If we have multiple interrupt resources, like for Z8530 clones on the mac-io bus, we cannot setup FAST interrupt handlers. This because we use spinlocks to protect the hardware and all interrupt resources are assigned the same interrupt handler. When the interrupt handler is invoked for interrupt X, it could be preempted for interrupt Y while it was holding the lock (where X and Y are the interrupt resources corresponding a single instance of this driver). This is a deadlock. By only using a MPSAFE handler in that case we prevent preemption.
If we have multiple interrupt resources, like for Z8530 clones on the
mac-io bus, we cannot setup FAST interrupt handlers. This because we
use spinlocks to protect the hardware and all interrupt resources are
assigned the same interrupt handler. When the interrupt handler is
invoked for interrupt X, it could be preempted for interrupt Y while
it was holding the lock (where X and Y are the interrupt resources
corresponding a single instance of this driver). This is a deadlock.
By only using a MPSAFE handler in that case we prevent preemption.

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