[linux-audio-user] APIC is bad?

Clemens Ladisch clemens at ladisch.de
Fri Jul 16 12:06:33 EDT 2004

Mark Knecht wrote:
> No, you are exactly right. Clemens and I have some small technical
> disagreements on this subject. We had a conversation that was
> interesting but I think didn't yield any clear answers, at least in my
> mind, so I freely admit I'm still confused on the subject.
> [...] As I remember the conversation the interrupt order was from
> the highest interrupt number and going down from there, so if
> Florin sees 25 interrupts, then #25 gets serviced first and that
> would be where I'd want my sound card, if I could get it there.

What I tried to explain was that the priority is not determined by the
interrupt number but by the interrupt vector number.

Well, let's see if my explanation isn't too muddy this time :-)

The routing of interrupts from the hardware to the driver happens on
three levels.

First, there are the (PCI) interrupt lines, which are the input pins
of the interrupt controller chip.  These are usually called
PIRQA/B/C...  The only way to choose the interrupt line for the sound
card is to put it into another PCI slot.  The interrupt line of
integrated devices is usually fixed.

The interrupt controller (PIC or I/O-APIC) has a number of hardware
interrupts (PIC 16, I/O-APIC 24? or more).  Each interrupt line is
mapped to one interrupt.  In PIC mode, the routing of interrupt line
to interrupt number can be chosen by software (but is done by the BIOS
in practice).  In I/O-APIC mode, the routing of interrupt lines to
interrupts is fixed: each PCI interrupt line gets one of the
interrupts above 15.
These are the interrupt numbers shown in /proc/interrupts.

The interrupts are mapped to interrupt vectors.  Intel CPUs have 256
of them, but some are used for other purposes than hardware
interrupts.  The interrupt vector numbers are chosen by the Linux
kernel, and are fairly well hidden, but you can find them in the dmesg
output if I/O-APIC is enabled.

When the I/O-APIC is not enabled, interrupt priority is determined by
the interrupt number (0 1 8 9 10 11 12 13 14 15 3 4 5 6 7).

When the I/O-APIC is enabled, interrupt priority is determined by the
interrupt vector number (lower vectors numbers have higher priority).

(My patch to change vector numbers doesn't work with current kernels.)


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