Hello, I have a question regarding how interrupts are routed on Intel platforms. I am not sure whether kernelnewbies is the right place to ask the question but I couldn't find a mailing list that discusses intel chipset related issues. If someone can point me there, that will be really helpful. My system has Intel Xeon CPU, E7520 (MCH) and 82801ER (ICH). The MCH has three PCI express ports and is interfaced to ICH through HL1.5. A syskonnect PCIe ethernet card is installed on one of the slots of MCH. It don't have an IOAPIC. The ICH has the IOAPIC. My questions are: 1. The syskonnect driver, by default uses INTx mechanism. I installed the driver and from /proc/interrupts, I find that IRQ number 169 is allocated for this card. Please see cat /proc/interrupts below: 0: 124 88320 IO-APIC-edge timer 1: 0 9 IO-APIC-edge i8042 8: 0 0 IO-APIC-edge rtc 9: 0 0 IO-APIC-level acpi 14: 0 1836 IO-APIC-edge ide0 169: 0 0 IO-APIC-level eth2 177: 0 293 IO-APIC-level eth0 NMI: 88671 88644 LOC: 88257 88310 ERR: 0 MIS: 0 It also shows that the interrupt is delivered by the IO-APIC. How this is possible? As I mentioned above, the only link I could find between MCH and ICH is HL1.5. I looked into the spec of MCH and ICH and couldn't find any specific registers that will let the routing of interrupts from MCH to ICH IOAPIC. (You may find the specs from the following URL: http://www.intel.com/design/chipsets/E7520_E7320/documentation.htm) 2. Then I modified the driver and added pci_enable_msi() to use MSI instead of INTx. At that time, I got a different IRQ (209) number cat /proc/interrupts CPU0 CPU1 0: 124 407539 IO-APIC-edge timer 1: 0 9 IO-APIC-edge i8042 8: 0 0 IO-APIC-edge rtc 9: 0 0 IO-APIC-level acpi 14: 0 1950 IO-APIC-edge ide0 177: 0 738 IO-APIC-level eth0 209: 0 0 PCI-MSI eth2 NMI: 408756 408795 LOC: 407492 407490 ERR: 0 MIS: 0 I think I understand the reason. MSI interrupts bypass the IOAPIC and goes directly to the LocalAPIC of the CPU (at address 0xFEExxxxx). Am I right? Thanks. Sadik. -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
