I'm getting ready to write the kstat intr support for a revised audio
driver, and I noticed that it only reports hard, not spurious
interrupts. That got me thinking.
On an x86 system at least, the interrupt chain logic keeps retrying the
interrupt until all devices report DDI_INTR_UNCLAIMED. So, a typical
device on an unshared bus will have at least 1 spurious interrupt for
each hard interrupt. (Ignoring the case where successive calls into the
drivers ISR return DDI_INTR_CLAIMED, of course.)
On a system with shared interrupts (more and more common, *especially*
on x86 systems) the spurious interrupt count is likely to be much higher
for some devices, because a busy device will cause spurious interrupts
(or the appearance thereof) in the less busy device(s) on the same
interrupt.
The upshot of all this is, it seems to me that drivers cannot reliably
count spurious interrupts.
The other observation is that very few drivers bother to use
kstat_intr. (Some drivers, such as NIC drivers, report interrupts
differently, such as in a named kstat.)
There's the intrstat program which uses dtrace, but from what I can
tell, it seems to record entries into the interrupt handler, which
doesn't correctly count hard interrupts (see above for why not!)
So what I'm thinking is that the interrupt framework itself could count
interrupts and keep track of this in a kstat. This is useful for debug,
but it would also be able to provide a more accurate picture than
intrstat does. (Furthermore, as much as I like dtrace, it might make it
possible to build a portable intrstat, that was able to report not just
PCI interrupts but other kinds of interrupts -- hard, soft, pci, pciex,
isa, usb, firewire, sbus, and maybe other kinds in the future (for
example SDcard can support IO devices that interrupt, although the
framework and nexus drivers I just putback into 97 don't have support
for SDIO yet.)
The framework could also count spurious interrupts -- a spurious
interrupt being one where the top-level CPU or nexus specific interrupt
handler dispatched, but didn't find *any* handler that could service the
interrupt. (This could be counted as spurious interrupt for *each*
device on the bus, or it could be counted in a separate top-level
spurious interrupt counter for the bus.)
Thoughts?
-- Garrett
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