Plug-and-Play-HOWTO

• Index
• Introduction
• What PnP Should Do: Allocate "Bus-Resources"
• Setting up a PnP BIOS
• How to Deal with PnP Cards
• Tell the Driver the Configuration ??
• How Do I Find Devices and How Are They Configured?
• PCI Interrupts
• PnP for External and Plug-in Devices
• Error Messages
• Interrupt Sharing and Interrupt Conflicts
• Appendix

4. How to Deal with PnP Cards

4.1 Introduction to Dealing with PnP Devices

Today almost all new internal boards (cards) are Plug-and-Play (PnP). Thus, the configuring of bus-resources should, in almost all cases be entirely automatic. If a device is not working, see if it was detected, possibly by rebooting. If the device driver can't resource-configure it, then hopefully one or more of methods 2-6 will:

1. Device Driver Configures
2. /sys User Interface Configures kernel 2.6 + (not for PCI yet, other severe limitations)
3. BIOS Configures (For the PCI bus you only need a PCI BIOS, otherwise you need a PnP BIOS)
4. ISA cards only: Disable PnP by jumpers or DOS/Windows software (but many cards can't do this)
5. ISA Bus: Isapnp is a program you can always use to configure ISA PnP devices
6. PCI Utilities is for configuring the PCI bus but the device driver should handle it
7. Windows Configures and then you boot Linux from within Windows/DOS. Use as a last resort

Any of the above will set the bus-resources in the hardware but only the first one (and possibly the second) tells the driver what has been done. How the driver gets informed depends on the driver. You may need to do something to inform it. See Tell the Driver the Configuration

4.2 Device Driver Configures, Reserving Resources

Device drivers (with the help of code provided by the kernel) can be written to use PnP methods to set the bus-resources in the hardware but only for the device that they control. But many device drivers just accept what the BIOS or Linux has configured and use code provided by the kernel to find out how this device has been configured. Since the driver has checked the configuration and possibly reconfigured it, it obviously knows the configuration and there is no need for you to tell it this info. This is obviously the easiest way to do it since you don't have to do anything if the driver does it all.

If you have old pre-PnP ISA hardware, the Linux PnP software may not know about it and the bus-resources it requires. So it might erroneously allocate the resources that this old hardware needs to some other device. The result is a resource conflict but there's a way to try to avoid it. You can reserve the resources that the old ISA card needs by configuring the BIOS at boot-time (usually), the isa-pnp module or to the kernel (if the PnP is built into the kernel). For example, to reserve IRQ 5 give this argument to the isa-pnp module (or to the kernel): isapnp_reserve_irq=5. See BootPrompt-HOWTO. Instead of ..._irq there are also _io, _dma, and _mem.

For PCI devices, most drivers will configure PnP. Unfortunately, a driver could grab bus-resources that are needed by other devices (but not yet allocated to them by the kernel). Thus a more sophisticated PnP Linux kernel would be better, where the kernel did the allocation after all requests were in. See How Linux Does PnP.

4.3 /sys User Interface Configures

Starting with kernel 2.6 there's supposedly a new way for the user to resource configure using the /sys directory tree. But as of Aug. 2004, it can't be used for configuring in most cases. See The /sys Directory Tree.

4.4 BIOS Configures

Intro to Using the BIOS to Configure PnP

If you have a PnP BIOS, it can configure the hardware. If the driver can't do it, the BIOS probably can. This means that your BIOS reads the resource requirements of all devices and configures them (allocates bus-resources to them). It is a substitute for a PnP OS except that the BIOS doesn't match up the drivers with their devices nor tell the drivers how it has done the configuring. It should normally use the configuration it has stored in its non-volatile memory (ESCD). If it finds a new device or if there's a conflict, the BIOS should make the necessary changes to the configuration and may not use the same configuration as was in the ESCD. In this case it should update the ESCD to reflect the new situation.

Your BIOS needs to support such configuring and there have been cases where it doesn't do it correctly or completely. The BIOS may need to be told via the CMOS menu that it's not a PnP OS. While many device drivers will be able to automatically detect what the BIOS has done, in some cases you may need to determine it (not always easy). See What Is My Current Configuration? A possible advantage to letting the BIOS do it is that it does its work before Linux starts so it all gets done early in the boot process.

Most BIOS made after about 1996 ?? can resource-configure both the PCI and ISA buses. But it's been claimed that some older BIOSs can only do the PCI. And of course, for PCs with only the PCI bus, the BIOS only needs to do PCI. To try to find out more about your BIOS, look on the Web. Please don't ask me as I don't have data on this. The details of the BIOS that you would like to know about may be hard to find (or not available). Some old BIOS's may have minimal PnP capabilities and seemingly expect the operating system to do it right. If this happens you'll either have to find another method or try to set up the ESCD database if the BIOS has one. See the next section.

The BIOS's ESCD Database

The BIOS maintains a non-volatile database containing a PnP-configuration that it will try to use (if you claim that it's not a PnP OS). It's called the ESCD (Extended System Configuration Data). Again, the provision of ESCD is optional but most PnP-BIOSs have it. The ESCD not only stores the resource-configuration of PnP devices but also stores configuration information of non-PnP devices (and marks them as such) so as to avoid conflicts. The ESCD data is usually saved on a chip and remains intact when the power is off, but sometimes it's kept on a hard-drive??

The ESCD is intended to hold the last used configuration. But since Linux can change how devices are configured (including the user using isapnp or pci utilities) then the ESCD will not know about this and will not save this configuration in the ESCD. A good PnP OS might update the ESCD so you can use it later on for a non-PnP OS (like standard Linux). MS Windows9x does this only in special cases. See Using Windows to set ESCD. Starting with kernel 2.6, Linux is capable of modifying the ESCD but it's not used yet (as of Aug. 2004).

To use what's set in ESCD be sure you've set "Not a PnP OS" or the like in the BIOS's CMOS. Then each time the BIOS starts up (before the Linux OS is loaded) it should configure things this way. If the BIOS detects a new PnP card which is not in the ESCD, then it must allocate bus-resources to the card and update the ESCD. It may even have to change the bus-resources assigned to existing PnP cards and modify the ESCD accordingly.

There's a program that you may use to view the contents of the ESCD. It shows IRQs and IO addresses etc., but device names are missing (only EISA device-ID numbers). It's at: Index of /home/gunther.mayer/lsescd

If each device saved its last configuration in its hardware, hardware configuring wouldn't be needed each time you start your PC. But it doesn't work this way. So all the ESCD data needs to be kept correct if you use the BIOS for PnP. There are some BIOSs that don't have an ESCD but do have some non-volatile memory to store info regarding which bus-resources have been reserved for use by non-PnP cards. Many BIOSs have both.

Using Windows to set the ESCD

Eventually the Linux kernel may set the ESCD. Starting with kernel 2.6, a function in the new code could do it provided the kernel has been compiled with PNPBIOS. But it currently sits in the code unused.

If the BIOS doesn't set up the ESCD the way you want it (or the way it should be) then it would be nice to have a Linux utility to set the ESCD. One may resort to attempting to use Windows for this (if you have it on the same PC) to do this.

There are three ways to use Windows to try to set/modify the ESCD. One way is to use the ICU utility designed for DOS or Windows 3.x. It should also work OK for Windows 9x/2k ?? Another way is to set up devices manually ("forced") under Windows 9x/2k so that Windows will put this info into the ESCD when Windows is shut down normally. The third way is only for legacy devices that are not plug-and-play. If Windows knows about them and what bus-resources they use, then Windows should put this info into the ESCD.

If PnP devices are configured automatically by Windows without the user "forcing" it to change settings, then such settings probably will not make it into the ESCD. Of course Windows may well decide on its own to configure the same as what is set in the ESCD so they could wind up being the same by coincidence.

Windows 9x are PnP operating systems and automatically PnP-configure devices. They maintain their own PnP-database deep down in the Registry (stored in binary Windows files). There is also a lot of other configuration stuff in the Registry besides PnP-bus-resources. There is both a current PnP resource configuration in memory and another (perhaps about the same) stored on the hard disk. To look at this in Windows98 or to force changes to it you use the Device Manager.

In Windows98 there are 2 ways to get to the Device Manager: 1. My Computer --> Control Panel --> System Properties --> Device Manager. 2. (right-click) My Computer --> Properties --> Device Manager. Then in Device Manager you select a device (sometimes a multi-step process if there are a few devices of the same class). Then click on "Properties" and then on "Resources". To attempt to change the resource configuration manually, uncheck "Use automatic settings" and then click on "Change Settings". Now try to change the setting, but it may not let you change it. If it does let you, you have "forced" a change. A message should inform you that it's being forced. If you want to keep the existing setting shown by Windows but make it "forced" then you will have to force a change to something else and then force it back to its original setting.

To see what has been "forced" under Windows98 look at the "forced hardware" list: Start --> Programs --> Accessories --> System Tools --> System Information --> Hardware Resources --> Forced Hardware. When you "force" a change of bus-resources in Windows, it should put your change into the ESCD (provided you exit Windows normally). From the "System Information" window you may also inspect how IRQs and IO ports have been allocated under Windows.

Even if Windows shows no conflict of bus-resources, there may be a conflict under Linux. That's because Windows may assign bus-resources differently than the ESCD does. In the rare case where all devices under Windows are either legacy devices or have been "forced", then Windows and the ESCD configurations should be identical.

Adding a New Device (under Linux or Windows)

If you add a new PnP device and have the BIOS set to "not a PnP OS", then the BIOS should automatically configure it and store the configuration in ESCD. If it's a non-PnP legacy device (or one made that way by jumpers, etc.) then here are a few options to handle it:

You may be able to tell the BIOS directly (via the CMOS setup menus) that certain bus-resources it uses (such as IRQs) are reserved and are not to be allocated by PnP. This does not put this info into the ESCD. But there may be a BIOS menu selection as to whether or not to have these CMOS choices override what may be in the ESCD in case of conflict. Another method is to run ICU under DOS/Windows. Still another is to install it manually under Windows 9x/2k and then make sure its configuration is "forced" (see the previous section). If it's "forced" Windows should update the ESCD when you shut down the PC.

4.5 ISA cards only: Disable PnP ?

PCI devices are inherently PnP so it can't be disabled. But a few ISA devices once had options for disabling PnP by jumpers or by running a Windows program that comes with the device (jumperless configuration). If the device driver can't configure it, this will avoid the possibly complicated task of doing PnP configuring. Don't forget to tell the BIOS that these bus-resources are reserved. But since Linux support for PnP has improved, you usually don't want to disable PnP. Here's some more arguments in favor of PnP:

1. If you have MS Windows on the same machine, then you may want to allow PnP to configure devices differently under Windows from what it does under Linux.
2. The range of selection for IRQ numbers (or port addresses) etc. may be too limited unless you use PnP.
3. You might have a Linux device driver that uses PnP methods to search for the device it controls.
4. If you need to change the configuration in the future, it may be easier to do this if it's PnP (no setting of jumpers or running a Dos/Windows program).

Once configured as non-PnP devices, they can't be configured by PnP software or a PnP-BIOS (until you move jumpers and/or use the Dos/Windows configuration software again).

4.6 ISA Bus: Isapnp (part of isapnptools)

The isapnp standalone program is only for PnP devices on the ISA bus (non-PCI). It was much needed prior to the 2.4 kernels. After the 2.4 kernel, which provided functionality to allow drivers deal with ISA PnP, the isapnp standalone program is less significant. Also, the BIOS may configure ISA PnP satisfactory. But the isa-pnp module (or the equivalent built into the kernel) is now very significant since various ISA device drivers call on it to configure bus-resources. Prior to kernel 2.6 it resulted a /proc/isapnp "file" which may be used to manually configure (see isapnp.txt in the kernel documentation).

In some cases Linux distributions have been set up to run isapnp automatically at startup. It's still done in 2004 but it isn't really needed if the device drivers work well. If you need to set it up yourself much of the documentation for isapnp is difficult to understand unless you know the basics of PnP. This HOWTO should help you understand it as well the FAQ that comes with isapnp. Running the Linux program "isapnp" at boot-time will configure such devices to the resource values specified in /etc/isapnp.conf. Its possible to create this configuration file automatically but you then should edit it manually to choose between various options. Then to let the driver know the resources, you often need to specify them as parameters to the appropriate modules (drivers). This is done with configuration files, often in the /etc directory. Look there for files named mod*, etc. If the driver is built into the kernel, then they may sometimes be given as a parameter to the kernel. See BootPrompt-HOWTO.

With isapnp there once was a problem where a device driver which is built into the kernel may run too early before isapnp has set the address, etc. in the hardware. This resulted in the device driver not being able to find the device. The driver tries the right address but the address hasn't been set yet in the hardware. Is this still a problem ??

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.

If the configuration file is wrong or doesn't exist, you can use the program "pnpdump" to help create the configuration file. It almost creates a configuration file for you but you must skillfully edit it a little before using it. It contains some comments to help you edit it. While the BIOS may also configure the ISA devices (if you've told it that you don't have a PnP OS), isapnp will redo it.

The terminology used in the /etc/isapnp.conf file may seem odd at first. For example for an IO address of 0x3e8 you might see "(IO 0 (BASE 0x3e8))" instead. The "IO 0" means this is the first (0th) IO address-range that this device uses. Another way to express all this would be: "IO[0] = 0x3e8" but isapnp doesn't do it this way. "IO 1" would mean that this is the second IO address range used by this device, etc. "INT 0" has a similar meaning but for IRQs (interrupts). A single card may contain several physical devices but the above explanation was for just one of these devices.

4.7 PCI Utilities

The package PCI Utilities (= pciutils, sometimes called "pcitools"), allows one to manually PnP-configure the PCI bus (with difficulty). "lspci" or "scanpci" lists bus-resources while "setpci" sets resource allocations (except IRQs) in the hardware devices. It appears that setpci is mainly intended for use in scripts and one needs to understand the details of the PCI configuration registers in order to use it. That's a topic not explained here nor in the manual page for setpci.

People have used this to configure PCI devices where the driver failed to do it. An example is found in my Modem-HOWTO and Serial-HOWTO in the subsection "PCI: Enabling a disabled port". However, enabling a device is of no use unless you have a working driver for the device.

4.8 Windows Configures

This method uses MS Windows to configure and should be used only if all else fails. If you have Windows9x (or 2k) on the same PC, then just start Windows and let it configure PnP. Then start Linux from Windows (or DOS) using, for example, loadlin.exe. But there may be a problem with IRQs for PCI devices. As Windows shuts down (without any messages) to make way for Linux, it may erase (zero) the IRQ which is stored in one of the PCI device's configuration registers. Linux will complain that it has found an IRQ of zero.

The above is reported to happen if you start Linux using a shortcut (PIF file). But a workaround is reported where you still use the shortcut PIF. A shortcut is something like a symbolic link in Linux but it's more than that since it may be "configured". To start Linux from DOS you create a batch file (script) which starts Linux. (The program that starts Linux is in the package called "loadlin"). Then create a PIF shortcut to that batch file and get to the "Properties" dialog box for the shortcut. Select "Advanced" and then check "MS-DOS mode" to get it to start in genuine MS-DOS.

Now here's the trick to prevent zeroing the PCI IRQs. Click "Specify a new MS-DOS configuration". Then either accept the default configuration presented to you or click on "Configuration" to change it. Now when you start Linux by clicking on the shortcut, new configuration files (Config.sys and Autoexec.bat) will be created per your new configuration.

The old files are stored as "Config.wos and Autoexec.wos". After you are done using Linux and shut down your PC then you'll need these files again so that you can run DOS the next time you start your PC. You need to ensure that the names get restored to *.sys and *.bat. When you leave Windows/DOS to enter Linux, Windows is expecting that when you are done using Linux you will return to Windows so that Windows can automatically restore these files to their original names. But this doesn't happen since when you exit Linux you shut down your PC and don't get back to Windows. So how do you get these files renamed? It's easy, just put commands into your "start-Linux" batch file to rename these files to their *.bat and *.sys names. Put these renaming commands into your batch file just before the line that loads Linux.

Also it's reported that you should click on the "General" tab (of the "Properties" dialog of your shortcut) and check "Read-only". Otherwise Windows may reset the "Advanced Settings" to "Use current MS-DOS configuration" and PCI IRQs get zeroed. Thus Windows erases the IRQs when you use the current MS-DOS configuration but doesn't erase when you use a new configuration (which may actually configure things identical to the old configuration). Windows does not seem to be very consistent.

4.9 PnP Software/Documents

• Isapnptools homepage
• Proposal for a Configuration Manager for Linux 1999 (Never got into kernel but Linux is slowly "evolving" in this direction).
• PnP Specs. from Microsoft
• Book: PCI System Architecture, 4th ed. by Tom Shanley +, MindShare 1999. Covers PnP-like features on the PCI bus.
• Book: Plug and Play System Architecture, by Tom Shanley, Mind Share 1999. Details of PnP on the ISA bus. Only a terse overview of PnP on the PCI bus.
• Book: Programming Plug and Play, by James Kelsey, Sams 1995. Details of programming to communicate with a PnP BIOS. Covers ISA, PCI, and PCMCIA buses.