twi/isync/nop sequence in newer ppc linux kernels

Hi,

I'm programming an embedded device (with PPC405GP/EP) that has to access the
pci bus and there may be cases that there is no one listening there and a
timeout occurs which results in a machine check. Until now I used a eieio
operation with an exception handler after the actual io operation and this
seems to work. Current kernels contain a twi/isync/nop sequence. What are
the reasons for this sequence instead of an eieio? Is it only needed on
some processors? Is there a document that describes this special
sequence in more detail?

Thanks in advance,
Ronald

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