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Old 10-08-08, 06:47 PM   #9
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Join Date: Sep 2006
Posts: 20
Default Re: What better, PCI-X MSI or Wired IRQ? [177.80]

Originally Posted by logan View Post
Does this mean that only my PCIe ports support MSI?
It depends. It's been part of the PCI spec since version 2.2. I have a QStor PCI SATA card that supports MSI. You might want to do a command like
lspci -vvv |grep -E '[0-9]:[0-9][0-9]\.[0-9]|Message|Address'
and look for bridge and other devices that have the Message Signalled Interrupt capability.
I think there's also some chips on a blacklist. So you should check the boot log for any messages that indicate MSI is being disabled.

Originally Posted by logan View Post
What if my video card is sharing with things that aren't MSI-aware/enabled? Can I still make use of EnableMSI=1?
Yes. When MSI is enabled, the kernel assigns a new IRQ number/route to the device and it is removed from the initial IRQ route. With MSI enabled, the boot log shows this as the initial IRQ when the driver is loading.

[ 7.578952] nvidia 0000:01:00.0: PCI INT A -> GSI 18 (level, low) -> IRQ 18

Then after the driver switches to MSI, /proc/interrupts looks like:

           CPU0       CPU1       
  0:    8869393        244   IO-APIC-edge      timer
  1:         13          9   IO-APIC-edge      i8042
  8:         51          1   IO-APIC-edge      rtc0
  9:          0          4   IO-APIC-fasteoi   acpi
 14:          0          0   IO-APIC-edge      pata_atiixp
 15:          0          0   IO-APIC-edge      pata_atiixp
 16:     104814          1   IO-APIC-fasteoi   ohci_hcd:usb1, HDA Intel
 17:          1          1   IO-APIC-fasteoi   ohci_hcd:usb2, ohci_hcd:usb4
 18:          1          1   IO-APIC-fasteoi   ohci_hcd:usb3, ohci_hcd:usb5
 21:          2          1   IO-APIC-fasteoi   ohci1394
 22:     530004         28   IO-APIC-fasteoi   ahci, ohci_hcd:usb6
218:        291      74047   PCI-MSI-edge      nvidia
219:       1339     566655   PCI-MSI-edge      eth0
220:          0          4   PCI-MSI-edge      sata_qstor
221:       1272     253824   PCI-MSI-edge      ehci_hcd:usb7
Originally Posted by logan View Post
I see that ajw1980 just posted about problems resuming with EnableMSI=1. Is anyone else using this? How's it so far?

On one system I have (AMD 770 / 7600GS), it works quite well.

On a another system I have (MCP51 / 6150PV), it works but Bad Things(TM) under heavy load. I haven't had time to figure out what's up with that one yet as it's a MythTV box as most of the IRQs are unshared and the box is usually busy.

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