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7. PCI Interrupts

7.1 Introduction

Each PCI device that needs an interrupt comes with a fixed PCI interrupt that can't be changed. It's designated by a slot number and a letter A, B, C, or D. Example 3:B. But this PCI interrupt is mapped (routed or redirected) to an interrupt number like say 21 by a chip on the motherboard.

This routing is done by a "programmable interrupt router" = PIR. Alternatively, an interrupt line may just routed directly (without any PIR). If there's a PIR (router) it can be programmed by the BIOS or by Linux. Thus a PCI device's interrupt may be sometimes be changed, not by sending the interrupt on a different wire but by changing the routing of the pulse on that wire by programming the PIR. When the routing changes, the interrupt provide by this new routing is written in a configuration register located in the device chip.

7.2 History: From ISA to PCI Interrupts

Before the PCI bus, PCs used the ISA bus and then during the transition to the newer PCI bus, most PC computers used both the PCI and ISA busses. The ISA bus had all interrupt lines going to every card so any card could change its irq number just by sending out its interrupt signal on a different line (on a different pin). All the interrupt signals were sent to the in interrupt controller which then signalled the CPU to temporarily stop whatever it was doing and run driver code to service the interrupt.

When PCI first appeared, the simple solution was just to map the PCI interrupts to available ISA interrupts that weren't being used. This required the use of "programmable interrupt router" = PIR (hardware) to do this mapping. But since there were only 15 such interrupts, it was common to put many PCI devices on just the few available interrupts. To solve this problem is simple: provide new hardware to increase the number of interrupts. The result was the APIC. But it was slow to be adopted since the ability of the PCI bus to share interrupts eased the interrupt shortage problem. So APIC was mostly used where it was needed for dual processors.

7.3 Advanced Programmable Interrupt Controller (APIC)

This can provide (depending on the model) 16, 24, 32, or 64 interrupts, etc. It also can handle the routing of interrupts from one CPU to another for cases of multiple CPUs. See the file "IO-APIC" in the i386 directory of the kernel documentation and the ACPI-HOWTO. Don't confuse APIC with ACPI (Advanced Configuration and Power Interface) which may be used by the kernel to configure the APIC.

The actual APIC controller that is connected to the interrupt lines is an I/O APIC (or IO-APIC or IOAPIC). By using more than one IO-APIC one may obtain more interrupts and they are numbered so as to be unique. For example, the first controller could have input pins 0-23 and the second would call its input pins 24-47, resulting in 48 interrupts numbered 0-47. But a few people find they have high interrupt numbers. Could it be that the second IO-APIC is starting with a higher base number than it should, leaving a long gap of non-existent irqs?

Besides IO-APICs there are local APICs (LAPIC) which are part of each CPU. The IO-APIC does it work by communicating with the LAPICs inside the CPUs.

When APIC was introduced, the old ISA PICs were also retained giving one a choice of whether or not to use APIC or ISA's PIC (which is sometimes just called PIC or XT-PIC in /proc/interrupts; the "XT" comes from IBM's XT PC which was IBM's second model PC in 1983). It's possible to tell the kernel (on the kernel command line) to not use APIC in which case it will use the old XT-PIC if its available. But since APIC can have more interrupts than the 15 provided by XT-PIC, there could be problems ??

To see if you have PIC or APIC look at /proc/interrupts. If you see XT-PIC for just irq 2 but IO-APIC for the others, it may mean that you have the old XT-PIC but it isn't being currently used. Well, irq 2 is available for communication between two old XT-PICs just in case you might need to use them if you were to disable APIC. There are two XT-PICs since each only supports 8 interrupts.

7.4 Message Signalled Interrupts (MSI)

Another development is Message Signalled Interrupts (MSI) where the interrupt is just a message sent to a special address over the main computer bus (no interrupt lines needed). But the device that sends such a message must first gain control of the main bus so that it can send the interrupt message. Such a message contains more info than just "I'm sending an interrupt". It contains an index for the address of program that needs to be run to service the IRQ. That index, such as 3, would mean the the cpu find the address it must jump to in the 3rd element of a special table that the cpu knows about.

Since cards must support MSI and many cards don't, it seems that the conventional methods of interrupt hardware support (called INTx) will be around for a long time.

7.5 Sharing PCI Interrupts

PCI interrupts may be shared, meaning that two or more PCI devices will generate the same IRQ. If feasible, it's usually better not to share. Sharing doesn't work right for: 1. very old PCI hardware (before 1995 ??) 2. defective PCI hardware which may have a factory defect (it was made that way). For example, if a PCI device on IRQ9 falsely claims that any IRQ9 was intended for it, then other devices using IRQ9 may wind up having all IRQs they issue ignored since the bad device is falsely claiming their IRQs. With no sharing, this problem is avoided.

For an example of sharing the same IRQ between two PCI devices. see PCI interrupt sharing This sharing ability is built into the hardware and all device drivers are supposed to support it. Note that you usually can't share the same interrupt between the PCI and ISA bus.

7.6 Looking at Routing Tables

Some info is provided by the boot-time messages which may be viewed by typing dmesg. The following ways of looking at tables involve software which you may not have (or doesn't exist yet). To check routing where PCI routes to the 16 ISA interrupts use pirtool which shows the $PIR routing table. If you have APIC with hard-wired routing (no PIR), use mptable to look at the MP table. For routeable APIC, a table is accessed by ACPI _PRT methods (but is there a command-line command to do this?)

7.7 For More Information

Detailed technical information about interrupts is at PCI Interrupts for x86 Machines under FreeBSD. Microsoft has The Importance of Implementing APIC-Based Interrupt Subsystems on Uniprocessor PCs

7.8 PCI Interrupt Linking

Here are some of the details of the PCI interrupt system. Each PCI card (and device mounted on the motherboard) has 4 possible interrupts: INTA#, INTB#, INTC#, INTD#. From now on we will call them just A, B, C, and D. Each has its own pin on the edge connector of a PCI card. Thus for a 7-slot system (for 7 cards) there could be 7 x 4 = 28 different interrupt lines for these cards. Devices built into the motherboard also have additional interrupts. But the specs permit a fewer number of interrupt lines, so some PCI buses seem to be made with only 4 or 8 interrupt lines. This is not too restrictive since interrupts may be shared. For 4 interrupt line (wires, traces, or links) LNKA, LNKB, LNKC, LNKD there is a programmable "interrupt router" chip that routes LNKA, LNKB, LNKC, LNKD to selected IRQs. This routing can be changed by the BIOS or Linux. For example, LNKA may be routed to IRQ5. Suppose we designate the B interrupt from slot 3 as interrupt 3B. Then interrupts 3B and 2A could both be permanently connected to LNKA which is routed to IRQ5. These 2 interrupts: 3B and 2A are permanently shared by hardwiring on the motherboard.

One may type "dmesg" on the command line to see how interrupt lines like LNKA are routed (or linked) to IRQs (*5 means that it's linked to IRQ 5). Look for "PCI Interrupt Link". Note that "link" is used here with two meanings: 1. the linking (routing) of PCI interrupt lines to IRQs. 2. the label of an interrupt line such as LNKB (link B). The interrupt line labels seem to be provided by the Bios ?? and they may have many different names like: LNKC, LNK2, APCF, LUBA, LIDE, etc. Question: When a large number of interrupt lines are shown disabled, do they all physically exist on the motherboard? Or do they just exist only in the ACPI BIOS software so that the BIOS can work with motherboards which have more interrupt lines?

One simple method of connecting (hard-wiring) these lines from PCI devices (such as 3B) to the interrupts LNKA, etc. would be to connect all A interrupts (INTA#) to line LNKA, all B's to LNKB, etc. This method was once used many years ago but it is not a good solution. Here's why. If a card only needs one interrupt, it's required that it use A. If it needs two interrupts, it must use both A and B, etc. Thus INTA# is used much more often than INTD#. So one winds up with an excessive number of interrupts sharing the first line (LNKA connected to all the INTA#). To overcome this problem one may connect them in a more random way so that each of the 4 interrupt lines (LNKA, LNKB, LNKC, LNKD) will share about the same number of actual PCI interrupts.

One method of doing this would be to have wire LNKA share interrupts 1A, 2B, 3C, 4D, 5A, 6B, 7C. This is done by physically connecting wire W to wires 1A, 2B, etc. Likewise wire LNKB could be connected to wires 1B, 2C, 3D, 4A, 5B, 6C, 7D, etc. Then on startup, the BIOS maps the LNKB, LNKA, LNKC, LNKD to IRQs. After that, it writes the IRQ that each device uses into a hardware configuration register in each device. From then on, any program interrogating this register can find out what IRQ the device uses. Note that just writing the IRQ in a register on a PCI card doesn't in any way set the IRQ for that device.

A practical use for this info is that, as a last resort, one may change the IRQs of a PCI card by inserting it in a different slot. In the above example, INTA# of a PCI card will be connected to wire LNKA if the card is inserted into slot 1 (1A maps to LNKA) but INTA# will be connected to wire LNKB when it's inserted into slot 4 (4A maps to LNKB).

A card in a slot may have up to 8 devices on it but there are only 4 PCI interrupts for it (A, B, C, D). This is OK since interrupts may be shared so that each of the 8 devices (if they exist) can have an shared interrupt. The PCI interrupt letter of a device is often fixed and hardwired into the device. The assignment of interrupts is done by either the BIOS or Linux mapping the PCI interrupts to the ISA-like interrupts as mentioned above.

If there are only 4 lines (LNKA, LNKB, LNKC, and LNKD) as in the above example, the mapping choices that the PCI BIOS has are limited. Some motherboards may use more lines and thus have more choices. For example LNKA-LNKH (8 lines). The boot-time messages (and dmesg) may display them and how they are mapped. The BIOS knows about how they are wired.

On the PCI bus, the BIOS (or Linux) assigns IRQs (interrupts) so as to avoid conflicts with the IRQs it knows about on the ISA bus. Sometimes the CMOS BIOS menu may allow one to assign IRQs to PCI cards or to tell the BIOS what IRQs are to be reserved for ISA devices. The assignments are known as a "routing table". In MS Windows it's called "IRQ steering" but this also covers the case of dynamic IRQ routing after boot-time. The BIOS may support it's own IRQ steering.

If your PC uses PCI interrupts which are mapped to ISA interrupts, you right think that interrupts might be slow since the ISA bus was slow. 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 old ISA address and data buses are slow to get to the CPU, the IRQ interrupt signals get there very fast.


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