Change IRQ assignments

[pelckyboy]

After my motherboard died a few weeks ago, I bought a replacement board from AsRock. This board does not allow me to modify the interrupt assignments in the bios. Standard bios interrupt assignments are :

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  0:      27719          XT-PIC  timer
  1:        147          XT-PIC  i8042
  2:          0          XT-PIC  cascade
  3:          0          XT-PIC  ehci_hcd:usb1
  4:          0          XT-PIC  ohci_hcd:usb3
  5:       6069          XT-PIC  ohci_hcd:usb2, saa7134[0], saa7134[0], saa7146 
(0), ICE1712, eth0
 11:       5023          XT-PIC  libata
 12:        170          XT-PIC  i8042
 14:      11100          XT-PIC  ide0
 15:         24          XT-PIC  ide1

3 of my 4 PCI cards are assigned to IRQ5. The 4th PCI card is a second video card that gets IRQ10 (I can see that from lspci -v).

As this did not seem a very healthy situation to me, I recompiled my kernel with IO-APIC and ACPI enabled. Now I get following interrupt assignments :

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  0:    4453508    IO-APIC-edge  timer
  1:       5244    IO-APIC-edge  i8042
  9:          0   IO-APIC-level  acpi
 12:      20706    IO-APIC-edge  i8042
 14:      35046    IO-APIC-edge  ide0
 15:         24    IO-APIC-edge  ide1
 17:     986149   IO-APIC-level  saa7146 (0), matroxfb
 18:    1810352   IO-APIC-level  ohci_hcd:usb2, eth0
 19:      27526   IO-APIC-level  libata, ohci_hcd:usb3
 20:          0   IO-APIC-level  ehci_hcd:usb1
 21:          0   IO-APIC-level  saa7134[0], saa7134[0]
 22:       7889   IO-APIC-level  ICE1712

This looks great except that the video card now gets IRQ 16 (which is ok), which interferes with my sound card on IRQ 22. The sound stutters whenever a widget is displayed. After some googling and reading about interrupt handling, I think that the PCI video card on 16 is generating lots of interrupts, so I should try and give it a higher interrupt compared to the IRQ 22 for the sound card.

I could switch the cards physically, but I am afraid that the APIC will still assign a lower IRQ to the video card as it is first detected. In the kernel documentation I read that it is possible to assign IRQs by appending pirq=10,5,9,11 to the kernel boot parameters, but it does not seem to work.

Can this be done for a nVidia nForce2 ?

[pelckyboy]

Ok, I switched the cards and put the video card (an old Matrox Millenium II) in the 4th PCI slot. After reboot, I got the same interrupt assignments as before, but the stuttering was gone !!! Then I had to do another kernel modification and rebooted, and it was back... Dmesg output, lspci -vvv was same, except for cat /proc/interrupts :

GOOD :

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17:         56   IO-APIC-level  saa7146 (0)

BAD :

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17:     541687   IO-APIC-level  saa7146 (0), matroxfb

So the matroxfb is the culprit. Now it is a mistery to me why it was not loaded when I first booted. I'm running DirectFB, X and QT-embedded over framebuffers all the time and I was able to start without them ?