devices as they now need to. The solution would be a standardized version of Linux that supports Plug-and-Play. 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''. 3.3. Buses PnP is intended to work on any bus such as ISA and PCI. 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. The PCI bus reserved many configuration addresses on the bus for configuring (by PnP or otherwise). Linux uses some of these addresses to find out what PCI devices you have and puts information about them in the /proc/pci "file". For the ISA bus there is a real problem with implementing PnP since unlike the PCI bus which was designed for PnP, 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. As a result, the way PnP is handled on the ISA bus is complicated and 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'' in the Appendix for the complex details. 3.4. Configuring a PnP BIOS When the computer is first turned on, the BIOS runs before the operating system is loaded. Newer BIOSs are PnP and will configure some or all of the PnP devices. For most PnP BIOSs there is no way to disable PnP so you have to live with it. Here are some of the choices which may exist in your BIOS (sometimes called CMOS) menu: · ``Do you have a PnP operating system?'' · ``How are resources to be controlled?'' · ``Reset the configuration?'' 3.4.1. Do you have a PnP operating system? If you say yes, then the PnP BIOS will start to PnP-configure the hard-drive, video card, and keyboard to make the system bootable. But it will leave it up to the operating system to finish the configuration job. It may do an ``Isolation'' on the ISA bus leaving the devices disabled but ready to be configured by the operating system. If your operating system doesn't do configuring (some Linux patched versions do), you probably should not say yes since the BIOS might leave the ISA devices it hasn't configured in a disabled state ?? If you say no, then the BIOS will do the configuring itself. Unless you have added new PnP devices, it should use the previous configuration which it has stored in non-volatile memory. If the last session on your computer was with Linux, then there should be no change in configuration. But if the last session was with Windows 95 or 98 (which are PnP) then Windows may have set up the configuration differently than what you want under Linux. You may check on how it's configured when running Windows and then assume that under Linux it remains the same. This is one way to deal with PnP: Just accept whatever configuration Windows 9x gives it. When you are first configuring under Windows9x, you may tell the BIOS that you have a PnP OS and then later on change the answer to no. When you add a new device, the BIOS alone may be able to configure it too without help from Windows but it might not do the configuring as well ?? You can always have isapnp run at the startup of Linux to change the ISA configuration to what you want under Linux. This is of no help for PCI cards but you can always check how they are configured by looking at /proc/pci. Then you need to make sure that this matches your Linux configuration files ("setserial", lilo.conf, etc.) for the driver being used. See ``BIOS Configures PnP'' for more info. 3.4.2. How are resources to be controlled? This may involve just deciding how to allocate IRQ's and DMA's. It set to "auto", the bios will do the allocation. If set to manual, you enter another menu and are able to reserve some IRQ's for use on "legacy" (non-pnp) cards. Now the BIOS may or may not know about your legacy cards. If it knows about them, then try using "auto". If it doesn't know about them then manually reserve the IRQ's needed for the legacy ISA cards and let the rest be for the BIOS PnP to allocate. The BIOS will only know about your legacy cards if you ran ICU (or the like) under Windows to tell the BIOS about them. The BIOS will then store this information in its non-volatile data-base. 3.4.3. Reset the configuration? This will erase the BIOSs data-base of how your PnP devices should be configured as well as the list of how legacy (non-PnP) devices are configured. Never do this unless you are convinced that this data- base is wrong and needs to be remade. It was stated somewhere that you should do this only if you can't get your computer to boot. If the BIOS loses the data on legacy devices, then you'll need to run ICA again under windows to reestablish this data. 4. How To Deal With PnP Cards 4.1. Introduction Today most all new internal boards (cards) are Plug-and-Play (PnP). Some of these have jumpers (or this like) which may be changed to disable PnP. Since software exists in Linux (and Dos/Windows) to handle PnP, it is sometimes best to keep PnP enabled even when you have the option of disabling it. If you patch the kernel for Plug- and-Play it not only configures the hardware by putting resource information into its registers but it also tries to provides this information to the driver software so that you may not need to configure it. For example, for a serial port you may not need to use "setserial". Unfortunately, most drivers don't yet support it. If you have a PnP card, then your have one or more of the following options for configuring it: · ``Disable PnP'' by jumpers (but many cards can't do this) or special software · ``BIOS Configures PnP'' (only if you have a PnP BIOS) · ``Isapnp''(is a program you can always use to configure PnP devices on the ISA bus (not for PCI devices) · ``Patch Kernel'' to transform it Linux into a PnP operating system 4.2. Disable PnP ? Many devices are PnP only with no option for disabling it. If possible, it may be a lot simpler to just disable PnP but don't forget to tell the BIOS the resources that it uses. There are also some reasons why you might not want to disable PnP: 1. If you have MS Windows on the same machine, then you may want to allow PnP under MS Windows to configure your devices differently. 2. The range of selection for IRQ numbers (or port addresses) etc. may be quite limited unless you use PnP. 3. If it requires the use of Dos/Windows software to configure a non- PnP configuration, then someday you might not have Dos/Windows around anymore and will thus have difficulty changing the configuration. 4. You have (or will have) other PnP devices that need configuring so that you'll need to provide for PnP anyway. Once configured as non-PnP devices, they can't be configured by PnP software or the BIOS (until you move jumpers and/or use the Dos/Windows configuration software again to enable PnP). 4.3. BIOS Configures PnP Your BIOS must obviously support PnP to do this. To find out more about your BIOS, look on the Web. Some BIOS may have minimal PnP capabilities and try to turn over the difficult parts of the configuration task to Window utilities (which can't happen under Linux). In this case if the BIOS does maintain a configuration data base (in non-volatile memory), you might try to help set up this data base by using the ICU under DOS/Windows or if you have Windows9x (which are PnP OS's) you could just let Windows do it without using ICU. Before you run Linux you set up the BIOS for "Not a PnP Operating System" (or the like). See ``Configuring a PnP BIOS'' This will make the BIOS do the configuring instead of leaving it to the operating system. The BIOS's non-volatile data-base is called the ESCD (Extended System Configuration Data). The ESCD not only stores the configuration of PnP devices but stores configuration information of non-PnP devices so as to avoid conflicts. When you install a new non-PnP device you are supposed to tell the BIOS's ESCD about it by running the Dos/Windows ICU (Intel Configuration Utility) program prior to installation. You will also need to run this to tell the ESCD about the existing non-PnP devices on the PC (unless someone else has already done so). The non-volatile ESCD configuration is usually saved on a chip and remains intact when the power is off, but sometimes it's put on a hard-drive?? Each time the BIOS starts up under Linux it should configure things this way. You may want to take notes on how ICU (and the BIOS) has configured things (or print them out). However, if you add a new non-PnP device you need to run the ICU program again to let the BIOS put that info into it's data base. If it's PnP then the BIOS will automatically configure it and hopefully not change the configuration on any of the other existing devices on your PC. But it might have to reconfigure some of the existing devices in order to allocate the required resources to the new device. How has the BIOS configured it? To determine this under Linux may not be easy. But it is easy for the PCI bus: just look at /proc/pci. For the ISA bus it's not so easy. Using pnpdump one may find PnP cards on the ISA bus and also how they would like to be configured but there is no guarantee that they have been actually configured like this. One may run some isapnp commands using the PEEK keyword to find out what the I/O address and IRQ is but this is complicated. Don't confuse the read-port address which pnpdump "trys" (and finds something there) with the I/O address of the found device. They are not the same. Note that under Dos/Windows the configuration used is also put in a Windows file on the hard disk so the the operating system knows where everything is. This doesn't happen in Linux which usually auto- detects devices. 4.4. Isapnp Unfortunately, the documentation for isapnp was difficult to understand unless you knew the basics of PnP. This HOWTO should help you understand it. isapnp is only for PnP devices on the ISA bus (non-PCI). Running the Linux program "isapnp" at boot-time will configure such devices to the resource values you set in /etc/isapnp.conf. You need the package isapnptools, available in many distributions of Linux. Type "locate pnp" to see what you may already have available for isapnp. If your Linux distribution automatically installed isapnptools, isapnp may already be running at startup. In this case, all you need to do is to edit /etc/isapnp.conf per "man isapnp.conf". Note that this is like manually configuring PnP since you make the decisions as to how to configure as you edit the configuration file. You can use the program "pnpdump" to help create the configuration file. If you use "isapnp" like this and have a PnP BIOS, should you tell the BIOS (when you set it up) that you have a PnP operating system ?? If you only ran isapnp once to configure ISA-PnP devices but failed to run isapnp each time the computer starts up then if you have Windows 9x on the same PC the following problem might occur: When you use Windows 9x, Windows may configure your PnP cards differently in such a way that it doesn't work right (if at all) when you go back to use Linux. 4.5. Patch the Kernel to Make Linux PnP David Howells has created a patch to do this called "Linux Kernel Configuration/Resource Manager. The resulting kernel is is claimed to be stable but bugs have been reported. It includes documentation: serial.txt to show how to deal with the serial port. It provides "files" in the /proc tree so that you can see what is going on and can echo commands into one of these files for custom configuration. One problem is that most device drivers don't know about it so that you still may have to use the traditional configuration files, etc. for configuration. See 4.6. PnP Software and Documents · See for pointers to specifications of PnP · Book: PCI System Architecture, 3rd ed. by Tom Shanley +, MindShare 1995. Covers inherent PnP on the PCI bus. · Book: Plug and Play System Architecture, by Tom Shanley, Mind Share 1995. Details of PnP on the ISA bus. Only a brief overview of PnP on the PCI bus. 5. Appendix 5.1. Addresses There are three types of addresses: main memory addresses, I/O addresses and configuration addresses. On the PCI bus configuration addresses constitute a separate address space just like I/O addresses do. Except for the case of ISA configuration addresses, whether or not an address on the address bus (or shared address-data bus for PCI) is a memory address, I/O address, or configuration address depends only on the voltage on other wires (traces) of the bus. 5.1.1. ISA Bus Configuration Address (Read-Port etc.) For the ISA bus, there is technically no configuration address space, but there is a special way for the CPU to access PnP configuration registers on the PnP cards. For this purpose 3 @ I/O addresses are allocated. This is not 3 addresses for each card but 3 addresses shared by all cards. Each card listens at the same address called a write-port The 3 addresses are named read-port, write-port, and address-port. The ports are one byte in size. Each PnP card has a number of registers so that just 3 addresses are not even sufficient for a single card. To communicate with a certain card, the card number (handle) is sent to all cards at the write-port address. After that only that one card is listening. Then the address of the register which is to be dealt with is sent to the address-port (of all cards --but only one is listening). Next either a read is made on the read- port or a write is made on the write-port. The write-port is always A79 and the address-port is always 279. But the read-port is not fixed but is set by the configuration software at some address that will supposedly not conflict with any other ISA card. If there is a conflict, it will change the address. All PnP cards get "programmed" with this address. Thus if you use say isapnp to set or check configuration data you need to know the read-port address. 5.1.2. Address ranges The term "address" is sometimes used in this document to mean a contiguous range of addresses. Since addresses are given in bytes, a single address only contains one byte but I/O (and main memory) addresses need more than this. So a range of say 8 bytes is often used for I/O address while the range for main memory addresses allocated to a device is much larger. For a serial port (an I/O device) it's sufficient to give the starting I/O address of the device (such as 3F8) since it's well known that the range of addresses for this device is only 8 bytes. The starting address is known as the "base address". 5.1.3. Address space For ISA, to access both I/O and (main) memory address "spaces" the same address bus is used (the wires used for the address are shared). How does the device know whether or not an address which appears on the address bus is a memory address or I/O address? Well, there are 4 dedicated wires on the bus that convey this information and more. If a certain one of these 4 wires is asserted, it says that the CPU wants to read from an I/O address, and the main memory ignores the address on the bus. The other 3 wires serve similar purposes. In summary: Read and write wires exist for both main memory and I/O addresses (4 wires in all). For the PCI bus it's the same basic idea also using 4 wires but it's done a little differently. Instead of only one or the four wires being asserted, a binary number is put on the wires (16 different possibilities). Thus more info may be conveyed. Four of these 16 numbers serve the I/O and memory spaces as in the above paragraph. In addition there is also configuration address space which uses up two more numbers. Ten extra numbers are left over for other purposes. 5.1.4. Communicating Directly via Memory Traditionally, most I/O devices used only I/O memory to communicate with the CPU. For example, the serial port does this. The device driver, running on the CPU would read and write data to/from the I/O address space and main memory. A faster way would be for the device itself to put the data directly into main memory. One way to do this is by using ``DMA Channels''. Another way is to allocate some space in main memory to the device. This way the device reads and writes directly to main memory without having to bother with DMA. Such device normally have both an I/O address and a main memory address. 5.2. Interrupts --Details Interrupts convey a lot of information but only indirectly. The interrupt signal (a voltage on a wire) just tells a chip called the interrupt controller that a certain device needs attention. The interrupt controller then signals the CPU. The CPU find the driver for this device and runs a part of it known as an "interrupt service routine" (or "interrupt handler"). This "routine" tries to find out what has happened and then deals with the problem such a transferring bytes from (or to) the device. This program (routine) can easily find out what has happened since the device has registers at addresses known to the the driver software (provided the IRQ number and the I/O address of the device has been set correctly). These registers contain status information about the device . The software reads the contents of these registers and by inspecting the contents, finds out what happened, and takes appropriate action.. Thus each device driver needs to know what interrupt number (IRQ) to listen to. On the PCI bus (and for the serial ports on the ISA bus starting with Kernel 2.2) it's possible for two (or more) devices to share the same IRQ number. This is done by the CPU running all interrupt service routines for all devices using that interrupt. The first thing the first service routine does is to check to see if an interrupt actually happened for its device. If there was no interrupt (false alarm) it likely will exit and the next service routine starts, etc. 5.3. PCI Interrupts PCI interrupts are different but since they are normally mapped to IRQ's they behave in about the same way. A major difference is that PCI interrupts may be shared. For example IRQ5 may be shared between two PCI devices. This sharing ability is automatic: you don't need special hardware or software. There have been some reports of situations where such sharing didn't work, but it's likely due to a defect in the device driver software. All device drivers for PCI devices are supposed to provide for interrupt sharing. Note that you can't share the same interrupt between the PCI and ISA bus. However, illegal sharing will work provided the devices which are in conflict are not in use at the same time. "In use" here means that a program is running which "opened" the device in it's C programming code. You may need to know some of the details of the PCI interrupt system in order to set up the BIOS's CMOS or to set jumpers on old PCI cards. The PCI bus has INTA# thru INTD# (A, B, C, and D) which are 4 interrupts. The specs provide for these 4 interrupts for each card so that for a 7-slot system there could be 7 x 4 = 28 different interrupt lines. But the specs permit a fewer number of interrupt lines. This is not too restrictive since interrupts may be shared. Many PCI buses seem to be made with only 4 interrupt lines. Call these lines (wires or traces) W, X, Y, Z. Suppose we designate the B interrupt from slot 3 as interrupt 3B. Then wire W could be used to share interrupts 1A, 2B, 3C, 4D, 5A, 6B, 7C. This is done by physically connecting wire W to wires 1A, 2B, etc. Likewise wire X could be connected to wires 1B, 2C, 3D, 4A, 5B, 6C, 7D. Then on startup, the BIOS maps the X, W, Y, Z to IRQ's. After that it writes the IRQ that each device is mapped to into a hardware register in each device, so that the device (and anything interrogating the device) will know what IRQ it uses. The above mentioned wires X, W, Y, Z are labeled per PCI specs as INTA#, INTB#, INTC# and INTD#. This official PCI notation is confusing since now INTA# has 2 possible meanings depending on whether we are talking about a slot or the PCI bus. For example, if 3C is mapped to X then we say that INTC# of slot 3 is cabled to INTA# (X) of the PCI bus. Confusing notation. There's another requirement also. A PCI slot must use the lower interrupt letters first. Thus if a slot only uses one interrupt, it must be INTA#. If it uses 2 interrupts they must be INTA# and INTB#, etc. A card in a slot may have up to 8 devices on it but there are only 4 PCI interrupts for it. This is OK since interrupts may be shared so that each of the 8 devices (if they exist) can have an interrupt. The PCI interrupt letter of a device is often fixed and hardwired into the device. Since the BIOS maps this to an IRQ, how it does this mapping determines the IRQ it uses. You might reason that using IRQ's is something from the ISA bus and might be slow, etc. Not really. The ISA Interrupt Controller Chip(s) has a direct interrupt wire going to the CPU so it can get immediate attention. While signals on the ISA address and data buses need to go thru the PCI bus to get to the CPU, the IRQ interrupt signals go there almost directly. 5.4. Isolation This is only for the ISA bus. Isolation is a complex method of assigning a temporary handle (id number or Card Select Number = CSN) to each PnP device on the ISA bus. Since there are more efficient (but more complex) ways to do this, some might claim that it's a simple method. Only one write address is used for PnP writes to all PnP devices so that writing to this address goes to all PnP device that are listening. This write address is used to send (assign) a unique handle to each PnP device. To assign this handle requires that only one device be listening when the handle is sent (written) to this common address. All PnP devices have a unique serial number which they use for the process of isolation. Doing isolation is something like a game. It's done using the equivalent of just one common bus wire connecting all PnP devices and the isolation program. For the first round of the "game" all PnP devices listen on this wire and send out simultaneously a sequence of bits to the wire. The allowed bits are either a 1 (positive voltage) or an "open 0" of no voltage (open circuit or tri-state). Each PnP device just starts to sequentially send out its serial number, bit-by-bit, starting with the high-order bit, on this wire. If any device sends a 1, a 1 will be heard on the wire by all other devices. If all devices send an "open 0" nothing will be heard on the wire. The object is to eliminate (by the end of this first round) all but highest serial number device. "Eliminate" means to cease to listen anymore to the write address that all devices still in the game are still listening to. This is also called "dropping out". (Note that all serial numbers are of the same length.) First consider only the high order bit of the serial number which is put on the wire first by all devices which have no handle yet. If any PnP device sends out a 0 (open 0) but hears a 1, this means that some other PnP device has a higher serial number, so it temporarily drops out of this round and doesn't listen anymore until the round is finished (when a handle is assigned to the winner: the highest serial number). Now the devices still in the game all have the same leading digit (a 1) so we may strip off this digit and consider only the resulting "stripped serial number" for future participation in this round. Then go to the start of this paragraph and repeat until the entire serial number has been examined for each device (see below for the all-0 case). Thus it's clear that the highest serial number will not be eliminated from the game. But what happens if the leading digits (of the possibly stripped serial numbers) are all 0? In this case an "open 0" is sent on the line and all participants stay in the game. If they all have a leading 0 then this is a tie and the 0's are stripped off just like the 1's were in the above paragraph. The game then continues as the next digit (of the serial number) is sent out. At the end of the round (after the low-order bit of the serial number has been sent out by whatever participants remain) only one PnP device with the highest serial number remains. It then gets assigned a handle and drops out of the game permanently. Then all the dropouts from the last round (that don't have a handle yet) reenter the game and a new round begins with one less participant. Eventually, all PnP devices are assigned handles. It's easy to prove that this algorithm works. Once all handles are assigned, they are used to address each PnP device and send it a configuration as well as to read configuration info from the PnP device. Note that these handles are only used for PnP configuration and are not used for normal communication with the PnP device. When the computer starts up, all of the handles are lost so that a PnP BIOS usually does the isolation process again each time you start your PC. END OF Plug-and-Play-HOWTO