[linux-audio-user] IRQ settings and low latency

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Subject: [linux-audio-user] IRQ settings and low latency
From: Greg Jednaszewski (greg_AT_attenuated.org)
Date: Thu Mar 11 2004 - 22:15:41 EET


Hi,

I've been working on getting my linux system set up for recording with
jack/ardour. I came across the "Low Latency mini howto":
http://www.djcj.org/LAU/guide/Low_latency-Mini-HOWTO.php3

At the end, it talks about setting up IRQs such that the sound card is
at a high priority relative to other devices in the system. However,
all the information in the doc pertains to non-APIC machines. What
about an APIC-enabled machine?

The output of 'cat /proc/interrupts' shows my sound card (ICE1712) dead
last:

           CPU0 CPU1
  0: 6628091 0 IO-APIC-edge timer
  1: 6 0 IO-APIC-edge keyboard
  2: 0 0 XT-PIC cascade
  9: 0 0 XT-PIC acpi
 12: 803 0 IO-APIC-edge PS/2 Mouse
 15: 5 0 IO-APIC-edge ide1
 16: 0 0 IO-APIC-level usb-uhci, usb-uhci
 18: 4178 0 IO-APIC-level libata, usb-uhci
 19: 320 0 IO-APIC-level usb-uhci
 20: 6406 0 IO-APIC-level eth0
 21: 0 0 IO-APIC-level ICE1712
NMI: 0 0
LOC: 6628191 6628246
ERR: 0
MIS: 0

output of: lspci -v -s 02:01.0
02:01.0 Multimedia audio controller: IC Ensemble Inc ICE1712 [Envy24]
(rev 02)
        Subsystem: IC Ensemble Inc: Unknown device d634
        Flags: bus master, medium devsel, latency 32, IRQ 21
        I/O ports at 8000 [size=32]
        I/O ports at 8400 [size=16]
        I/O ports at 8800 [size=16]
        I/O ports at 8c00 [size=64]
        Capabilities: [80] Power Management version 1

I've tried manually setting IRQs in the BIOS, moving the card around,
etc. No amount of tweaking can move it any higher in the list. (It's
set to IRQ 10 in the BIOS)

I guess my question is: Does the IRQ priority matter when IRQs are
abstracted by the APIC? If so, how does one change the priority in this
situation? I'm running 2.4.22, with the low latency and kernel
preemption patches installed.

Thanks,
Greg

-- 
Greg Jednaszewski
greg_AT_attenuated.org


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