[linux-audio-user] IRQ conflicts, acpi, and linux audio

Mark Knecht markknecht at gmail.com
Sun Nov 27 10:44:30 EST 2005

On 11/27/05, Paul Davis <paul at linuxaudiosystems.com> wrote:
> On Sat, 2005-11-26 at 17:40 -0800, Mark Knecht wrote:
> > Changing interrupt priorities of internal devices on a laptop is not
> > possible. The devices are on the motherboard and are hard wired to
> > specific inputs on the PIC. You cannot change them at all.
> i am not sure that this is strictly true. my impression of modern intel-
> based architecture is that the inputs to the (A)PIC do not map
> deterministically to IRQ lines feeding the CPU.

This is true. APIC inputs != IRQs. If a PC (laptop, desktop, etc.)
uses an APIC then the idea is that each hardware device can have it's
own input on the APIC and eventually it's own APIC IRQ.

However, if a PC (laptop, desktop, etc.) shares interrupts going into
an APIC then these (TTBOMK) cannot be separated. If they are the same
trace on the motherboard then they map to a single shared IRQ.

> the (A)PIC and its
> cousins can be programmed to do many different things. the problem with
> laptops is generally that they come with a BIOS that offers no options
> to "reprogram" the (A)PIC, and then you boot into a kernel that
> generally seems to want to leave this stuff alone. this is increasingly
> the case on non-laptops as well, which is quite depressing.

You raise a very valuable point. Whether using a PIC or an APIC some
BIOS's do allow the user to:

1) Disable the APIC in favor of an old style PIC
2) Change the IRQ number of a limited number of internal hardware devices.

As far as I know Linux does leave BIOS IRQ settings alone, or can be
told to.Possibly thewade should look at what BIOS control, if any, his
machine gives him.

- Mark

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