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2004/01/30 spurious interrupt

© January 2004 Tony Lawrence

A spurious interrupt is an interrupt the driver didn't expect.

Briefly, devices such as disks, parallel ports, serial ports, etc. use interrupts to signal back to the cpu that they have completed the last task given them or that the device has recieved data that the kernel needs to put away somewhere. The rationale behind this is that devides are very slow; you can hand them data, and there is room for thousands or even millions of other cpu instructions to be done before that device is ready to handle more data. Interrupts can also signal a need for attention: a serial port recieving characters sends interrupts letting the driver know that new data has arrived.

So a spurious interrupt is unexpected. You didn't give the damn thing any data, so why is it tapping you on the shoulder looking for attention? You could also have the case of an interrupt on a line that isn't configured at all: say the kernel hasn't been told of any device using interrupt 11, but whoops, there it is. That generates a different message, though.

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I have a pentium IV with a FasTrak RAID adaptor, and linux redhat 9, and 3 additional serial port PCI cards. When I use these serial ports with modems, I have the syslog messages:
kernel: ttyS: 1 input overrun(s)
kernel: spurious 8259A interrupt: IRQ7
and on my other server which has RAID included in the Gigabyte motherboard:
kernel: IT8212Interrupt: Spurious interrupt (Interrupt bit is not on)
These issues disturb serial transmissions.
Does anybody knows why, and what to do?

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