_IO_IS_ISA question

[Paul Mackerras]

> Hmmm. include/asm-ppc/io.h   ... twi ... isync ...
>
> I hope that we can do something that doesn't require either twi or isync,
> or sync for that matter, as the cure seems worse than the disease.

The twi/isync sequence has nothing to do with exceptions.  It is there
to ensure that subsequent instructions are not executed until the data
has come back from the device.  It would be nice not to have to do
that, but there are quite a few drivers in the kernel that assume that
I/O operations are synchronous, and we avoid a lot of subtle bugs by
doing this.

Paul.

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