Return value from IOFilterInterruptEventSource::Filter

Hello everyone,

 

 We're using IOFilterInterruptEventSource for one of our device drivers. On some systems this device shares its interrupt with other devices, and on some it gets its own interrupt.

 As I understand from the documentation Filter() should return a true/false value indicating whether the interrupt originated from our device or not, and when Filter() returns true, the secondary interrupt handler will be called in workloop context to handle the interrupt.

 

 In many cases, however, we can easily finish the handling of the interrupt within the filter function without requiring the invocation of the secondary handler, e.g. by simply acknowledging the interrupt to the HW and incrementing a counter in memory.

 Theoretically it could be ok to return false from the Filter() in such a case and waive the secondary handler, but this contradicts with the implied role of Filter() which is to determine where the interrupt came from. This could result in OS code seeing an interrupt that nobody claims responsibility for; a paranoid OS could assume that the interrupt remains unacknowledged in the HW and choose to protect itself by disabling the interrupt source altogether (to prevent an infinite interrupt loop).

 

 So the question is, what is the requirement from the return value of Filter()? Do I always need to return true if my device generated the interrupt, or is it safe to return false if the interrupt was completely handled by the Filter()? Following the documentation to the letter it seems that the latter is true, but I still can't be certain about it.

 

 Thanks,

   Udi.